1. business and industrial
2. construction

Technical
Extra
October 2014 | Issue 15
In this issue:
NHBC STANDARDS
Focus on cavity trays and DPCs
page 3
Movement joints in external masonry walls page 7
Slating and tiling for pitched roofs – BS 5534 page 9
REGULATION AND COMPLIANCE
Type Approvals: Certificate changes page 11
Building Regulations – Regulation 38 page 12
Publication of Category 4 Screening Levels for
land affected by contamination page 14
GUIDANCE AND GOOD PRACTICE
Copings and cappings to brick chimneys page 18
Suspended beam and block concrete floors page 20
Installation of GRP dormer windows
page 24
Audible cracking noises in intermediate floors
page 25
INFORMATION AND SUPPORT
Information and support
page 26
Technical news
page 27
For technical advice and support, call 01908 747384 or visit www.nhbc.co.uk
Technical Extra | Issue 07 | July 2012 | Page 1
Foreword
Welcome to Technical Extra 15
Our lead article in this edition focuses on the importance of correctly installing
cavity trays and DPCs, particularly around openings and at abutments,
expanding on an area we touched on in the annual claims review published in
Technical Extra 14.
The importance of these critical areas can’t be stressed enough, and this article
highlights how apparently small deficiencies in detailing can lead to both costly
repairs and considerable disruption to homeowners. If you’ve had to revisit your
sites, do take the opportunity to learn from the repairs you’ve had to undertake
and ensure current work practices address any inherent deficiencies of the past.
Within Regulation and compliance, we discuss the transfer of Fire safety
information to the responsible person – Regulation 38 of the Building Regulations.
If you are uncertain of your requirements in this regard, please contact your
NHBC Building Surveyor.
Other articles highlight changes to the code of practice relating to slating and
tiling for pitched roofs and vertical cladding, and the publication of Category 4
screening levels for land affected by contamination.
Among the articles in Guidance and good practice, we review GRP dormer windows
and copings and cappings to brick chimneys, two additions to the roof line where
shortcomings in detailing and construction can greatly affect long-term
performance. We also draw attention to suspended beam and block floors and, in
particular, the need to ensure that adequate structural performance of floors is
not impaired by the method of achieving sufficient levels of thermal insulation.
Finally, whilst technical content has been regularly updated, the current format of
the Standards has remained largely unchanged for over 20 years. NHBC is
therefore pleased to announce that we have embarked on a full review of the
Standards format. Work is advancing well and we intend to publish the newly
formatted Standards during 2015; for now, the current edition remains in place
until the new Standards are launched. In the meantime, we’ll be publishing a new
Chapter that will set the benchmark for waterproofing of basements and below
ground structures. The new Chapter will be released shortly and accompanied by
a special edition of Technical Extra.
Mark Jones
Head of House-Building Standards
NHBC STANDARDS
Focus on cavity trays and DPCs
Learning lessons from the wind and rain storm conditions of 2013/14
Who should read this: Technical and construction directors and
managers, architects, designers and site managers.
INTRODUCTION
As we now enter autumn, it seems wise to reflect on the somewhat unprecedented weather conditions in terms
of wind and rain that were experienced during the winter of 2013/14.
BACKGROUND
During a three-month period in the winter of 2013/14,
claims made to NHBC increased by over 40%
compared with normal levels for the same period in
previous years. Around half of the claims reported
during this period were in relation to external wall
issues, with an increase of over 150% above ‘normal’
levels experienced. NHBC received circa 1,200 claims
related to inadequate cavity trays and DPCs in the
three-month period. This compares with 1,700 in total
over the previous three full years.
Analysis of the findings of investigations into these
defects has revealed very few, if any, cases of
construction that had been built in accordance with
NHBC Standards having failed; in most cases, the
cause was found to be directly related to
workmanship. This indicates that the Standards,
including classification of exposure areas, are suitable
for purpose, but that ensuring construction is in
accordance with the Standards is the major issue.
Cavity trays and DPCs
The issues surrounding cavity trays include trays
being installed at the incorrect height, not linking
with flashings or being omitted completely.
Further analysis in relation to external walls and
NHBC's ongoing campaign to raise the standard of
pitched roofs also shows a significant increase in
flashing and upstand failures at abutments. This
highlights that a clear understanding between trades
as to who will do what is necessary to ensure correct
installation and satisfactory construction quality.
A clear understanding and co-ordination of
individual trade's responsibilities is vital to ensure
correct installation and satisfactory
construction quality.
lead flashing
wedged into joint
below wall dpc
at least 75mm
underlay
turned up
behind
flashing
Internal damage due to horizontal tray not being
lapped with flashing, causing water ingress within
roof void and damp to ceiling
at least 150mm
clip free edge of
flashing - method
depends on
exposure
NHBC Standards 7.2 - S12(f)
For technical advice and support, call 01908 747384 or visit www.nhbc.co.uk
Technical Extra | Issue 15 | October 2014 | Page 3
Focus on cavity trays and DPCs
REQUIREMENTS
Horizontal cavity trays – in more detail
In addition to the above, NHBC sees simple workmanship issues with horizontal trays – from the flexible material
being too narrow, weep holes being omitted or trays being bedded down onto brickwork, making cutting out for
flashings necessary and damage inevitable. We also see issues with horizontal trays above meter boxes and flues
or ducting, including examples where flues and ducts have been installed through the tray.
Extract duct core drilled through cavity tray, which is
also installed too high
Flexible material too narrow for cavity width
The following highlight good practice when installing cavity trays above openings
and at abutments:
n
Weep holes should be provided and spaced at a maximum of 450mm intervals; each opening should
have at least two weep holes.
nCavity trays or combined lintels should have stop ends. Where cavity trays are used, they should extend at least 25mm beyond the outer face of the cavity closer and cover the ends of the lintel.
nAt abutments, the cavity tray should be linked to a flashing to prevent water penetrating into the enclosed area.
nMortar should be raked out whilst still green to allow the flashing to be tucked under the cavity tray by a minimum of 25mm.
Good practice
stop end
at least 2 weep holes
per opening, not more
than 450mm apart
at least
140mm
total rise
in cavity
tray
at least
100mm rise
in cavity tray
from front
of cavity
cavity
tray
cavity
tray
combined lintel
projects at least
25mm beyond
the outer face
of cavity closure
AIR BRICK
dpc oversails
lintel to protect
timber door and
window heads
groove in window
head prevents rain
penetration
For technical advice and support, call 01908 747384 or visit www.nhbc.co.uk
Page 4 | October 2014 | Issue 15 | Technical Extra
METER BOX
Focus on cavity trays and DPCs
REQUIREMENTS (CONTINUED)
Stepped cavity trays – in more detail
To install a stepped cavity tray correctly, careful consideration should be given to the setting out. For this to be
successful, trade collaboration is paramount. It is good practice to use a template, e.g. an appropriate roof truss
or frame (with guide lines), to assist with setting out the stepped trays. As a rule of thumb, the back of the tray
should be 170-200mm above this truss (NHBC Standards require stepped flashings to have a minimum upstand of
85mm above the top of the finished roof line). Utilising an appropriate truss or frame, as per the example, should
enable bricklayers to install stepped trays in the correct position and alignment.
Stepped tray not linked with flashing
Stepped tray incorrectly installed below roof line
Ensuring that the trays are in the correct position and aligned is only half the battle.
There are a number of other requirements that must also be adhered to:
n To comply with the guidance within NHBC Standards, stepped cavity trays should be preformed.
If any other method is used, this will need to be agreed with your Inspector together with the method
by which you will ensure correct installation.
n The lowest tray should project beyond the eaves line of the roof; there should be a starter tray
(stop end at both ends) and a weep hole should be included.
n Each intermediate tray should overlap the previous with the stop end closest to the roof line.
n A ridge tray, with open ends, should be provided at the ridge.
n Mortar should be raked out whilst still green to allow the flashing to be tucked under the cavity tray by a minimum of 25mm.
n Tile selection will dictate if a secret gutter, proprietary soakers or soakers, or a cover flashing is
required at the abutment.
preformed
stepped
cavity trays
at least
85mm
Example of a truss for setting out
NHBC Standards 6.1 – D6(c) and 6.1 – S4(f)
For technical advice and support, call 01908 747384 or visit www.nhbc.co.uk
Technical Extra | Issue 15 | October 2014 | Page 5
Focus on cavity trays and DPCs
REQUIREMENTS (CONTINUED)
Summary
Parapets and copings
Experience during the ‘storm period’ has reinforced
previous analysis regarding the number of claims
related to superstructure; with some 33% of all
section 3 claims (years 3-10 of the Buildmark Policy)
dealt with by NHBC in 2013 being in relation to
external walls. This article has focused on cavity tray
and DPC issues as the dominate cause; however, our
review has highlighted further areas requiring
attention. A brief summary of these issues follows;
more substantial guidance and support will be made
available in 2015 (see Next steps).
