NHBC’s technical newsletter
December 2003
Issue 28
This edition includes:
2
Using vapour permeable underlays?read the fine print
Read the assessment carefully before using the product
A load of rubbish
NHBC highlights the consequences of the recent Landfill Directive
4
Floating rafts
A case study where raft foundations were unable to deal with the ground conditions
5
Keeping the lid on changes to fixing requirements for slates and tiles
New fixing requirements for vulnerable roof tiles
6
Registration of new homes
Care is needed when completing your HB3 application and registration forms
EXTRA
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STANDARDS EXTRA
1
CONTENTS
2 Using vapour permeable
underlays? - read the
fine print
2 Chapter 6.9
‘Cladding/Enclosures’ we need your help
3 Initial Notices timing is everything
3 A load of rubbish
4 Floating rafts
Using vapour permeable
underlays? - read the fine print
Standards Extra 21 (April 2001) discussed
the use of vapour permeable underlays
(VPUs) in pitched roofs where insulation is
laid at horizontal ceiling level and the roof
space is not ventilated - commonly referred
to as ‘cold pitched roofs’.
5 Keeping a lid on - changes
to fixing requirements for
slates and tiles
5 ‘Guide to your new home’
gets a makeover
6 Registration of new homes
6 Chapter 4.2 ‘Building near
trees’ new training course
digs deeper
7 Questions and answers
Non-ventilated cold pitched roof
At that time the use of VPUs was limited to
dwellings with simple rectangular plan roofs.
Assessments by BBA and BRE Certification
have now been revised and include the use
of VPUs in most cold pitched roof designs
including: duo pitched, mono pitched,
hipped, mansard and gable ended, and with
features such as valleys, dormers, rooms in
the roof etc.
VPUs should always be installed in
accordance with conditions set out in their
assessments. The assessments are all based
on the roof void below the VPU being
unventilated but this is where the similarity
ends. At least one assessment requires
ventilation of the tile batten space, between
the top of the VPU and the underside of the
roof covering, whilst others rely on the
fortuitous air gaps in the roof covering. The
exception to this is where the roof covering
is considered to be ‘tight jointed’ (e.g. fibre
cement slates and reconstituted interlocking
slates) - then the tile batten space should be
ventilated. Some VPUs have been assessed
for use over solid timber sarking but not
other forms of sheet sarking (plywood, OSB
and chipboard etc.)
A common requirement for all VPUs used in
cold pitched roofs is the need to limit the
rate of water vapour passing from the living
spaces into the roof void above. To do this it
is essential that gaps in the ceiling are
avoided, services which pass through the
ceiling are well sealed and loft hatches are
effectively draught proofed.
ACTION:
If you are considering using a vapour
permeable underlay, check that the
material has a third party assessment,
read it carefully and ensure that you are
able to meet all of its conditions. The
risk of not doing so is a roof space
which suffers from condensation and
the risk of timber decay.
Chapter 6.9 ‘Cladding/Enclosures’ We need your help!
As mentioned in Standards Extra 27, NHBC is preparing a new chapter of the Standards to
deal with curtain walling, rainscreen cladding and insulated render systems. The work is
being carried out by a task group specially chosen for their expertise in cladding and curtain
walling and we are expecting to have a final draft ready for an industry consultation in
Spring 2004.
If you have dealt with projects involving curtain walling, rainscreen cladding or insulated
render systems and would like to comment on the proposals, we would like to hear from you.
Please contact Graham Perrior, Technical Manager:
Tel: 01494 735706 Fax: 01494 735717 or Email: [email protected].
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Initial Notices - timing is everything
Where NHBC is providing your building control, submitting the Initial Notice to the local authority is crucial to the process. If
any work has commenced before the Initial Notice has been accepted, it is classed as ‘unauthorised’ and the local authority
may take enforcement action.
ACTION:
Ensure that Initial Notices are completed fully and given to NHBC at least 4 weeks before you start work on site.
A load of rubbish
New legislation has recently been introduced which will
make it increasingly difficult and more costly to dispose of
certain wastes from construction sites.
The Landfill Directive supported by Regulations was
implemented in June last year. The purpose of the new
legislation is to reduce the overall amount of material
finding its way into landfill sites. In addition, it will no longer
be possible for certain materials, such as flammable or
corrosive wastes, to be disposed of in this way.
