Download Standards Extra

NHBC’s technical newsletter
April 2003
Issue 26
This edition includes:
A root and branch review of Standards Chapter 4.2
An introduction to the significant changes in the Chapter rewrite.
3
Concrete changes to Chapter 2.1
A better understanding of concrete and chemical attack leads to a Chapter update.
4
New guidance for underfloor heating systems
NHBC introduces guidance on the design and installation of these systems.
5
Changes to Part E of the Building Regulations
A summary of the changes to regulations governing sound insulation.
6
NHBC gives support to the joist debate
EXTRA
2
We evaluate the two methods used for the support of floor joists.
STANDARDS EXTRA
1
CONTENTS
2 A root and branch review of
Chapter 4.2
3 Concrete changes to
Chapter 2.1
3 NHBC publishes Good
Craftsmanship Guides
3 NHBC launches Trade Talks
A root and branch review
of Chapter 4.2
One of the major areas of claim against Buildmark is foundation failure. All too often
these claims involve homes which have been constructed on shrinkable clay soil
in close proximity to trees or shrubs.
4 Adequate fixings for tiles
and slates
4 New guidance for
underfloor heating systems
5 Changes to Part E of the
Building Regulations
6 NHBC gives support to the
joist debate
7 Reminder - Notification of
energy ratings
8 Questions and Answers
Shrinkable clays change in volume as their
moisture content varies. This can result in
subsidence or heave to the home where the
tree causes the moisture content to vary and
the foundations have not been designed to
take account of the possible movement.
The problems associated with clay soils were
highlighted in the late '60s when NHBC first
published Practice Note 3 which provided
guidance to avoid damage caused by trees
near dwellings. This was revised in 1974 and
again in 1986. This version subsequently
became Chapter 4.2 of the Standards, which
was itself revised in 1992.
Over the years our knowledge of the way in
which trees and foundations interact has
increased and the time has come to review the
Chapter again. This review has been based on
both our claims data and up-to-date advice
from engineers and arboriculturalists.
The 2003 version expands the guidance to
include shrubs. NHBC Claims data confirms
that some claims are caused by shrubs;
Pyracantha, Cotoneaster and climbers such
as Ivy, Virginia Creeper and Wisteria can
be particularly damaging. The guidance asks
for minimum foundation depths to be
increased where shrubs are planted unless
they are excluded within stated distances
from the foundation.
Other changes include:
■
the addition of an introduction to explain
the philosophy behind Chapter 4.2
■
better guidance about where heave is
likely to occur
■
guidance about the Plasticity Index of
shrinkable clay soils
■
limitations on the maximum depths of
trench fill foundations - where they are
greater than 2.5m deep then an engineer
will need to become involved. This is
because of the uncertainty of such
foundations at these depths.
The actual foundation depths given in the
Chapter remain unchanged. However, the
graphical solution to finding the correct
foundation depth has been improved. The
alternative method – by use of tables – has
also been improved by combining tables. A
D/H table has been included so it is no longer
necessary to work this out when determining
foundation depths. A number of tree species
have been added and two species have been
changed to a higher water demand category.
Hawthorn has been moved from Moderate
to High and Beech has been moved from
Low to Moderate.
The Chapter will become effective for homes
where the foundations are commenced on or
after 1 July 2003.
ACTION:
Familiarise yourself with the new Chapter. If you have used very deep trench fill
foundations in the past, then these will not be acceptable in the future unless
they are designed by an Engineer.
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STANDARDS EXTRA
Concrete changes to Chapter 2.1
Image courtesy of RMC UK
ACTION:
Familiarise yourself with the
new Chapter and ensure that
the correct grade of concrete
is specified. Ready mixed
suppliers are gearing up to
supply the new mixes.
NHBC Standards Chapter 2.1 - Concrete
and its Reinforcement - has been
revised to take account of the better
understanding of concrete specification
needed to resist chemical attack from
the ground, as detailed in the new
BRE Special Digest 1, (see page 7 of
Standards Extra 23).
These include:
■
whether the ground water is static
or mobile
■
whether the ground water is acidic
or not
■
whether the site is brownfield
or natural
In addition, the revisions bring the
Chapter in line with the European
Standard for concrete, BS EN 206-1, and
its complementary British Standard, BS
8500-Parts 1 and 2. In December 2003
these Standards will replace the current
British Standard for concrete, BS 5328
which was revised in September 2002
to take account of BRE Special Digest 1.