Issues with parapets and copings is almost exclusively
caused by inadequate fixings. Northern Ireland has
very few (if any) failures. England and Wales and
Scotland see failures in one in 2,000 homes built.
Proportionately, it is a much bigger issue in Scotland.
Render
Over the last three years, the top three failures of
render in the UK have consistently related to
deboning from substrate, shrinkage (render or
substrate) and number of coats or thickness. A render
finish is commonly adopted in Scotland and a
prominent area of failure, with an average of eight
homes per 1,000 finalled having experienced render
issues.
Mortar
70% of mortar claims are due to workmanship issues.
These include poor pointing, unfilled joints and
irregular or inadequate bonding. A further 25% can
be attributed to the mix specification.
NHBC has already taken some action and released
additional guidance in Technical Extra 14 for the fixing
of copings to gable walls. This will be included within
the NHBC Standards 2015.
Next steps
In order to assist the industry to reduce the
number of defects in superstructures, NHBC is
in the process of developing appropriate and
sustainable actions to raise awareness and
improve construction quality in these areas.
This is due to be launched in early 2015 and is
likely to include:
nUpdated Standards and guidance.
nFree nationwide training seminars for site
managers and trades.
nOn-site trade talks.
nTechnical articles within trade press.
YOU NEED TO…
■■ Review your construction details and make sure that all trades involved understand the importance of
getting the detail right.
■■ Make sure that installation is right first time – physically check the detail as constructed, paying
particular attention to the first unit constructed for each house type or by different sub-contractors.
■■ Ensure that there is a clear understanding and co-ordination of individual trade's responsibilities;
this is vital in achieving correct installation and satisfactory construction quality.
■■ Look out for more information on NHBC initiatives, including site manager training seminars, to be
held in the New Year.
For technical advice and support, call 01908 747384 or visit www.nhbc.co.uk
Page 6 | October 2014 | Issue 15 | Technical Extra
NHBC STANDARDS
Movement joints in external masonry walls
Who should read this: Technical and construction directors and
managers, architects, designers and site managers.
INTRODUCTION
As previously reported, the use of concrete facing bricks has significantly increased. Providing that they are
installed correctly, they can provide a durable and aesthetically pleasing part of the building envelope. But, as
with clay bricks, it is extremely important to ensure that the detailing is correct, particularly in relation to the
accommodation of movement.
REQUIREMENTS
During late 2013 and early 2014, NHBC inspectors
reported an increase in the use of concrete facing
bricks. This type of product has been used
successfully for many years, but the more widespread
use has highlighted a need to review what should be
considered for the accommodation of movement,
which is different to that required for walls
constructed with clay bricks.
In March 2014, NHBC published Technical Alert 01,
which provided general guidance for the spacing of
vertical movement joints in walls constructed from
concrete facing bricks. The alert suggested that
movement joints should be spaced at no more than
6m intervals unless specific evidence is provided to
justify wider spacing. Since then, we have received
some enquiries asking in what circumstances it may
be acceptable to increase the spacing.
Guidance for avoiding cracking can be found in British
Standards as well as NHBC’s Technical Standards.
However, the guidance does not clearly distinguish
between what should be considered specifically for
high or low density concrete masonry. Complying with
the guidance and adopting the minimum distance of
6m for the spacing of vertical movement joints should
provide a robust solution, but it can be overly onerous
in situations where higher density concrete masonry
is used.
Movement of concrete masonry can be attributed to
thermal and moisture movement, the effects of which
can be more significant when low density materials,
which are more prone to movement than higher
density materials, are used. As with walls constructed
with clay bricks, the ratio of width to height of the wall
panels will also have an effect which needs to be
considered in the design, along with the localised use
of bed joint reinforcement where required.
Lightweight concrete masonry units are generally
made of aggregates that have a gross density not
exceeding 1,500kg/m3. Dense concrete masonry units
are generally made of aggregates that have a gross
density exceeding 1,500kg/m3. Typically, concrete
facing bricks have a higher density in excess of
this figure.
The next edition of NHBC’s Standards will be updated
to include the revised table overleaf. This provides
guidance for the spacing of vertical movement joints
that distinguishes between dense and lightweight
concrete blocks and bricks.
For technical advice and support, call 01908 747384 or visit www.nhbc.co.uk
Technical Extra | Issue 15 | October 2014 | Page 7
Movement joints in external masonry walls
REQUIREMENTS (CONTINUED)
Material
Joint width (mm)
Normal spacing
Clay brick
16
12m (15m maximum)
Calcium silicate brick
10
7.5 to 9m
Lightweight concrete block and brick
(autoclaved or using lightweight aggregates)
10
6m
Dense concrete block and
brick (using dense aggregates)
10
7.5 to 9m
Any masonry in a parapet wall
10
Half the above spacing and 1.5m from
corners (double the frequency)
In addition to the revised table and the guidance published in British Standards, such as BS 5628-3 and PD 6697,
the manufacturer’s advice should also be considered in the design of walls, particularly the accommodation of
movement.
YOU NEED TO…
■■ Ensure that walls are designed to accommodate movement safely without cracking by following the
guidance in NHBC Standards, British Standards and the manufacturer’s recommendations.
■■ Revised guidance will be published in the next edition of NHBC Standards, but the above guidance can
be adopted now.
For technical advice and support, call 01908 747384 or visit www.nhbc.co.uk
Page 8 | October 2014 | Issue 15 | Technical Extra
NHBC STANDARDS
Slating and tiling for pitched roofs – BS 5534
Who should read this: Technical and construction directors and
managers, architects, designers and site managers.
INTRODUCTION
This article explains the main changes made in the revised BS 5534 ‘Slating and tiling for pitched roofs and
vertical cladding – Code of practice’, which was published at the end of August 2014.
REQUIREMENTS
The revised BS 5534 has been re-structured and
includes some significant changes that will affect
the way slated and tiled roofs are constructed
going forward.
One of the main changes relates to the specification
and use of roof underlays. An unsupported roof
underlay provides a secondary barrier to water ingress
through the roof cover, but it also provides the majority
of the resistance to wind uplift acting across a roof.
The type and number of fixings for the roof coverings
are calculated taking account of the wind loads taken
by the underlay. It is therefore important that the
underlay does not balloon under wind conditions and
make contact with the roof tiles, as this would transfer
more wind loads to the tiles, which could cause them
to lift or even dislodge and come off the roof.
Resistance to wind uplift figures, currently quoted by
underlay manufacturers, are difficult to understand
and apply. To address this, the British Standard now
includes a new wind uplift resistance test and
classification system for underlays. The classification
system uses the wind uplift resistance test figures and
relates them to the batten gauge being used and
where in the country the site is located in relation to
design wind pressures.
This information will appear on a ‘zonal-classification’
label (Table 1) provided by the underlay manufacturer
and, subject to a few parameters, will enable the
designer or installer to identify easily which underlays
are suitable for a particular roof. The classification
system will suit most roof designs. For others,
a calculation to determine the required wind uplift
resistance should be carried out and used to identify
appropriate underlays that can resist those forces.
The underlay manufacturer will be able assist in
these situations.
The underlay classification system will enable users to
identify the weaker underlays that are only suitable
for sheltered areas of the UK or roofs where the
batten gauge is small, or where the underlay is
supported on solid sarking or insulation boards,
which themselves provide the wind uplift resistance.
The new wind uplift resistance tests include the
provision of a lap in the underlay. The new tests have
highlighted the importance of securing the laps to
prevent them opening up and touching the roof tiles.
The British Standard describes how laps should
be secured by covering with a separate batten,
or extending the underlay to the nearest slating or
tiling batten or sealing the laps with a proprietary
sealant. If sealants are used, they must be durable
and perform to the same standards as the underlay
and should therefore be provided by the underlay,
manufacturer and installed strictly in accordance with
the manufacturer’s recommendations.
Changes have been made to take account of wind
loadings in accordance with Eurocodes. As a result,
calculated wind loads have increased, which in turn
requires additional fixing of the roof tiles. It is likely
that all roof tiles will now require fixing either by
nailing or clipping, or a combination of the two. The
current ‘Zonal Method’, which could be used to
establish roof tile fixings without requesting a
calculation and fixing specification from the roof tile
manufacturer, has not been updated to take account
of these new wind load requirements and should no
longer be used. Therefore, all fixing specifications
should now be provided by the tile manufacturer in
line with the revised British Standard.