Landfill sites will be categorised as ‘inert’, ‘non-hazardous’
or ‘hazardous’ and each type of site will only be able to
receive that specific type of waste. The number of hazardous
sites will be reduced from 182 to 12 public sites and there will
potentially be no hazardous waste sites in the South-East or
Wales.
It will also be necessary for waste to be treated before it is
sent to a landfill site in order to change the characteristics
of the waste, reduce its volume, reduce its hazardous nature,
make it safer to handle or enhance recovery.
Hazardous waste will need to meet Waste Acceptance
Criteria (WAC) although these have not yet been determined
by the Environment Agency. (These should not be confused
with the standards and guidance for levels of contaminants
that are allowed to be present in land used for housing.)
Hazardous waste from the house-building industry is
expected to be small compared with other industries but the
Waste Acceptance Criteria will need to be defined before
volumes can be assessed. Nonetheless, builders will need to
find ways of reducing waste on site, as it becomes more
difficult and costly to dispose of.
The Landfill Directive (Council Directive 1999/31/EC) was
implemented by The Landfill (England and Wales)
Regulations 2002.
The regulations may be viewed on the HMSO website at;
www.hmso.gov.uk/si/si2002/20021559.htm
Further information relating to the Landfill Directive may be
viewed on the Environment Agency website at:
http://www.environment-agency.gov.uk/business/444217/444
663/landfill/?version=1&lang=_e
STANDARDS EXTRA
3
Floating rafts
Raft foundations are widely used on sites where it is
necessary to spread loads over the footprint of the home.
Once cast, the raft provides a stable platform from which
the homes can be constructed.
Raft foundations were used at one site in Scotland where
the ground varied across the site. The ground conditions
were relatively good in some areas, and traditional strip
foundations were used, which have generally proved
satisfactory. However, other areas had peat strata, which the
engineer’s design required should be removed and replaced
with imported material, compacted in layers.
Soon after moving in a number of homeowners contacted
NHBC and the extent of the problem became apparent. We
appointed a consultant to investigate and report back with
his findings.
The consultant’s report concluded that properties in the
central part of the site had suffered substantial movement.
Some homes were tilting and others settling vertically. The
worst case was a tilt of 265mm from corner to corner of one
bungalow. Its floor was sloping at 1 in 28! Many of the walls
were also well out of plumb. Although many of the raft
foundations had remained intact and the buildings were still
holding together, others had suffered damage, causing
significant cracking to the brickwork outer leaf.
The damage was not limited to the homes; the detached
garages were also affected. The worst affected single
detached garage showed a tilt of 208mm!
Note the differences in the line of the ridges
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STANDARDS EXTRA
So why has the problem occurred?
Although the design was correct, it was not carried out on
site. With hindsight, it may have been more appropriate to
have used a pile and beam system rather than rafts.
Whereas the design required the removal of the peat, the
report showed that it still remained in places. This had
caused settlement to the homes and garages as well as
paths and drives. Also some drains had settled to such an
extent that they had a backfall.
Lessons to be learned
To avoid similar problems, you should:
■ Carry out a site investigation in accordance with NHBC
Standards Chapter 4.1.
■
Determine if the investigation is representative of the
conditions across the site.
■
Ensure the design proposals are appropriate for the site
conditions.
■
Follow the design proposals on site.
■
Consult the designer and the NHBC inspector if the
conditions on site are different to what the site
investigation identified.
ACTION:
Ensure that adequate site investigations are made and
follow the design. If unforeseen situations occur in
ground conditions, consult with the designer and NHBC
before proceeding.
General view of the development
Keeping the lid on - changes to fixing
requirements for slates and tiles
From past experience, we all know
which tiles are dislodged from roofs
during high winds. It’s always those
around the perimeter and the end ridge
tiles that are vulnerable. If tiles are
dislodged during the first two years of
the warranty, then builders are
responsible for replacement which, with
health and safety considerations, can be
an expensive business.
The slating and tiling Code of Practice
(BS 5534) has recently been amended
and there are two aspects which will
impact builders.
