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ANHBC’s technical newsletterApril 2004 Issue 29

This edition includes:

Batten down the hatchesCheck the minimum fixing requirements for your roof coverings

Don’t cut corners with gas safetyThe HSE reports a growth in the number of unregistered installers carrying out gas work

Nothing like a cold shower to get the blood boilingMake sure your heating and shower systems can cope with demand

Getting out and aboutNHBC introduces changes to Standards Chapter 9.2 “Drives, paths and landscaping”

Flat roofs and balconies – changes to Chapter 7.1An accident triggers changes to the acceptable use of timber in balcony construction

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2 Giving timber the treatment

3 I-joists and metal web joists– whatever next?

3 Batten down the hatches

4 NHBC training courses

5 Put your foot down – insiston the right grade ofexpanded polystyrene (EPS)for flooring applications

6 Don’t cut corners with gas safety

7 Nothing like a cold showerto get the blood boiling

8 Getting out and about

9 Basements – below the surface

10 Flat roofs and balconies –changes to Chapter 7.1

11 Questions and answers

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Giving timber the treatment

NHBC Standards only require timber to be preservative treated where it is necessary fordurability. Items such as trussed rafters and floor joists generally do not require treatment,whereas studs for timber framed walls do.

Chapter 2.3 “Timber preservation (natural solid timber)” may not be a Chapter which youmake reference to on a regular basis. However, it does provide information on those elementsof the home which do need treatment.

British and European Standards for treatments have changed and these changes have nowbeen included in Chapter 2.3. NHBC has taken the opportunity to simplify the chapter andnow it contains only two tables, from which it is possible to establish whether treatment isnecessary or is not required because the species is naturally durable.

If in the past you have used copper, chrome, arsenic (CCA) treatments in the construction ofhomes, then please note this is proscribed by a European Directive from 1 July 2004. Youshould therefore consult with your timber supplier or treater to find a suitable alternative.

ACTION:The new version of Chapter 2.3 can be used immediately but is applicable for all homesfrom 1 July 2004. CCA treated timber should not be used at all in the construction ofhomes from that date.

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NHBC Standards Chapter 6.4 “Timber and concrete upperfloors”, originally written in 1992 when solid timber joistswere the norm, has been revised to reflect the rise in thepopularity of I-joists and metal web joists.

There have been issues with certain proprietary joists usedover long spans, typically over 4.5m, where a floor may feel‘bouncy’ when walked on. Although there is no question ofthe floor being structurally inadequate, it can feeluncomfortable to a homeowner and not what they expectedin their new home.

After discussions with representatives of the joistmanufacturers, NHBC has decided to impose a limitingdeflection of 0.003 times the span with a maximum of 12mmif strutting is not provided or 14mm if it is. It is likely that

joist manufacturers will opt for the 12mm deflection limitwhich will avoid builders having to install strutting.

Although this is in excess of what the British Standarddesign code currently says, NHBC has made representationsto the BSI Committee responsible and is hopeful that thecode will be amended with the same requirement.

In Standards Extra 28 we advised that the British Standard(BS 5534) for tiling and slating had been revised regardingthe fixings to roof coverings and also batten sizes.

Both items have now been incorporated into Chapter 7.2“Pitched roofs” and the changes can be found in the update pack.

Builders should familiarise themselves with the newminimum fixing requirements and check with the tilemanufacturer what, if any, additional fixings are necessary.Don’t forget that all ridge tiles need to be mechanically fixedfor at least 900mm from the end and either side of a partywall. Tiling batten sizes are either 25 x 38mm or 25 x 50mmdepending on the tile design - 19 x 38mm battens are nolonger suitable (See Appendix 7.2-F).

Batten down the hatches

I-joists and metal web joists– whatever next?

ACTION:

Ensure you are familiar with the new Chapter 6.4 if youuse or intend to use I-joists or metal web joists. Adviseyour designer and buyer of the new deflection limits inClause 6.4 – D4. These will apply to all homescommenced on or after 1 July 2004.