A third major change is that mortar bedding can no
longer be relied upon as a method of fixing. In other
words, any tensile strength provided by mortar
For technical advice and support, call 01908 747384 or visit www.nhbc.co.uk
Technical Extra | Issue 15 | October 2014 | Page 9
Slating and tiling for pitched roofs – BS 5534
REQUIREMENTS (CONTINUED)
bedding should be ignored, and all bedded ridge and
hip tiles now require mechanical fixings. This change
has brought the British Standard in line with NHBC
Standards, which have called for mechanical fixings of
bedded ridge and hip tiles since January 2012. The
ratio of soft and sharp sands in mortar mixes has also
been revised in line with NHBC Standards.
Other changes include a requirement for a minimum
of two mechanical fixings for perimeter tiles (subject
to meeting the wind load requirements). One of the
fixings can be a tile clip or dry fix unit if it can be
shown by the clip or unit manufacturer to provide the
necessary wind uplift resistance. There is also advice
on the avoidance of creating small cut tiles at
perimeters by the use of double, tile-and-a-half or half
tiles where available from the manufacturer. It is also
recognised that, where small cut single lap tiles (less
than half tile width) cannot be avoided, they should be
mechanically fixed or bonded with adhesive to an
adjoining full tile. This is in line with discussions NHBC
has been having with the roof tile industry and it is
important that any adhesive fixings should only be
made with adhesive recommended by the roof tile
manufacturer and used strictly in accordance with
that tile manufacturer’s instructions.
To allow time to implement the changes, the previous
edition of BS 5534 (2003+A1:2010) will not be withdrawn
until 28 February 2015, but NHBC strongly recommends
that the changes be adopted as soon as is practicable.
Product
Identification
Manufacturer
Website
Top underlay
LR
Underlaymakers Ltd.
www.underlaymakers.com
Batten gauge
Declared wind uplift
resistance, PD (N/m2)
Zone suitability
Wind zone map
<345mm
1 200
1 to 3
<250mm
2 200
1 to 5
<100mm
>2 200
1 to 5
Note 1
In this example, light grey indicates that the zone is suitable and dark grey indicates that it is not suitable. In practice, suitability might be indicated by the use of colour, e.g. green for suitable and red for not suitable.
Note 2
Zone suitability applies only for underlays in applications where a well-sealed ceiling is present, ridge height is not greater than 15m, roof pitch is between 12.5o and 75o, site altitude is not greater than 100m and no significant site topography is present. Other applications might require underlays with greater wind uplift resistance, and it is advisable to seek professional advice.
Note 3
Zones 3 and 4 apply to Northern Ireland.
Table 1 – Illustration of a zonal classification label for an underlay
Permission to reproduce extracts from British Standards is granted by BSI Standards Limited (BSI). No other use of this material is permitted.
YOU NEED TO…
■■ Make yourself familiar with the revised BS 5534 and start to incorporate the new requirements in
your roof designs and construction as soon as is practicable.
■■ Make sure roof underlays are suitable for the intended location – refer to the zonal classification label.
■■ Make sure that laps in underlays are adequately secured.
■■ Ensure that you have a tile-fixing schedule in all cases – note that the zonal method of fixing should
no longer be used.
For technical advice and support, call 01908 747384 or visit www.nhbc.co.uk
Page 10 | October 2014 | Issue 15 | Technical Extra
REGULATION AND COMPLIANCE
Type Approvals: Certificate changes
Who should read this: Technical and construction directors, architects,
design teams and site managers.
INTRODUCTION
Type Approvals can simplify and speed up the design and approval processes, and give you earlier certainty
about Building Regulations compliance. NHBC Building Control offers Type Approvals free to our building control
customers. They are helpful for house, flat and garage types, and can also be used for standard details and
specifications that will be repeated on different sites.
GUIDANCE
Certificate changes
Building regulations in Wales have now devolved to
the Welsh Government. As a result, Type Approval
Certificates for Regulations issued since the
introduction of Part L 2014 will now include the
following information:
n
Region: this will state whether the approval applies
to England or Wales, or both England and Wales
n
Approved Documents (ADs): the applicable ADs
will be referred to and fully listed in Annex A.
These changes will ensure that customers and NHBC
surveyors and inspectors can quickly recognise the
regulations and jurisdiction that the Type Approval
has been designed to meet.
YOU NEED TO…
■■ This article is for information. If you require any further advice please contact your
NHBC building surveyor.
For Building Regulations advice and support, call 0844 633 1000 and ask for ‘Building Control’ or visit www.nhbc.co.uk/bc
Technical Extra | Issue 15 | October 2014 | Page 11
REGULATION AND COMPLIANCE
Building Regulations – Regulation 38
Transfer of fire safety information to the responsible person
Who should read this: Technical and construction directors, architects,
design teams, site managers, building owners and managing agents.
INTRODUCTION
This article provides guidance on complying with Regulation 38 of the Building Regulations in respect of the
transfer of fire safety information to the responsible person to help them operate and maintain the building
with reasonable safety. Regulation 38 applies in both English and Welsh Building Regulations.
GUIDANCE
Whenever a building to which the Regulatory Reform
(Fire Safety) Order 2005 applies is built, formed by a
change of use, altered or extended, compliance with
Regulation 38 of the Building Regulations (English
and Welsh versions) is required.
Regulation 38 places a duty on the person who
carries out the work (usually the principal contractor)
to provide the responsible person (the employer,
owner or person who has control of the premises as
defined by Section 3 of the Order) with fire safety
information to help them operate and maintain the
building with reasonable safety. It is an expectation of
the Order that this information will also be used by
the responsible person as the basis for their fire
safety risk assessment and ongoing fire safety
management of the building.
The Building Regulations also place a duty on the
building control body overseeing the work to take
reasonable steps to ascertain that the fire safety
information (as required by Regulation 38) has been
provided to the responsible person before issuing a
completion certificate or final certificate.
nLocation of fire doors.
nLocation of fire detectors.
nAny sprinkler system(s).
n Any smoke control system(s).
nAny high-risk areas.
nSpecifications of any fire safety equipment .
n
Any design assumption made regarding the
ongoing management of the building.
n
Any provisions incorporated for the evacuation of
disabled persons.
For more complex buildings, or buildings incorporating
fire-engineered solutions, more detailed records are
likely to be required. These records should include all
the items detailed above and, where appropriate, may
include the following:
nThe fire safety strategy.
nAny assumed fire loading in the design of the fire safety systems.
Best practice
nAny risk assessments undertaken.
For less complex buildings, all that might be required
is a set of ‘as-built’ plans of the building, detailing the
following fire protection measures as stated within
Approved Document B, Appendix G:
nAny risk analysis undertaken.
nAll assumptions in the design of the fire safety systems regarding ongoing management.
nEscape routes, and escape strategy, including
nEscape routes.
nCompartmentation and separation.
nAll passive fire safety measures.
muster points.
For Building Regulations advice and support, call 0844 633 1000 and ask for ‘Building Control’ or visit www.nhbc.co.uk/bc
Page 12 | October 2014 | Issue 15 | Technical Extra
Building Regulations – Regulation 38
GUIDANCE (CONTINUED)
nAll active fire safety measures.
nSprinklers systems, design and controls.
nSmoke control systems, design and controls.
nAll high-risk areas and particular hazards.
nAny other details appropriate for the
specific building.
Further detailed guidance is also available within
BS 9999:2008, Annex H (formerly BS 5588-12:2004,
Annex A).
of every building covered by the Fire Safety Order,
and it is essential that all safety-critical information is
made available to the building occupiers before
occupation so that they may fulfil their duties under
the Order. Fire brigades and Building Control bodies
would encourage every builder to take actions to
ensure that sufficiently detailed information is passed
over to the responsible person, and would advise that
it is far better to provide too much information
about the design and construction of a building than
not enough.
The undertaking of a fire risk assessment is the
cornerstone of the ongoing fire safety management
YOU NEED TO…
■■ Be aware of your responsibilities in respect of Regulation 38. If you require any further advice, please contact
your NHBC building surveyor.
For Building Regulations advice and support, call 0844 633 1000 and ask for ‘Building Control’ or visit www.nhbc.co.uk/bc
Technical Extra | Issue 15 | October 2014 | Page 13
REGULATION AND COMPLIANCE
Publication of Category 4 Screening Levels for
land affected by contamination
Who should read this: Technical and construction directors and
managers, architects, designers, consultants, specialist remediation
companies, contaminated land professionals and land buyers.