■
the size and service life of the
structural elements
■
what aggregates are to be used in
the concrete
■
whether additional protective
measures are to be applied to the
concrete e.g. protective coatings or
sacrificial layers.
The revised NHBC Chapter lists
Designated mixes which are supplied
ready mixed or Standard Prescribed
mixes for site mixing which are suitable
for non-aggressive and the lower range
of ground chemical aggressiveness.
In general terms a concrete that
satisfied the requirements of Class 1 or
Class 2 sulfate conditions under the old
BRE Digest 363 is also likely to satisfy
the ACEC (Aggressive Chemical
Environment for Concrete) Classes 1
and 2 as shown in table 4b, Appendix B
of the Chapter.
Specialist advice is required for the more
aggressive ground environments because
a range of items needs to be considered
before the concrete can be specified.
NHBC publishes Good
Craftsmanship Guides
common pitfalls and provide
advice on avoiding them.
The problems are those
most often identified during
NHBC’s inspection of homes
under construction. All of
the photographs were taken
on real building sites.
Enclosed with this issue of
Standards Extra are Good
Craftsmanship Guides for:
■
Brickwork and Blockwork,
and
■
Carpentry and Joinery –
Carcassing
These easy-to-use booklets
are practical reference guides,
which use photographs and
illustrations to highlight
Further copies of the Good
Craftsmanship Guides are
available from NHBC
Distribution (phone 01494
735328, fax 01494 765788)
at £25.00 for 50 copies
(including postage &
packing).
Building Inspectors hold a
limited number of copies
and may be able to provide
small quantities, on request,
during scheduled visits
to your sites.
The revised Standards Chapter 2.1 is
effective from July 2003.
NHBC launches Trade Talks
NHBC has recently launched its troubleshooting ‘Trade Talks’ for
site tradespeople. Based on the NHBC Standards and delivered by
NHBC Building Inspectors, these one-hour talks are designed to
highlight the most common reportable items or defects
encountered by inspectors and claims investigators on residential
developments. Practical guidance is then given on how to prevent
these problems.
The emphasis is on preventing defects through getting it right first
time. This type of training will help cut costs in terms of repair
work, and will also have a positive impact on customer satisfaction
and the levels of aftercare service needed to manage problems.
Everyone who attends a Trade Talk will receive his or her own copy
of the relevant NHBC Good Craftsmanship Guide.
We are currently offering Trade Talks on the following
subject areas:
Brickwork and Blockwork, and
Carpentry and Joinery – Carcassing
Trade Talks can be delivered on or near your site, provided that a
suitable room is available to seat the attendees.
Cost is £150 + VAT per Talk for a maximum of 12 attendees.
A 10% discount is offered for two or more Talks run
consecutively at the same venue.
Bookings can be made by calling Lisa Jones on 0870 241 4308.
STANDARDS EXTRA
3
Adequate fixings
for tiles and slates
Some readers may have seen press articles about the
need for extra fixings for tiles and slates. These have
come about because of changes to the British Standards
dealing with roofing (BS 5534-1 Code of practice for
slating and tiling) and wind loading (BS 6399-2 Code of
practice for wind loads).
The effect of these changes is that fixings arrangements
are now different. This is due to the way wind loads are
calculated and applied to different tiles, different roof
configurations and geographical location of the site. How
can builders be sure they are providing an adequate
number of fixings? Tile manufacturers we have spoken to
provide a free design service. They have computer
programmes to determine wind pressures for a specific
location. From that information, they can specify the
number of fixings for the tiles.
Image courtesy of Redland Roofing
ACTION:
Ensure that your specification for fixing roofing tiles
and slates meets with BS 5534 and BS 6399 for
each site. It is necessary to have each location
checked, as fixing requirements will vary.
New guidance for underfloor
heating systems
Underfloor heating systems may not be
widely used in mainstream house
building but they are popular with
certain sectors of the industry.
Underfloor heating systems need to be
designed and installed carefully to
ensure they are capable of heating the
home in accordance with NHBC’s
requirements. As the heat output is
determined by the length of the
heating loop, systems which fail to
work correctly cannot be put right
simply by fitting a larger radiator.