ACTION:

Check that you are using the correct number of fixingsfor coverings and mechanical fixings for ridges.

Ensure tiling battens are increased in size where appropriate.

I-joists and metal web joists– whatever next?

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NHBC Training CoursesIn Standards Extra 28 we advised about two ‘Building neartrees’ courses;

‘Building near trees for Site Managers’ looks at on-siteconstruction and workmanship best practice. It is suitablefor construction managers and contracts managers aswell as site managers.

‘Building near trees for Engineers and Designers’looks at design and geotechnical issues such asshrinkage potential of soil types and the effects ofdesiccation and heave. It is suitable for technicalmanagers and building control professionals in additionto engineers and designers.

Another course being launched is designed to get yourEcoHomes scores into shape well before the assessmentstage. ‘How to Improve Your EcoHomes Scores’ is forprocurers and users of the EcoHomes environmental ratingsscheme. So whether you are a designer, architect,consultant, house builder, developer or work for a housingassociation, this one-day workshop will help you:

gain a clear understanding of the scheme and its objectives

undertake a ratings prediction checklist

understand how specifications and site designs can beimproved to optimise scores

know what measures are required in a development toachieve optimal scores.

NHBC has a large team of EcoHomes assessors and, havingsponsored and been involved in the development of theEcoHomes scheme with the Building Research Establishment(BRE), we have a specialist and unique knowledge, offeringyou unparalleled technical expertise.

Finally health and safety is an ongoing concern within theindustry and although courses in this area are easy enoughto come by, this ‘IOSH Directing Safely’ course is run byprofessionals familiar with the house-building environmentand the problems faced when planning, organising andmanaging sites.

This one-day training workshop for senior levels ofmanagement within small to medium sized enterprises, isbeneficial when considering your overall health and safety strategy or when implementing and amending yourexisting system.

All of these courses are being run regionally and on variousdates until June 2004. For further information or to requesta booking form please contact: Leah O’Donnell on 01908 22 3573 or at lodonnell@nhbc.co.uk.

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To the untrained eye, one sheet of polystyrene may look likeanother, but this is far from the case and manufacturers ofEPS have invested heavily to ensure that their productscomply with appropriate standards. NHBC Standards requireboards to be manufactured to BS EN 13163, grade EPS 70 (seelatest updates).

Polystyrene boards used in flooring should be packed andlabelled with this Standard and grade. Floor systems,incorporating polystyrene, should be assessed byorganisations such as BBA or BRE Certification. Packaginglabels should clearly identify the product. Packs that are notmarked should be rejected as they could result in the floor

becoming defective over a period of time and potentiallylead to severe inconvenience for the homeowner and acostly repair for the builder.

Insist on the right grade of expandedpolystyrene (EPS) for flooring applications

ACTION:

Ensure that any EPS boards used for flooring are froma reputable manufacturer with clear markings or labellingto indicate what standards they meet and their grade.Don’t use unmarked products that could be unsuitableand lead to future problems.

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Don’t cut corners with gas safetyThe Health and Safety Executive (HSE) believes there hasbeen an increase in the number of builders using non-registered installers to fit gas appliances and pipework.Although such installations are usually thentested/commissioned by a member of CORGI, they haveasked us to remind registered builders that this method ofworking is illegal.

On one occasion the CORGI member was a friend of theinstaller and wrongly assumed that the installer had carriedout all the necessary safety checks. He issued a Gas SafetyCertificate but it turned out the installation did have faults.Both were successfully prosecuted.

The Gas Safety (Installation and Use) Regulations 1998 applyto all house building and renovation work, and require thatany company (or self employed person) which does gas fittingwork must be CORGI registered. The way in which the lawdefines both ‘gas fitting’ and ‘work’ means that there is verylittle that can be done by a non-registered installer withoutbreaking the law.

Those who are not registered can do work on the ‘wet’ side,i.e. installing radiators, and water pipes, as this is not within

the definition of gas work.Similarly any electrical workwhich has no potential to affectgas safety may be carried out bythose who are not registered, butany which could affect gas safety (e.g. electrical work on gascontrol systems) must only be done by a registered personwith the required competence.