Firstly, the Code sets out minimum
fixings for both slates and tiles. Builders
and their roofing contractors need to
check with the manufacturer what
additional fixings should be provided for
a given building in a particular location.
However, the minimum requirement for
additional fixings includes mechanically
fixing ridge tiles for at least 900mm
from the end or either side of party
walls. Fixing all ridge tiles is
recommended good practice.
Secondly, the British Standard has
increased the sizes of tiling battens and
suppliers are now recommending
their use.
NHBC Standards are in the process of
being revised and it is intended that
revisions to Chapter 7.2 will be
published early in 2004.
ACTION:
Ensure that you are using the correct
number of fixings for tiles and slates
by checking with the manufacturer.
Mechanical fixing is always required
for some ridge tiles.
Image courtesy of Lafarge Roofing
Guide to your new home
gets a makeover
It’s not only rooms that get makeovers as by now you should
be receiving NHBC’s newly revamped, re-designed and
updated ‘Guide to your new home’ booklet - a practical guide
for homeowners, full of advice and top tips on how to look
after their new home.
The booklet was introduced a number of years ago and is
regarded as essential reading for new homeowners who
have just moved in and want to know how to make the best
of their new purchase. The booklet provides general
guidance to homeowners on issues such as how to
run-in their new home and bleed a radiator, as well as
providing practical tips on decoration, and
maintenance advice on everything from chimneys
to paintwork.
NHBC recently took the opportunity to redesign the
booklet, as well as reviewing and updating the content,
to ensure you continue to provide the very latest advice
to homeowners.
STANDARDS EXTRA
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Registration of new homes
It is now just over a year since NHBC
introduced the revised HB3 ‘Buildmark
– Application and Registration’ form.
The reason for the change was
explained in Standards Extra 25
(December 2002). Questions 9 (is this
a mixed use development?) and 10
(what is the type of construction?)
need to be completed properly for the
application to progress. Where either
question is not fully answered, it
means we have to come back to you
for more information.
In the case of conventional cavity
masonry or conventional timber
framed construction, a simple tick is all
that is required. For other types of
construction, we need to know precisely
what it is we are being asked to
register so that we can determine
whether it is acceptable.
Over the past year, we have had a
number of forms which have been
completed incorrectly, including one
case where the HB3 said it was ‘cavity
masonry’ but on site, a steel framed
building appeared. Needless to say,
that caused much confusion and
regrettably some delay for the builder.
In another case the HB3 was ticked to
say that the homes were conventional
timber frame. Further questioning of
the builder revealed the intended use
of structural insulating panel (SIPs)
construction. Clearly this is not standard
timber frame construction! Again this
caused delay for all concerned.
Where the HB3 is ticked to confirm
that ‘other type of construction’ is to
be used, it is important that as much
detail as possible is put in the line below
e.g. “steel frame with curtain walling &
cladding,” or perhaps “r/c frame with
cavity masonry external walls”.
ACTION:
Complete the HB3 carefully and,
where appropriate, support the
application with details of what it is
you intend to build.
Chapter 4.2 ‘Building near trees’
New training course digs deeper
In April this year new guidance on ‘Building near trees’ was
issued in the revised edition of NHBC Standards Chapter 4.2.
A serious area for many builders, foundation failures in
shrinkable soil due to the effects of trees are a consistent
cause of warranty claims, and can also affect builder liability
and premium ratings.
Builders’ technical staff should be aware of the new
guidance on shrubs, the limitations of deep trench fill, where
heave may occur and precautions to take, variations in tree
water demand and further advice on Plasticity Index values.
In spring 2004, NHBC will be running 1-day training
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STANDARDS EXTRA
courses to cover this new guidance. Two types of course
will be available: one covering site issues, suitable for
site-based personnel, and another more theoretical course
for engineers, designers, technical managers and
construction professionals.
The courses will outline the rationale behind Chapter 4.2,
the changes in the latest revision, sitework issues, case
studies, soils identification and also repair solutions.
If you would like to be sent further information on these
courses when this is available, please register your interest
by calling NHBC Training Services on 0870 241 4323.
Questions & Answers
Below are the answers to a selection of questions we have recently been asked in NHBC Technical.
Installation of lead flashings
Question
Where lead flashings are to be installed under dpcs and cavity trays in masonry walls, should the mortar joints be raked
out as the masonry work proceeds or can the joint be cut out later?