INTRODUCTION
In 2012 revised Statutory Guidance for Part 2A of the Environmental Protection Act (1990) came into force for
England and Wales. This introduced a new four category approach for classifying land affected by contamination
to assist decisions by regulators in cases of Significant Possibility of Significant Harm (SPOSH) to specified
receptors, including humans, and significant pollution of controlled waters.
Following publication of the revised Statutory Guidance, DEFRA commissioned a research project to develop new
Category 4 Screening Levels (C4SLs) to provide a simplified test for regulators to aid decision-making on when
land was suitable for use and definitely not contaminated land under the statutory regime.
The output from this research project was published by CL:AIRE in December 2013, with Policy Companion
Documents published in England by DEFRA in March 2014 and the Welsh Government in May 2014. The
culmination of this work was the development of a framework and methodology for deriving C4SLs and the
publication of final C4SLs for use as new screening values for six common contaminants. This article provides
background information on the new C4SLs and their relationship to development and planning.
GUIDANCE
In March 2014, DEFRA published a Policy Companion
Document for England to accompany the research
project, which endorsed the draft methodology and
framework for the derivation of C4SLs. DEFRA
anticipated that regulators and risk assessors would
use this in line with the Statutory Guidance. A
separate Policy Companion Document was published
by the Welsh Government in May 2014.
The four-category approach introduced by the revised
Statutory Guidance for Part 2A is indicated in Fig 1.
The C4SLs represent estimates of contaminant
concentrations in soils that can, when used in
accordance with defined risk management decisions
in the methodology, be used to determine whether
Risk
Contaminated land
In December 2013, the output of a DEFRA-funded
research project (SP1010 – Development of Category 4
Screening Levels for the assessment of land affected
by contamination) was published by CL:AIRE and
included a draft methodology for determining Human
Health C4SLs.
land is clearly not contaminated land for human
receptors for specified land uses within the context of
Part 2A of the Environmental Protection Act (1990).
Non contaminated land
Background
1
2
Point above which land is
‘contaminated land’ under
Part 2A
3
Amount of land
Fig 1 – Land categories as defined by Statutory
Guidance and relationship of C4SLs and former SGVs
C4SLs therefore also have the potential to impact on
the approach used by risk assessors and
contaminated land professionals for assessing and
remediating site(s) affected by contamination for the
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Page 14 | October 2014 | Issue 15 | Technical Extra
C4SL
SGV/GAC
4
Publication of Category 4 Screening Levels for
land affected by contamination
GUIDANCE (CONTINUED)
purposes of demonstrating compliance with
NHBC Standards Chapter 4.1 ‘Land Quality –
managing ground conditions’.
(VI) and cadmium. In addition, the existing ‘idealised
standard land uses’ of residential use (with and
without) plant uptake, allotments, commercial and
industrial have been extended to include two specified
public open space uses.
Whilst considering the use of C4SLs, it is important to
understand that the methodology used in their
development represents a departure from the
previous approach adopted by the Environment
Agency for the generation of the soil guideline values
(SGVs). SGVs were developed on the basis of ‘minimal’
risk levels, but in the context of Part 2A, these were
generally considered to be ‘too precautionary’ to be a
useful screening tool to aid in the determination of
sites as contaminated land and they were withdrawn
by the Environment Agency in 2011.
The full research project report, along with the Policy
Companion Document for England, is published by
DEFRA as Technical Report SP1010 and is available on
the DEFRA website (http://randd.defra.gov.uk/Default.
aspx?Menu=Menu&Module=More&Location=None&Co
mpleted=0&ProjectID=18341#RelatedDocuments). The
report includes technical annexes for each of the six
contaminants and details the selection of appropriate
inputs, toxicological and exposure assessment
parameters and statistical reviews. The report should
be considered together with the Policy Companion
Document for England, which details the outputs (the
final C4SLs) and the risk management (policy-based)
decisions behind their derivation.
The methodology developed for determining C4SLs is
described as being strongly precautionary, but has
been based on a pragmatic approach to contaminated
land risk assessment adopting an ‘acceptably low’,
rather than a ‘minimal’, level of risk.
A separate Policy Companion Document was
published by the Welsh Government in May 2014 and
is available on the Welsh Government website
(http://wales.gov.uk/topics/environmentcountryside/
epq/contaminatedland/screening-levels-contaminatedland-assessment-/?lang=en).
The derivation of C4SLs also introduced the concept
of a low level of toxicological concern (LLTC),
which represents the estimated concentration of a
contaminant that would pose an ‘acceptably low’ risk
to human health that, alongside modifications to
exposure modelling, were used to develop the new
screening criteria.
The Policy Companion Documents also provide
guidance to risk assessors on the consideration of
normal background concentrations of contamination
in England and Wales when using C4SLs. The full
reports are considered essential reading for risk
assessors and contaminated land practitioners,
particularly where C4SLs are being considered for
use on new residential development(s).
One of DEFRA's requirements for the development
of C4SLs was that these should allow a higher
(though still ‘acceptably low’) level of risk than the
previously available SGVs, while maintaining a
precautionary approach.
To test the framework and methodology developed for
deriving C4SLs, the research project generated new
screening values for six common contaminants:
arsenic, lead, benzene, benzo(a)pyrene, chromium
There is currently no change to the existing regimes
for land affected by contamination in Northern Ireland
and Scotland.
Former residential
Residential
Residential
(with
home-grown
produce)
(without
home-grown
produce)
Arsenic
37
40
49
640
79
168
32
Cadmium
26
149
4.9
410
220
880
10
Chromium (VI)
21
21
170
49
23
250
4.3
Lead
200
310
80
2330
630
1300
450
Benzo(a)pyrene
5
5.3
5.7
76
10
21
0.83
Benzene
0.87
3.3
0.18
98
140
230
0.33
Substance
Allotments
Commercial
Public open
space (near
residential)
Public open
space (parks)
(with plant uptake)
SGV/GAC (for comparison
purposes)
Table 1: Category 4 Screening Levels (including previously published SGVs/GACs for residential land use for
comparative purposes); quoted concentrations are presented as mg/kg.
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Technical Extra | Issue 15 | October 2014 | Page 15
Publication of Category 4 Screening Levels for
land affected by contamination
GUIDANCE (CONTINUED)
Published C4SLs
The final C4SLs developed during the research
project for the six common contaminants considered
are shown in Table 2 on page 15. The previous SGVs
for residential end use with plant uptake have also
been included for comparison purposes.
It can be seen that the framework developed using
the new C4SL methodology has provided screening
values for England and Wales for the contaminants
considered that are above the previously used, and
withdrawn, SGV values. The exception to this is lead,
as the report concluded that current evidence from
worldwide toxicological studies and blood testing
suggested that the withdrawn SGV level did not
represent an ‘acceptably low’ risk to human health.
of land affected by contamination under the
respective planning regimes.
Use of published C4SLs
The respective Policy Companion Documents for
England and Wales provide the background to the
development of C4SLs and the final published
screening values for the six selected contaminants.
The documents also provide guidance on the use of
C4SLs by risk assessors and regulators when
assessing land affected by contamination. In relation
to the use of C4SLs for residential developments,
NHBC considers that:
n
C4SLs may be used for schemes in England
and Wales as generic screening levels for
contaminants in soils, as long as they are
justifiable and defensible in the conceptual
site model for the site. Where representative
contaminant concentrations exceed C4SLs,
remediation or further detailed assessment will
normally be required.
Planning and development
The Policy Companion Documents published for
England and Wales consider ‘wider issues’ in the
content of the C4SLs, including their relationship with
the planning regimes.
For England, the Policy Companion Document advises
that the C4SLs were developed on the basis that they
could be used under the planning regime as they
would be in investigations under Part 2A of the
Environmental Protection Act 1990, but that planning
policy falls within the remit of the Department for
Communities and Local Government (DCLG). In June
2014, the Planning Practice Guidance for England
(http://planningguidance.planningportal.gov.uk/
revisions/33/007/) was updated and placed on the
Planning Portal by DCLG. It makes reference to the
Policy Companion Document published by DEFRA and
the use of C4SLs in providing a simple test for
deciding when land is suitable for use and definitely
not contaminated land. This position was affirmed in
DEFRA correspondence to local authorities on
3 September 2014, stated “revision of the Part 2A
Statutory Guidance was developed on the basis that
C4SLs could be used under the planning regime as
well as within Part 2A” and that “exceeding a C4SL
means that further investigation is required, not that
the land is necessarily contaminated”.