Extending the heating loop is bound to
involve major upheaval to the property.
The Building Services Research and
Information Association (BSRIA) has
recently completed a project looking at
underfloor heating systems. This has
culminated in the publication of two
guides – one for designers and the
other for the assessment of installations.
NHBC Standards Chapter 8.1 now
recognises these two publications
where underfloor heating is proposed.
By following the guides, it will ensure
that a consistent approach is adopted
by system designers and installers and
systems meet home owners’ expectations.
Image courtesy of Wavin UK
ACTION:
If you are proposing to use an underfloor heating system, then ensure it meets with the BSRIA guides, both in terms
of the design and installation.
Copies are available from BSRIA, Old Bracknell Lane West, Bracknell, Berkshire, RG12 7AH.
Tel: 01344 426511 Fax: 01344 487575 Email: [email protected] www.bsria.co.uk
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STANDARDS EXTRA
Changes to Part E of the Building Regulations
The Government has introduced changes to Part E of the Building Regulations (England & Wales), which currently sets the
standard for sound insulation between dwellings. The new regulations are designed to improve the standard of sound
insulation in dwellings and have been extended to encompass other building types such as 'rooms for residential purposes'
(e.g. hostels and hotels), and schools.
The proposals are also backed up by new requirements
for sound testing.
What are the changes?
In summary the significant changes are:
Improvement of sound insulation between dwellings and
between 'rooms for residential purposes' by:
■ setting a new performance standard for separating
walls and floors that takes greater account of low
frequency noise
■
promotion of substantial constructions more likely to
comply with the new standard
■
introduction of mandatory sound testing to prove
compliance of each construction type used
(see “sound testing”).
Improvement of sound insulation within dwellings and
'rooms for residential purposes' by:
■
controlling sound insulation between bedrooms and
rooms containing wcs and other rooms
■
controlling sound insulation of internal floors.
Limiting reverberation in common parts of buildings
containing flats or 'rooms for residential purposes' by:
■
designing the common parts of buildings to prevent
excessive reverberation (e.g. by providing acoustic
ceiling linings).
When?
The new Part E comes into effect for all relevant building
work from 1 July 2003.
Sound testing
For new houses and flats the requirements for sound testing
come into effect on 1 January 2004.
For converted houses and flats and new and converted
'rooms for residential purposes' the requirement for sound
testing comes into effect on 1 July 2003. From then at
least one set of “pre-completion tests” will need to be
carried out for each ten units.
Builders should be aware
that the new
provisions will not
apply where work
on a plot has
started, or a Plans
Certificate for the
work has been
issued by NHBC
or unconditional
plans approval
has been given
by the Local
Authority prior to
1 July 2003.
The following table provides additional clarification as to which regulations apply where work is commenced on individual
buildings on or after 1 July 2003.
New build dwellings and flats
Approved Document E applicable
1992
Plans certificate* for work issued prior to 1 July 2003
Individual buildings commenced prior to 1 July 2003
Plans Certificate*
not issued prior to
01/07/2003
2003
Sound testing
required
✓
✓
Individual buildings commenced on or after 1 July 2003
and prior to 1 January 2004
✓
Individual buildings commenced on or after 1 January 2004
✓
✓
Conversions and rooms for residential purposes
Approved Document E applicable
1992
Plans Certificate* not
issued prior to
01/07/2003
Plans certificate* for work issued prior to 1 July 2003
Individual buildings commenced prior to 1 July 2003
Individual buildings commenced on or after 1 July 2003
✓
✓
2003
Sound testing
required
✓
✓
* Plans certificate issued by NHBC Building Control or unconditional plans approval issued by Local Authority Building Control.
Robust Standard Details
Pre-completion sound testing will not be required for new
homes until 1 January 2004. This delay is to allow the
industry the opportunity to develop Robust Standard Details.
A major House Builders Federation (HBF) project is currently
underway to develop forms of separating wall and separating
floor constructions, which provide consistently
good performance and so will not need testing. Given that
the project is successful, builders will be able to choose
between building in accordance with the Robust Standard
Details or undertaking sound testing.