Answers
■ NHBC Standards clause 6.1-S4(c) states it is easier to rake out joints as work proceeds.
■ The Lead Sheet Association provides guidance on installation of lead flashings which confirms this.
■ Cutting out the joint once the mortar had hardened is likely to cause damage to dpcs and cavity trays, leading to
water penetration.
To avoid damage to dpcs and cavity trays the joint for lead flashings should be raked out to a depth of at least 25mm
as work proceeds, as shown in the diagram below.
As an alternative, lead flashings can be built in as the work proceeds. They should be built in to a depth of 50mm and a
welted edge should be provided to form a key with the mortar as shown in the diagram below.
Lead flashings inserted into raked out joint
Lead flashings built in as work proceeds
Plastic plumbing - insulation
Question
Do plastic pipes supplying hot and cold water and central heating require thermal insulation?
Answers
■ Plastic pipes have a lower thermal diffusivity than copper pipes and therefore lose heat more slowly.
■ Approved Document L1 (England & Wales), Building Standards Part J (Scotland) and Building Regulations Part F
(Northern Ireland) require insulation of certain pipes to reduce unwanted heat loss.
■ Plastic pipes have increased resistance to damage from freezing compared with copper pipes. However, freezing of
water within the pipes may have a detrimental effect on the performance of the plumbing system.
■ NHBC Standards Appendix 8.1–B states the minimum insulation thickness to delay freezing inside domestic
premises for cold water systems.
■ The Water Supply Regulations (England & Wales), The Water Byelaws 2000 (Scotland) and Water Supply Byelaws
(Northern Ireland) require insulation of pipework against freezing.
Plastic pipes should be insulated to the same standard as required for copper pipes in the same location.
STANDARDS EXTRA
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Questions & Answers
Below are the answers to a selection of questions we have recently been asked in NHBC Technical.
Canopy flashings
Question
Should flashings be provided to canopy roofs?
Answers
■ NHBC Standards clause 7.2 – D8(g) says, ‘where a roof abuts a vertical surface, cover flashings, stepped flashings,
soakers and back or parapet gutters should be provided as necessary’.
■ NHBC Standards clause 7.2 – D8(g) says, ‘where a roof is over an enclosed area, the wall construction should
include cavity trays linked to the flashings’.
■ Seepage of rainwater between the canopy and wall may cause damage to the canopy structure.
Canopy roofs constructed on site should be provided with flashings at the abutment with the wall, regardless of
whether there is an enclosed or an open space below. Where there is an enclosed space below, the flashing should be
linked to a cavity tray.
Prefabricated (GRP) canopies located over an open space can be installed without flashings but should be sealed back
to the wall with a suitable sealant such as silicone, to minimise rainwater ingress. Prefabricated canopies located over
an enclosed space should be provided with flashings linked to cavity trays.
Strutting at the ends of floor joists
Questions
a) Is herringbone strutting acceptable where joists bear into the web of steel joists or are supported in hangers?
b) Where joists are in hangers is it acceptable to use 38mm x 38mm noggings at the top of the joists (for support of the
floor decking) and at the bottom of the joists (for fixing the plasterboard ceiling) as the strutting between joists?
Answers
■ Strutting is needed to prevent rotation of the joists.
■ NHBC Standards clause 6.4 – S8 refers to solid strutting between solid joist ends bearing into the web of steel joists
or supported in hangers.
a) Herringbone strutting is acceptable where joists bear into the web of steel joists or are supported in
joist hangers. At perimeters of floors and ceilings noggings, as required, should be provided to support the floor
decking and ceiling.
b) Noggings at the top and bottom of joists would not prevent rotation of the joists and provide load sharing.
They are not an acceptable alternative to solid or herringbone strutting.
Note: For timber I-joists
■ Strutting is necessary where joists bear onto the web of steel joists.
■ Strutting is not necessary where joists are supported in joist hangers of a type which restrain both
the top and bottom flanges.
NHBC Technical
Buildmark House, Chiltern Avenue, Amersham, Bucks HP6 5AP
Tel: 01494 735859 Fax: 01494 735717 Email: [email protected] www.nhbc.co.uk
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HB1954 11/03