For Wales, the Policy Companion Document states
that C4SLs may provide a useful means of assisting
local planning authorities in deciding whether land is
suitable for its proposed use.
In Northern Ireland and Scotland, there is currently no
change to the current requirements for consideration
nDevelopers should, however, check that the use of C4SLs would be accepted by regulators under the relevant planning regime.
n
In Scotland and Northern Ireland, care should
be exercised, as a ‘minimal risk' level is used by
regulators in these countries. Generic Assessment
Criteria (GAC) using a 'minimal risk' approach are
therefore still likely to be the starting point, and
developers would need to seek confirmation from
regulators in these countries on their approach
to lead.
n
Where a land use scenario covered by a C4SL
applies in England and Wales, that use of C4SLs
will satisfy NHBC Standards – Chapter 4.1
requirements. For lead, the C4SL value should be
adopted as the screening level, though normal
background concentrations can be considered
when appropriate.
nWhen the site or development deviates from the standard land uses or risk management assumptions covered by C4SLs, a site specific risk assessment should be undertaken to derive appropriate site-specific screening values for risk assessment and management purposes.
nDevelopers may still elect to take a more precautionary approach in line with their own risk management and liability policies (i.e. screening levels lower than C4SLs).
For technical advice and support, call 01908 747384 or visit www.nhbc.co.uk
Page 16 | October 2014 | Issue 15 | Technical Extra
Publication of Category 4 Screening Levels for
land affected by contamination
GUIDANCE (CONTINUED)
nWhere C4SLs do not exist for a contaminant, there are still around 132 publically available GACs for use from recognised land contamination sector bodies. Additionally, the C4SL methodology
could be used to develop screening values for
other contaminants.
nThough SGVs were withdrawn by the Environment Agency in 2011 for purposes of determining land as contaminated, they may be appropriate as reference levels. These are generally precautionary and conservative assessment criteria as they are based on ‘minimal’ rather than ‘low’ risk.
YOU NEED TO…
■■ Be aware of the changes to the Contaminated Land Statutory Guidance (England and Wales) and the
supporting guidance that regulators will use when determining whether land is contaminated land under Part
2A of the Environmental Protection Act 1990.
■■ Be aware that the framework and methodology used to generate C4SLs represents a departure from the
previous approach adopted by the Environment Agency for the generation of SGVs – SGVs were based on
‘minimal’ risk levels, whereas C4SLs represent an ‘acceptably low’ risk.
■■ Be aware that there are differences between regulatory approaches under planning for different countries in
the UK, and check that C4SLs will be accepted by the regulators under the relevant planning regime.
■■ Ensure that suitable consultants (conversant with the approach adopted for generating C4SLs) are appointed
for site investigation, risk assessments, remediation and validation, to ensure that the land for new
developments is appropriately assessed and remediated against appropriate standards.
■■ Ensure that your developments are assessed on a site-by-site basis for contamination, including the
development of a suitable conceptual site model basis using appropriate screening values for risk assessment
and the development of remediation strategies.
For technical advice and support, call 01908 747384 or visit www.nhbc.co.uk
Technical Extra | Issue 15 | October 2014 | Page 17
GUIDANCE AND GOOD PRACTICE
Copings and cappings to brick chimneys
Who should read this: Technical and construction directors and
managers, architects, designers, consultants, manufacturers,
specifiers and purchasers.
INTRODUCTION
Because of their exposed position on dwellings, brick chimneys are at high risk of rainwater penetration and frost
damage. Frost can damage both the mortar and the bricks themselves if they are not sufficiently frost resistant.
The greatest risk is the brickwork at the top of a chimney and, once damage has occurred, it is often difficult and
costly to rectify due to problems with gaining access to carry out repairs. In this article, we discuss these issues
and how to address them.
GUIDANCE
Careful consideration at the design and material
selection stages, together with correct installation on
site, can significantly reduce the risk of water
penetration and avoid saturation and subsequent
frost damage. Guidance on the provision of dampproof trays and flashings, where a chimney abuts or
passes through a roof, is provided in NHBC Standards
Chapter 6.8. Examples of chimney terminals and
cappings are also given, and what follows is a more
detailed explanation on the selection and construction
of both copings and cappings to provide good
weather protection.
NHBC Standards uses the term ‘capping’ for both
copings and cappings, but strictly speaking, they are
two different details and provide different levels of
protection. BS 5642 ‘Sills, copings and cappings’
defines copings and cappings as:
Coping – construction that protects the top of a wall
and sheds rainwater clear of the surfaces beneath.
Capping – construction that protects the top of a wall,
but does not shed rainwater clear of the surfaces of
the wall beneath.
A coping should have a projection beyond the
brickwork together with a drip which stops water
tracking back under the projection and on to the
brickwork. A capping, comprising any projecting
brickwork and a flaunching as described later, lacks a
drip feature and rainwater runs over the face of the
brickwork beneath.
Copings which throw rainwater away from the
brickwork are the preferred option. Ideally, a coping
should be in one unit to avoid having vertical joints
through which rainwater will eventually penetrate.
If a jointed coping is unavoidable, it should be bedded
on a DPC which should in turn be bedded onto the
brickwork below. Likewise, if a natural stone coping is
used, which could be slightly porous, it too should be
bedded on a DPC.
Because water will penetrate brickwork laid
horizontally, or at a low pitch, brick copings or
cappings, without a flaunching as described below,
should not be used to finish the top of a chimney
stack, even if a DPC is incorporated beneath the
brickwork. Such detailing is at a high risk of becoming
saturated and defective through frost action.
Although a projecting coping can offer the best
protection to the brickwork, cement flaunching has
been a traditional means to cap chimneys. Although
not the preferred option, if flaunching is selected,
it needs to have good tensile strength and durability.
A 1:3 cement sharp sand mix is therefore
recommended and should be trowelled to form a
smooth hard surface. The junction between the
flaunching and brickwork should be at least 25mm
thick to resist edge spalling, and because flaunching
can develop fine cracks over time, with the potential
for rainwater penetration, it should be laid over a
bedded DPC to protect the brickwork beneath.
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Page 18 | October 2014 | Issue 15 | Technical Extra
Copings and cappings to brick chimneys
GUIDANCE (CONTINUED)
Flaunched chimneys often have projecting or
corbelled feature brickwork at or near the top of the
stack. Corbelling cannot be relied upon to act as a
coping; because it lacks a drip feature, it can only be
considered as a capping. Where brickwork corbels
inwards, it forms an exposed horizontal ledge which
can hold water and increase the risk of saturating the
stack. Such ledges should be weathered by applying a
1:3 cement sharp sand fillet, as shown in Fig. 1.
To assist the keying in of the fillet, the mortar bedding
to the brickwork behind should be racked back slightly.
Projecting brick surface
weathered with fillet of mortar
1:3 Projecting
(cement
sharp sand) mix
brick surface weathered with
Unacceptable detail where
projecting brickwork forms a
ledge that holds water
n
Pointing should be bucket handle or weather
struck. Recessed and projecting joints which could
hold water should not be used.
The diagrams (Fig. 2) illustrate the principal features
that should be considered to ensure the tops of brick
chimney stacks are constructed to avoid saturation of
the brickwork and thereby reduce the risk of damage
through frost action. The advice of the brick
manufacturer on the selection of suitable bricks for
chimney stack construction should be sought at an
early stage.
Flue liner or chimney pot inserted 125mm min.
or quarter of the length of the terminal into the
stack, excluding the depth of the flauching
Unacceptable detail where projecting
brickwork forms a ledge that holds water.
fillet of mortar 1 : 3 (cement : sharp sand) mix
Mortar flaunching 1:3 (cement sharp sand)
with smooth finish and falls to shed water
25mm min.
High bond DPC
bedded in mortar
Capping
Frost resistant brickwork
F2, sulfate S2, mortar
M12, i.e. [designation (i)]
or as recommended by
brick manufacturer
PROJECTING BRICKWORK DETAIL
Fig. 1 – Projecting brickwork detail
In addition to the above, the following features should
be included:
nBricks used in a chimney stack should be frost M12 mortar or cement grout joint
between coping and flue/pot
Pre-cast coping, (preferably a
single unit). Any joints in coping
to be made with M12 mortar
resistant (F2).
Provide high bond DPC bedded in
mortar where a jointed or porous
coping is used
nCappings should be laid in a durable mortar mix M12 [designated mix (i)]. The same mortar mix should be considered for the whole stack.
n
M6 mortar [designated mix (ii)] may be suitable
for brickwork protected by a coping.