There is a duty on house builders to take all reasonablesteps to ensure that installers doing gas work on their behalfare CORGI registered. Installers registered with CORGI,should carry a card with them, which has their photographon the front. It also lists their competencies on the back. Ifsomeone is doing work that demands a competence that isnot on the list, they will be working outside CORGI’s rules.

ACTION:Only use CORGI registered installers, even if thepipework and appliances they are fitting are not yetconnected to the gas supply.

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Nothing like a cold showerto get the blood boilingImagine the delight of moving into your dream home - a largefour-bedroom family house with one en-suite and a familybathroom, set in a small picturesque village overlooking the countryside.

On the first morning you use the shower in the en-suite forthe first time. You set the temperature, put shampoo in yourhair. As you are relaxing under the warm spray, someonefills the kettle downstairs to make a cup of tea. Your showerslows to a trickle and then the water begins to scald – theshampoo begins to sting your eyes. The pressure thenreturns, but this time the water is freezing cold. You’re nota happy homeowner – but at least it happened to you andnot one of your children.

Sadly there is nothing fictitious about this story or whathappened next…

The builder had fitted a gas-fired combination boilerconnected directly to the mains water to provide hot waterthroughout the house. In addition two electric shower units had been fitted. There was no stored hot water orcold water.

Recognising that there was a problem the builder offered to replace the shower units with ones with thermostatictemperature controls. He also suggested that thehomeowner reported the water pressure problem to thelocal water authority.

The new showers were fitted but now when a tap was turnedon elsewhere in the house, they cut out completely (a safetyfeature) and had to be re-set. To make things worse, when abath was being run, the radiators throughout the house wentcold! Clearly the boiler was not capable of providing heatingand hot water at the same time.

The local water authority said that the builder should havefound out what the mains water pressure was and installedthe correct sized supply pipe to allow for the likely waterdemand in the home. They said the builder should havedesigned the system to work with the pressure available.

Next the boiler manufacturer became involved and said theboiler fitted was ‘suitable for a 2-bedroom starter home’ –totally unsuitable for a 4-bedroom home with two bathrooms.

To resolve the problem:

A new 32mm mains supply had to be installed involvingdigging up the drive

The kitchen had to be dismantled to remove the existingboiler and replace it with a suitable model

Carpets and flooring upstairs had to be taken up toupgrade some of the pipework from 15mm to 22mm, allat considerable cost and great inconvenience.

Now, it is possible to run both showers and fill the kettle atthe same time whilst the radiators stay hot.

ACTION:

When designing heating systems:Note the requirements of the NHBC Standards:

— The heating system shall be adequate,

— The hot water system shall be adequate, and

— The cold water system shall be adequate.

For homes with more than one bathroom have thesystem designed. Don’t leave it to chance that thesystem will provide adequate performance.

When designing a heating and hot water system,make sure it takes into account what facilities andoutlets are likely to be used simultaneously.

Check with the local water authority the MINIMUMmains pressure available and take into account thedrop in pressure over long runs of pipework. Is theresufficient flow rate to meet the demands ofoccupants and appliances?

Make sure the supplies to showers arethermostatically controlled or have been designedto prevent users from being scalded. This is anNHBC requirement.

Images courtesy of Pegler Limited

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NHBC Standards are mainly concerned with how the home isconstructed. However, one Chapter - 9.2 “Drives, paths andlandscaping” - deals with issues such as:

access for vehicles and pedestrians to the home

access around the home

garden areas.

These ‘soft’ issues are just as important to homeowners asthings inside the home.

Over the last few months, NHBC has been reviewing Chapter9.2 in the light of experience of our own staff as well ashouse builders.

What is new for 2004?

Access for vehicles and pedestrians

The Standards for vehicular access have generally been shownto be robust. But in recent years, most builders have improvedtheir specifications for drives to prevent deterioration of thesurface caused by on-the-spot turning of cars with powersteering. The specifications given in Appendix 9.2-B shouldavoid this happening in future.