Further information on the HBF project is available by visiting
www.rsd.napier.ac.uk.
STANDARDS EXTRA
5
NHBC gives support to the joist debate
A hot topic at the moment is the support of floor joists. Here we look at two alternative methods of support
in masonry construction. Each method needs to be carefully considered for very different reasons.
The first part of this article confirms the correct use of joist hangers. The second part details one method
of building-in joists, which satisfies Part L1 of the Building Regulations (England & Wales). We also look briefly
at the implications of the new Part E with respect to the building-in of joists.
1 Support of joists using joist hangers
The Health and Safety Executive (HSE)
has expressed concern that unless
hangers are used correctly, there is
a potential risk of floor collapse
during construction.
This is not a new issue and NHBC
highlighted the problem in 1997 in
Standards Extra 14. However, HSE
has asked NHBC to remind
registered builders of the need to use
hangers correctly because they have
had to continue investigating incidents
where floors have collapsed.
When using joist hangers:■ use joist hangers that comply with
BS 6178 Part 1, 1999*
■ obtain and follow the manufacturer’s
installation requirements
2 Building-in of joists
■ ensure that the hangers and
blocks are compatible. Do not use
hangers marked for use with a
particular block strength on
blocks of a lower strength, eg.
don’t use 3.5N/mm2 hangers on
2.8 N/mm2 blocks
The Building Regulations in England and Wales are performance based. Therefore,
alternatives are acceptable if they meet with the performance standard.
■ ensure that the hanger back-plates
are flush to the wall and that the
hangers are vertical
■ ensure joists are cut to length with a
maximum overall tolerance of 6mm
■ do not load the joists until the
required number of courses have
been laid above the masonry
flanges (usually 3 block courses
but consult the hanger
manufacturer) and the mortar
joints have achieved the required
strength. Alternatively install an
engineer designed support system
under the joists and have it
checked before loading the floor.
(*The above guidance is based on
hangers which meet with BS 6178.
A new European Standard (BS EN 845-1)
is available and some manufacturers
may be producing hangers to
this standard. If this is the case,
check the manufacturer’s
recommendations about use.)
6
STANDARDS EXTRA
In England and Wales the Robust Details, which support Approved Document L1, show
timber floor joists supported by joist hangers rather than being built into a masonry
wall. This is to reduce air leakage at the junction of the two elements.
NHBC has considered a number of proposals for the building-in of joists into a cavity
wall. The following alternatives are acceptable.
Solid timber joists
Solid timber joists may be built into the inner leaf of a cavity wall, provided that:
■ the mortar joints around each joist perimeter are recessed or struck, and
■ the joint between the masonry and the timber is carefully pointed
with silicone mastic:
Mortar joint struck or recessed
and filled with sealant
Mortar joint struck or recessed
and filled with sealant
Bead of sealant applied
between wall and joist
Mortar joint struck or recessed
and filled with sealant
Timber engineered joists
Points to watch
Timber engineered joists may be built into the inner leaf of a cavity wall, provided that:
The sealant should be applied after the
home is roofed so that the joists and
masonry are reasonably dry.
■ proprietary filler pieces are fitted on both sides of the web between the top and
bottom flanges. These filler pieces must not damage the joist flanges. The depth
of the filler pieces should be slightly less than the dimension between the joist
flanges to achieve a “loose fit”
■ the mortar joints around each joist perimeter are recessed or struck, and
■ the joint between the masonry and the timber and any other air paths are
carefully pointed with silicone mastic
Mortar joint struck or recessed
and filled with sealant
Bead of sealant applied to joint between
filler piece and top and bottom flanges
The workmanship needs to be good:
Masonry blocks should be cut accurately
(sawn, where possible) and a struck or
recessed mortar joint formed to allow a
good silicone mastic bead to allow for
shrinkage of both timber and masonry.
The use of expanding polyurethane
foam is not acceptable for sealing
around joist ends.
Joists and separating walls
Builders should be aware that the new
guidance contained in Approved
Document E - Resistance to the passage
of sound (2003 edition)* shows joists
supported on hangers to separating
walls. This is considered to be the most
reliable method of ensuring adequate
sound insulation for separating walls.
Proprietary
filler piece
Bead of sealant applied
between wall and joist
However, if builders have designs which
show joists built into separating walls
then the ends do require sealing, to
limit air leakage.