*
30mm
50mm
Coping
*
50mm min. for
concrete, cast stone or
natural stone
nSulfate-resisting cement
should be used in the Capping
mortar if smoke billowing engulfing the chimney stack is likely to occur.
nAll mortar joints should be fully filled.
Insulated concrete fill
Bucket handle or weather
struck pointing (no recessed
or projecting joints)
nWhere there is a risk of regular wetting of the brickwork, a low sulfate-resistant brick (S2) is advised to reduce efflorescence and sulfate attack.
Fig. 2 – Section through chimney with flaunched capping
or pre-cast coping (chimney stack construction may vary)
YOU NEED TO…
■■ Apply the design principles described above and select the correct quality of bricks and mortar for use in the
construction of brick chimney stacks.
For technical advice and support, call 01908 747384 or visit www.nhbc.co.uk
Technical Extra | Issue 15 | October 2014 | Page 19
GUIDANCE AND GOOD PRACTICE
Suspended beam and block concrete floors
Guidance on selection of products and materials
Who should read this: Technical and construction directors
and managers, architects, designers, manufacturers,
specifiers and purchasers.
INTRODUCTION
This article draws attention to the use of beam and block suspended concrete floors in residential construction,
and the need to ensure that adequate structural performance of floors is not impaired by the method of
achieving adequate levels of thermal insulation.
GUIDANCE
Suspended beam and block floors comprising precast
concrete beams and infill blocks with concrete topping
is a popular type of residential floor construction in
the UK. First introduced in the late 1970s, its use is
currently estimated to account for approximately
40% of the market. Suspended concrete ground
floors, when combined with an insulation material
such as polystyrene infill blocks or as sheets laid over
the top of blocks, can provide suitable thermal
insulation, but adequate structural performance of
the floor system should not be impaired by the chosen
method of satisfying thermal insulation requirements.
Specifications for these floors are covered by the
harmonised European Standard BS EN 15037 that is
published in five parts. Part 1 (BS EN 15037-1) covers
concrete beams, Parts 2 to 5 cover a range of block
types, i.e. concrete blocks, clay blocks, expanded or
extruded polystyrene blocks and lightweight blocks
for simple formwork, respectively. However, this
Standard does not include any guidance for
concrete topping.
The category of beam and the type of infill block
specified for the floor will affect the specification of
the concrete topping. This article is restricted to the
use of self-bearing and non-self-bearing beams
covered by BS EN 15037-1 and concrete or
polystyrene blocks, covered by BS EN 15037 Parts
2 and 4, respectively.
Beams – categorisation to
BS EN 15037-1:2008
Precast concrete beams may be either reinforced or
pre-stressed normal weight concrete according to
BS EN 1992-1-1:2004.
BS EN 15037-1 defines the beams as ‘self-bearing’ or
‘non-self-bearing’. Performance requirements are
described below. Guidance on appropriate structural
floor systems using concrete or expanded polystyrene
blocks is given in Tables 2 and 3 of this article.
Beam type
Definitions and
performance requirements
Self-bearing
Reinforced or pre-stressed concrete
beams, which alone provide the final
strength of the floor independent
of any other constituent part of
the system (i.e. blocks or
structural screed).
Non
self-bearing
Reinforced or pre-stressed concrete
beams, which provide the final
strength of the floor system in
conjunction with cast in situ concrete
screed and, possibly, with the top of
the blocks.
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Page 20 | October 2014 | Issue 15 | Technical Extra
Suspended beam and block concrete floors
GUIDANCE (CONTINUED)
Blocks
Screeds
Concrete blocks
Non-structural screeds
BS EN 15037-2 covers concrete blocks made of normal
or lightweight aggregate concrete. Concrete blocks
are defined as non-resisting (NR), semi-resisting (SR)
and resisting (RR). Performance characteristics,
described below. Guidance on appropriate structural
floor systems incorporating concrete blocks is given in
Tables 2 and 3 of this article.
Block type
Performance characteristics
Four types of non-structural screed can be used:
sand:cement screeds; enhanced sand:cement screeds;
anhydrite screeds, and liquid cementitious screeds.
If insulation material is laid over the top of the blocks,
the compressibility of the insulation material needs to
be verified, and the minimum thickness of screed
recommended is 65mm. If underfloor heating is used,
the minimum cover above the pipes for the particular
system needs to be met.
NR
Perform no mechanical function in
the final floor system, but act as
formwork during construction of the
floor.
All of the above screeds can incorporate the use of
polypropylene micro-fibres to control shrinkage
cracking, provided the specification is substantiated
by an independent assessment of use.
SR
Contribute to the mechanical
function of the final floor system.
RR
Contribute to the mechanical
function of the final floor system.
Table 2 gives the choices for concrete and EPS
blocks when used with self-bearing beams and
non-structural screed.
Expanded polystyrene blocks (EPS)
BS EN 15037-4 covers EPS blocks defined as type R1
(R1a or R1b) or type R2. Performance characteristics
are described below. Guidance on appropriate
structural floor systems incorporating EPS blocks is
given in Tables 2 and 3 of this article.
Block type
Performance characteristics
R1
Perform no mechanical function in
the final floor system, but may act as
formwork during construction of the
floor.
R2
Contribute to the mechanical
function of the floor system.
Concrete
block type
Minimum
characteristic
resistance to
concentrated load
[5% fractile] in kN
EPS block
type (Class)
NR
1.5
R1 (a & b)
SR
2.0
R2
RR
2.5
-
Structural screeds
Structural screeds should be designed by suitably
qualified persons in accordance with BS EN 206:2013
and its Complementary British Standard BS 85002:2006. Reinforcement consisting of welded steel
mesh should be in accordance with BS EN 15037-1.
Alternatively, an independent assessment of the
beam and block flooring, including the structural
concrete topping, may be acceptable (see Note 3 to
Tables 3 and 4).
In either case, structural screeds reinforced with
microfibres will not be acceptable to NHBC, but
polypropylene macrofibres or steel fibres could be
acceptable for use as reinforcement.
Tables 3 and 4 give the choices for concrete and EPS
blocks when used with non-self-bearing and selfbearing beams respectively, together with cast in situ
structural screed.
Table 1 – Approximate equivalent performance in
terms of characteristic resistance between
concrete and EPS blocks.
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Technical Extra | Issue 15 | October 2014 | Page 21
Suspended beam and block concrete floors
GUIDANCE (CONTINUED)
General
Self-levelling or self-compacting concrete can also be
used and should meet the minimum requirements of
standard concrete, including the provision for
reinforcement.
Since BS EN 15037 is a harmonised standard, the CE
marking for both the beam and the block is acceptable.
The CE marking should be based on the harmonised
standard with evidence on satisfying the required
Assessment and Verification of Constancy of
Performance (AVCP), formerly known as the
attestation of conformity system level, as specified in
Table ZA.2 of the respective parts of BS EN 15037.
Diaphragm action
Where a floor is required to act as a diaphragm,
the guidance given in BS EN 15037-1 Annex G should
be followed.
Garage floors
For all types of infill blocks where the floor is to be
used within a garage, the concrete shall be reinforced
with a minimum A142 mesh to resist a point load of
10kN minimum.
Choices for beams and blocks in suspended
concrete floors
Tables 2 and 3 give the choices for beams and blocks
when used with non-structural and structural concrete
toppings respectively, in accordance with BS EN 15037.
Table 4 provides an option for beams and blocks when
used with structural concrete topping, but not
specifically covered by BS EN 15037.
Blocks
Precast concrete
beam type
Self-bearing
Block
material
Concrete
EPS
Performance
type (min) (1)
SR
RR (ungrouted)
R2
Notes on the structural floor system
The precast beams alone provide the final strength of the
floor system. The blocks participate in the transfer of floor
loads to the beams. The finished surface will be non-structural
(e.g. screed, wood or floating screed).
Table 2 – Guidance on components for beam and block floor systems with self-bearing beams and non-structural
screed as concrete topping.
Blocks
Precast concrete
beam type
Block
material
Performance
type (min) (1)
Concrete
NR
EPS
R1
Non self-bearing
Concrete
RR
Notes on the structural floor system (3)
The beams act in conjunction with the cast in situ structural
concrete topping to provide the final strength of the floor
system. The concrete topping forms the compression flange of
the floor. The blocks perform no mechanical function in the
final floor system, but will act as formwork during the
construction of the floor.
The beams act in conjunction with the cast in situ structural
concrete topping to provide the final strength of the floor
system. The concrete topping acts compositely with part of
the top of the blocks to form the compression flange of the
final floor system as they participate in the transfer of loads.