Cars now come in many guises from sports saloons to four-wheel drive vehicles. Cars with lower ground clearancecan ‘bottom-out’ where gradients suddenly change. To preventthis from happening, transition lengths have been introducedfor certain drives.

Many access roads are no longer adopted by the HighwaysAuthority, but it is important that they are constructed totake the weight of vehicles likely to use them. Private roadsand shared drives may typically be used by refuse lorriesand delivery vehicles as well as by the white van mandelivering the Screw-fix order!

NHBC has introduced specifications for the different situationsthat arise where the road or shared private drive is notgoing to be adopted.

Access around the home

Again, the existing requirements for paths around the homehave generally been shown to be robust. Homeowners expectto have access around the property, to get to the kitchen dooror to take the wheelie bin from the rear of the property tothe front gate.

Paths and any steps need to be safe and if necessary, guardedwhere the ground level adjacent to it could be a hazard.

Garden areas

Garden areas are now described as the land within theproperty boundary extending up to 20m from the habitablepart of the home. Therefore, except for houses with verylarge plots, the whole garden will come within the Standard.

Within the 20m garden area, the requirements are similar toexisting requirements, i.e. that it should be stable andreasonably accessible. For the large plot situation, builders

Getting out and about

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will not be responsible for paddocks and woodlands whichthey have not touched during the construction of the dwelling.

Waterlogging

The ‘3m area’ around the home where waterlogging shouldnot occur has been retained. It is important thathomeowners are able to access the refuse bin position,patio areas or the rotary clothesline facility, without gettingwet feet.

Ground immediately around the home can be compacted byconstruction traffic during building. This compaction willreduce the ability of the subsoil to absorb surface water.There is now guidance on how this compaction can be alleviated.

Topsoil

It is reasonable for homeowners to expect that the gardenarea will be suitable for cultivation even if they only intendto create a lawn.

Some sites may have little existing topsoil and brownfieldsites may be devoid of topsoil altogether. The opportunityhas been taken to require 100mm of topsoil to all gardenareas. This has been made possible now that garden areashave been defined. Any topsoil needs to be clean and freefrom contaminants which could be a hazard.

Timber decking

The popularity of timber decking has been recognised andclauses giving a suitable specification have beenincorporated in the Chapter.

ACTION:

Familiarise yourself with Chapter 9.2 which becomesoperative for dwellings commenced on or after 1 July 2004.

ACTION:

If you are constructing basements, ensure that thedesign is appropriate for the conditions and thatrelevant personnel on site are aware of the need forgood workmanship in the construction.

Copies of the Basements for dwellings ApprovedDocument are available to Standards Extra readers at adiscounted price of £13.75 (normal price £17.25) fromBritish Cement Association, Century House, TelfordAvenue, Crowthorne, Berks, RG45 6YS (Tel: 01344762676) quoting reference 48.062 NHBC/SE. This offerends on 31 May 2004.

Basements can be difficult to make waterproof. Once wateror dampness becomes evident on the inside, it is difficult tostop – a small hole in the tanking or a badly performing sealcan allow a lot of water in! Basements are relatively small innumber but cost NHBC a sizeable amount in claims repaircosts. It is therefore necessary to pay proper attention totheir design and construction if they are to be successful.

NHBC has been looking at improving the Standards in respectof basement construction. We have chosen to makereference to the ‘Basements for dwellings’ ApprovedDocument. Although it relates specifically to the Englandand Wales Building Regulations, the guidance is equallyappropriate for all parts of the UK.

Basements which are used for habitable purposes clearlyneed to be dry but that is not quite so critical if thebasement is used only for underground car parking. TheStandards explain what constructions are appropriate fordifferent situations. It should be noted that in general wewon’t accept internal tanking systems in future for new-buildtanked protection where this is the sole water resisting measure.