*see page 5 for more on Part E
ACTION:
Mortar joint struck or recessed
and filled with sealant
REMINDER Notification of
energy ratings
In England and Wales the Building
Regulations require that the energy
rating of a new home be made known to
the home owner as soon as possible.
Builders may give the rating directly to
the purchaser or choose the most
appropriate time for posting the rating
notice in the completed home. The
intention of the legislation is to ensure
that purchasers are aware of the energy
efficiency of their new home.Research
by National Energy Services reveals
that this notification is not being carried
out. Their findings show that 98% of
unsold homes checked did not display
the SAP rating. Whilst the research
failed to identify whether builders were
making the rating known directly to
purchasers, a mystery shopping exercise
demonstrated that few sales staff
understood or even had knowledge of
the SAP rating.
There is a legal obligation to give details
of the SAP rating to home owners and
you should consider your approach to
complying with the requirement. It is
often not practicable to display the
energy rating notice prior to occupation
of dwellings; in these cases you should
ensure that home owners are given this
information, perhaps as part of an
information pack or via a separate
communication.
Whether you use joist hangers or
build joists in, follow the above
guidance carefully. Consider the
implications of building joists
into separating walls for buildings
that need to comply with the
new requirements of Part E.
inform purchasers regarding energy
efficiency and the relevance of the
energy ratings for new homes. NHBC
Training Services will be happy to advise
further on Building Regulation training
courses available for sales and other
non-technical staff (contact Dave Towell
on 0870 241 4323 or email
[email protected]).
ACTION:
Place an easy to read notice of the
energy rating in unsold properties members of the House Builders
Federation can obtain a blank notice
from the HBF website at www.hbf.co.uk
When selling off-plan, provide
information on the SAP rating to
purchasers and ensure sales staff
are briefed about the benefits of
energy efficient homes.
Sales staff are ideally positioned to help
STANDARDS EXTRA
7
Questions & Answers
Below are the answers to a selection of questions we have recently been asked in NHBC Technical.
Handrails to staircases - tapered treads/winders
Question
Where stairs have tapered treads/winders, is a handrail required to the outside of the stairs?
Answer
A safe handhold is required for the full rise of any stairs with a total rise greater than 600 mm.
A suitable newel post can provide a safe handhold.
Single newel
Double newels
In England, Wales and Scotland where the stairs have
between one and four tapered treads/winders and
the newel provides a safe handhold, a handrail is not
required to the outside of the stairs.
In England, Wales and Scotland where the newels
provide a safe handhold, a handrail is not required
to the outside of the stairs.
Additional
handrail
not needed
Additional
handrail
needed
Additional
handrail
not needed
In Northern Ireland a handrail should be fitted to the outside of the stairs in both cases (required by building regulations).
In all cases where a handrail is needed to the outside of the stairs, it should be continuous for the whole rise to
avoid the need to change hands.
Use of nail guns for fixing trussed rafter bracing
Question
The guage of the nails used in nail guns is less than that quoted in the NHBC Standards. Can these be used for fixing
trussed rafter bracing?
Answer
NHBC Standards clause 7.2 D5(c) asks for 3.35mm (10 guage) x 65mm long galvanised nails – nails used in nail guns are
not available to this specification.
When nail guns are used, 3.1mm x 75mm long annular ring-shank nails are acceptable. They do not need to be galvanised.
Roofing underlay at eaves level
Question
Can plastic dpc material be used at eaves level in place of a type 5U felt?
Answer
NHBC Standards clause 7.2 M5 (d) says ‘where the underlay is exposed, such as at the eaves, a type 5U felt
should be used’.
Plastic dpc materials are generally not designed for use in permanently exposed locations. The lack of suitable
ultra-violet stabilisers could lead to an early breakdown of these materials in such locations.
Dpc materials should not be used for the exposed sections of roof underlay at the eaves. A type 5U felt should be used
for the exposed sections unless the underlay used for the remainder of the roof has a third party assessment confirming
its suitability for use at exposed eaves. Alternatively, a proprietary eaves guard which extends under the unexposed
section of the roofing underlay and provides adequate projection into the guttering may be used.
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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HB1844 04/03