Table 3 – Guidance on components for beam and block floor systems with non-self-bearing beams and structural
screed as concrete topping.
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Page 22 | October 2014 | Issue 15 | Technical Extra
Suspended beam and block concrete floors
GUIDANCE (CONTINUED)
Blocks
Precast concrete
beam type
Self-bearing
Block
material
EPS
Performance
type (min) (1)
R1 (2)
Notes on the structural floor system (3)
The precast beams alone provide the final strength of the
floor system. The blocks perform no mechanical function in
the final floor system, but will act as formwork during the
construction of the floor. The concrete topping participates in
the transfer of floor loads to the beams.
Table 4 – Guidance on components for beam and block floor systems with self-bearing beams and structural screed
as concrete topping.
NOTES TO TABLES 2, 3 AND 4
1.
Tables 2, 3 and 4 show the minimum strength of block required to perform the function indicated in the right hand column of the table.
A stronger block may be used, but no improvement in performance may be assumed in the design of the floor system.
2.
The use of self-bearing precast concrete beams with EPS blocks type R1 is not covered by BS EN 15037. However, for residential purposes only,
NHBC will accept type R1 classification EPS blocks used in conjunction with self-bearing beams and cast in situ structural concrete topping for
residential suspended ground floor construction, provided that the adequacy of the complete structural floor system (including beams, blocks and
structural concrete screed) has been satisfactorily verified by calculation to BS EN 1992-1-1, together with full-scale testing (details to be agreed
with NHBC prior to testing) by an appropriate independent technical approval authority and in accordance with NHBC Standards Technical
Requirement R3.
3.
In all situations where the cast in situ concrete topping is required to perform the function of a structural screed, the reinforcement of the topping
should consist of welded mesh in accordance with BS EN 15037-1. Alternatively, cast in situ structural concrete topping reinforced with
polypropylene macrofibres or steel fibres may be acceptable to NHBC for residential suspended ground floor construction, providing that the
adequacy of the complete structural floor system (including beams, blocks and structural concrete screed) has been satisfactorily verified by
calculation to BS EN 1992-1-1, together with full-scale testing (details to be agreed with NHBC prior to testing) by an appropriate independent
technical approval authority in accordance with NHBC Standards Technical Requirement R3.
YOU NEED TO…
■■ Select the appropriate types of pre-cast concrete beam and block to suit the structural and thermal
requirements of the suspended floor construction and ensure that the beam type is compatible with the block
type specified, in accordance with BS EN 15037, Part 1 and Part 2 or 4 as appropriate.
■■ Ensure that the design of the screed is structurally suitable for use with the beam and block system specified.
■■ As an alternative to the first two bullet points above, and for residential purposes only, NHBC will accept
structural suspended beam and block floor systems verified in accordance with the guidance given in Note 1 to
Tables 2 and 3.
■■ Ensure that the design of the whole floor construction fulfils the structural requirements for its position in the
building in accordance with BS EN 1992-1-1.
■■ Ensure that all elements of the floor construction are installed in accordance with the manufacturers’
technical information.
For technical advice and support, call 01908 747384 or visit www.nhbc.co.uk
Technical Extra | Issue 15 | October 2014 | Page 23
GUIDANCE AND GOOD PRACTICE
Installation of GRP dormer windows
Who should read this: Technical and construction directors and
managers, architects, designers, consultants, manufacturers,
specifiers and purchasers.
INTRODUCTION
Quick to install, reliable and robust, low maintenance and aesthetically pleasing, it’s not difficult to see why
off-site fabricated GRP systems are a popular choice for building designers when considering dormer windows,
but there are a few common pitfalls to avoid.
GUIDANCE
Standards Extra 44 (April 2009) raised concerns with the design,
manufacture and installation of GRP products used to form building
elements, including dormer roofs. Subsequently, the Construction Glassfibre
Manufacturers Association (CGMA) was formed, as reported in Technical
Extra 07 (July 2012). Most of the major manufacturers now have products
that comply with the requirements of the CGMA scheme, or alternatively
hold independent third-party assessments.
The good news is that there seems to be a marked improvement in the
quality of products available and also in the way they are installed. However,
feedback from NHBC’s Inspection Service suggests a number of instances
where inspectors have found the detailing of the interface between the roof
covering and the GRP dormer has been less than satisfactory. Without the
intervention of our inspectors, these poor installations would have likely
resulted in water ingress, and the possibility of costly repairs for the builder,
NHBC or perhaps, in the longer term, for the homeowner.
Common defects include the underlay not being finished correctly and
being left to hang down into the rafter or roof void, and battens and tiles
abutting the dormer incorrectly. These could have easily been avoided if the manufacturer’s installation
instructions had been followed.
A reoccurring theme indicates that the installation instructions are not available at the time of installation,
which creates unnecessary risk. The correct detailing of underlay, tiles and battens at interfaces is critical to the
long-term performance of the roof. The detailing will vary between manufacturers, and therefore it is important
that the product-specific instructions are followed. Leaving these details to ‘get over’ on site could significantly
increase the risk of failure.
YOU NEED TO…
■■ Ensure that GRP dormers have a third-party assessment or alternatively a CGMA certification.
■■ Ensure fitting instructions are available at the time of installation and are followed for all phases of the
construction, including the roof covering.
For technical advice and support, call 01908 747384 or visit www.nhbc.co.uk
Page 24 | October 2014 | Issue 15 | Technical Extra
GUIDANCE AND GOOD PRACTICE
Audible cracking noises in intermediate floors
Who should read this: Technical and construction directors and
managers, architects, designers and site managers.
INTRODUCTION
In Technical Extra Issue 11 September 2013, we ran an article on audible cracking noises in intermediate floors
that can occasionally occur in a small number of homes. This article recaps research work that NHBC, Gypsum
Products Development Association and UK Timber Frame Association jointly funded to establish the cause and
possible solutions to the problem.
GUIDANCE
Research was carried out by Salford University
Acoustics Research Centre and involved both
laboratory and on-site investigations using homes
where the problem had occurred. The results showed
that the source of each cracking sound emanated from
locations where the drylining adhesive had extended
upwards and made contact with built-in joist ends. The
recommendation following the research was to ensure
that the gypsum plasterboard adhesive did not spread
up to set in contact with the floor joists, or where a
cracking noise was apparent, to form a cut through
the adhesive to debond the wall and ceiling drylining.
The various drylining installation instructions from
plasterboard manufacturers show the adhesive dabs
or ribbons at the top of the walls are kept down from
ceiling. Drylining installation methods found on sites
would suggest that not all dryliners have been achieving
this gap, either because they have installed the adhesive
tight to the ceiling or the adhesive has spread up too
far when the plasterboard sheets have been installed.
To achieve a gap between the adhesive and the
ceiling, it is suggested that the adhesive be kept
100mm down from the ceiling which, on application
of the plasterboard, should still maintain the required
isolation from the ceiling. Fig. 1 shows a suggested
drylining detail at the wall/ceiling junction that
has been adopted by some contractors with
apparent success.
Other feedback suggests that, where remedial work
has been carried out to ceilings with noise issues,
plasterboard not being fixed tight up to the floor joists
was a contributory factor. Fixing the plasterboard
tight to the joists resolved that noise problem.
Clear gap between adhesive and ceiling
board after installation of drylining
A continuous ribbon of adhesive
approximately 100mm clear of the ceiling
board prior to application of plasterboard
(the ribbon may coincide with the ribbon
around window and door openings)
Fig. 1 – Section through a drylined wall/ceiling junction
at an intermediate floor within a dwelling
YOU NEED TO…
■■ Ensure the plasterboard adhesive is kept far enough down from the top of the wall to ensure the adhesive
does not spread up to contact the ceiling board and joist ends. A distance of 100mm between the adhesive
and the ceiling would be a reasonable distance to adopt. Where a ceiling already has a noise problem that
it is suggested that checking the fixing of the existing plasterboard ceiling would be an appropriate first
course of action.
For technical advice and support, call 01908 747384 or visit www.nhbc.co.uk
Technical Extra | Issue 14 | May 2014 | Page 25
INFORMATION AND SUPPORT
IMPROVE CUSTOMER EXPERIENCE WITH ONLINE HANDOVER INFORMATION
In response to the growing demand for the NHBC Home
User Guide (HUG), we’ve announced a further series of
seminars to demonstrate this unique, online system and
showcase what it can offer to both builders and homeowners.
Date
Location
24 November 2014
Bristol
28 November 2014
Milton Keynes
The NHBC Home User Guide (HUG for short) is the
ultimate online home management tool and is available
free for every new plot registered with us for Buildmark
cover. It’s co-branded, accessible from computer or tablet,
and enables our registered builders to provide new home
buyers with all the information they need to move in and
run their new home.
1 December 2014
Cumbernauld, Scotland
4 December 2014
Nottingham
8 December 2014
York
11 December 2014
Croydon
UPCOMING TECHNICAL EVENTS
Building for tomorrow 2015
Date
Location
For 23 years, Building for tomorrow (Bft) has been
informing the industry on topics that directly impact
current and future house building.
26 February 2015
Shendish Manor, Hemel Hempstead
5 March 2015
Thistle Haydock Hotel, Haydock
Ahead of further information and an agenda being
published in the autumn, NHBC is pleased to announce
the dates for next year’s Bft seminars.
10 March 2015
Leigh Court, Bristol
12 March 2015
Sandown Park Racecourse, Esher
17 March 2015
York Racecourse, York
24 March 2015
Westerwood Hotel, Cumbernauld
26 March 2015
Cambridge Belfry, Cambourne
14 April 2015
National Motorcycle Museum,
Birmingham
16 April 2015
Hilton Templepatrick, Belfast
REPLACING THE EXTRANET – THE NHBC PORTAL
For some time, NHBC has been developing a significant
improvement to the way we work with builders to manage
site and plot information. This was driven by feedback
from our customers that we needed to make it much
easier for you to work with us. We launched the NHBC
Portal in May this year, as an upgrade to the NHBC
Extranet. The response has been very positive; it saves
you, our registered builder customers, considerable time
and effort. As most builders have moved to the NHBC
Portal, we will be de-commissioning the Extranet in
November 2014. If you haven’t moved across, we
therefore now need you to register for the
NHBC Portal as soon as possible – please go to
www.NHBC.co.uk/PortalLogin.
If you are not familiar with the NHBC Portal, we wish to
encourage you to share the success. The Portal enables
you to check the status of your plots and sites online,
making it much easier to manage them. You can now:
nSee any Reportable Items and other builder responsible items. By showing this online, it is much easier and quicker for any issues to be seen and resolved.
Page 26 | October 2014 | Issue 15 | Technical Extra
nUpload all the documentation about the site such as drawing, schedules and technical data, rather than waiting for the post and for them to be scanned.
nSee and change plot details, such as selling price, fabrication, etc. , making sure this is up to date and track Plot Product Schedules (PPS).
n
Submit Site Notification and Initial Notice (SNIN)
forms, so we can provide you with instant quotes
for Warranty and Building Control.
nDownload management reports about any or all of your sites; the consolidated reports for all your sites (or across all companies) make it easier to see site status at any time of your choice.
The NHBC Portal is provided to you as a registered
builder. It is a free service and is available 24 hours a day.
The feedback has been that it is a comprehensive portal;
it greatly improves our service to you, enabling you to
manage your sites easily and securely, reducing your
time and costs.
To see a video about the NHBC Portal and sign up,
click www.nhbc.co.uk/portallogin
TECHNICAL NEWS
COMPLIANCE ALERT FOR REBAR MATERIAL
As advised in the August edition of NHBC’s Clicks &
Mortar, the British Association of Reinforcement (BAR)
has recently issued a compliance alert after independent
tests identified some imported bars failed to comply
with BS 4449.
The bars were found not to comply with the geometrical
requirements of the British Standard. BAR has advised its
members always to check rebar material for British
Standard compliance.
Action: visit the BAR website below for a copy of the
alert in full at www.uk-bar.org/news/41.htm
Click below to view previous editions and subscribe to
NHBC’s monthly technical newsletter Clicks & Mortar
www.nhbc.co.uk/NewsandComment/Joinournewsletter/
AUTOMATIC GATES
Pressure to increase the safety of automatic gates has
intensified following a number of incidents involving
children. Designers and builders should consider the
following guidance when specifying powered residential
driveway gates.
The Door & Hardware Federation (DHF) Powered Gate
Group represents the UK’s leading designers,
manufacturers, installers and maintainers of powered
gates. The specialist Powered Gate Group was formed
with encouragement from the Health and Safety
Executive (HSE), which was anxious to see an industrywide trade body responsible for developing higher
standards of safety for automated gates.
The first output was the publication of a comprehensive
guide to powered gate safety to provide appropriate best
practice technical guidance to the industry – the DHF
Guide to Gate Safety Legislation and Standards. The guide
lists all of the current published standards for powered
gate performance and the design requirements that
should be followed to ensure every gate installation is
safe. The guide advises manufacturers on the safe design
and manufacture of safe powered gates and also makes
recommendations on current safety solutions which go
beyond the obligations set out in current standards.
The guide is available at: http://www.dhfonline.org.uk/
docs/1053-Gate-Guide-CURRENT.pdf
For maximum safety, not only must powered gates be
designed and manufactured to the highest standards,
they must also be correctly installed and maintained.
Member companies of the Powered Gate Group ensure
that their key personnel are sent on an intensive two-day
DHF Powered Gate Safety Diploma training course. It is a
condition of DHF membership that all members supplying
powered gates send their relevant personnel through the
training course.
Only when the training has taken place can member
companies apply the DHF Safety Assured mark to their
installations. This mark on a powered gate assures the
specifier, owner and user of the gate that the company
supplying it has:
n
Undergone the rigorous gate safety
training programme
n
Achieved the Powered Gate Safety
Diploma qualification
nSigned up to a rigorous and binding code of conduct
n
Committed to comply with or exceed all current
safety standards.
For more information on the DHF Powered Gate Group,
visit www.dhfpoweredgategroup.co.uk
Action: ensure automatic gates are correctly designed,
manufactured and installed.
BIM4 HOUSING CONFERENCES
Building Information Modelling (BIM) is widely seen as the
construction industry's answer to greater efficiency and
improvement. Whilst some house builders are starting to
realise the benefits of BIM, others are taking a more
cautious approach.
The BIM4 Housing group, which is formed from a wide
range of industry representatives to look at issues with
the use of BIM specific to house building, is now holding a
series of conferences across the UK. Speakers from a
range of disciplines involved with BIM will be drawing on
shared practical experience to dispel some of the myths
and uncertainties around BIM to help builders make a
more informed decision on its use.
A number of one-day conference dates remain:
nBirmingham – 30 October
nLondon – 12 November
nEdinburgh – 8 December
Conference places can be booked here:
http://constructingexcellencesw.org.uk/events
Technical Extra | Issue 15 | October 2014 | Page 27
Useful contacts for technical information and advice
NHBC technical advice and support
Training
Tel: 01908 747384
Email: [email protected]
Web: www.nhbc.co.uk/builders/technicaladviceandsupport
For information about training, please go to
www.nhbc.co.uk/training, call 0844 633 1000 and
ask for ‘Training’, or email [email protected].
Technical Extra
The Zero Carbon Hub
Previous editions of Technical Extra are available on our
website at www.nhbc.co.uk/Builders/ProductsandServices/
TechnicalExtra/
The UK Government has set out an ambitious plan for all
new homes to be zero carbon from 2016. The Zero Carbon
Hub helps you understand the challenges, issues and
opportunities involved in developing, building and marketing
your low and zero carbon homes.
NHBC Standards
Buy online at: www.nhbc.co.uk/nhbcshop/technicalstandards
or access the new digital format Standards Plus via the
NHBC Extranet at: www.nhbc.co.uk/builders/NHBCExtranet
www.zerocarbonhub.org
Building Regulations
NHBC regularly distributes information on a range of
industry topics, including new products and services,
the building industry market, house-building news and
house-building statistics. To receive this industry
information, please register at:
For guidance on issues relating to Building Regulations,
please visit NHBC’s TechZone at www.nhbc.co.uk/techzone
Building Control
For Building Control queries, please call
0844 633 1000 and ask for ‘Building Control’, or email
[email protected].
Engineering queries
NHBC Clicks & Mortar e-newsletter
www.nhbc.co.uk/newsandcomment/registerfore-news
General enquiries
For all other enquiries, including ordering products and
services, please call 0844 633 1000, and ask for ‘Sales’.
For Engineering queries, please call 0844 633 1000 and ask
for ‘Engineering’.
NHBC Foundation research
The NHBC Foundation facilitates research and
shares relevant guidance and good practice with the
house-building industry.
www.nhbcfoundation.org
Copyright© NHBC 2014
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NHBC, NHBC House,
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Milton Keynes,
Bucks MK5 8FP
Tel: 0844 633 1000
Fax: 0844 633 0022
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