If the homes you build have basements, familiarise yourselfwith the changes to Chapter 5.1 and obtain a copy of theBasements Approved Document even if you are in Scotlandor Northern Ireland.

Basements – below the surface

Flat roofs and balconies –Changes to Chapter 7.1

ACTION:

If you construct balconies or have designs thatincorporate flat roofs, then familiarise yourself withthe latest changes to Chapter 7.1 “Flat roofs andbalconies”. The changes are effective from 1 July 2004.

Balconies

An accident, which could have resulted in serious injury orworse, has caused NHBC to look at requirements for balcony constructions.

Balconies do form part of the structure and should thereforebe durable. Often they are exposed to wind-driven rain andoften the difficulty of maintenance can mean it’s not done.

All materials are subject to durability issues but timber isparticularly prone to problems if it is not naturally durable,has not been treated properly, detailing is poor orworkmanship is questionable.

The amendments to Chapter 7.1 “Flat roofs and balconies”limit the use of timber in the construction of balconies. Infuture timber should not be used for:

Gallows brackets supporting a balcony

Posts or columns supporting a balcony

Guard rails or their supports.

However, timber will still be permitted for secondaryelements such as:

Open balcony constructions with timber decking. Thedecking may be supported on solid timber treated joistswhich in turn are supported by materials or componentsother than timber

Balustrading.

Flat roofs

How flat is a flat roof? For many years, NHBC Standardshave recommended a 1:40 fall to flat roofs.

This figure has been amended to take account of BritishStandards and many BBA Certificates which refer to afinished fall of 1:80. Particularly with flat roofs using timberjoists, the design fall should be 1:40 to be reasonably certainthat the finished fall will be at least 1:80.

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Questions & AnswersBelow are the answers to a selection of questions we have recently been asked in NHBC Technical.

Marking of safety glass

QuestionIs it necessary for each pane of safety glass to be marked or is it sufficient for the large sheet to be marked oncebefore it is cut?

AnswerBS 6206 ‘Impact performance requirements for flat safety glass and safety plastics for use in buildings’ requireseach pane to be marked permanently in a position visible after installation. The mark should show:

a) an identifiable name or trademark.

b) the type of material.

c) “BS 6206”.

d) The classification relating to impact test behaviour e.g. (A, B or C).

Without a mark on each pane, it would not be possible to confirm that each pane is actually of safety glass orknow its performance classification.

QuestionCan fibre reinforced floor screeds be used in place of wire mesh reinforced screeds when used over resilient insulation materials?

AnswerNHBC Standards clauses 8.3 - D3(j) and 8.3 - S2(g) refer to the use of wire mesh reinforcement in cement : sandscreeds over resilient insulation materials.

Reinforcement is required to control drying shrinkage in the screed by transferring shrinkage to the edges of thefloor to avoid cracking within the floor area.

Fibre reinforced cement : sand screeds may be used in place of wire reinforced screeds over resilient slabs or quilts. The fibres should have independent assessment for use in the control of shrinkage in screeds or concrete.

QuestionIs a door opening limitation device required where the door has a glazed panel which enables the occupant to identifythe caller?

AnswerNHBC Standards clause 6.7 - D4 provides for a caller to be both seen and spoken to before the door is fully opened.

A door opening limitation device is required in this situation to enable the occupant to speak to the caller.

Fibre reinforced screeds

Door limitation devices

NHBC TechnicalBuildmark House, Chiltern Avenue, Amersham, Bucks HP6 5APTel: 01494 735859 Fax: 01494 735717 Email: technical@nhbc.co.uk www.nhbc.co.uk

HB1954 04/0412

Questions & AnswersBelow are the answers to a selection of questions we have recently been asked in NHBC Technical.

In-situ concrete suspended ground floors

Questionsa) Where an in-situ concrete suspended ground floor is laid on a dpm, should the dpm be dressed up the side and over the

slab (sketch A) or cut off at the bottom (sketch B)?

b) Is it necessary to provide a dpc between the top of the slab and the masonry inner leaf?

AnswerCutting the dpm off would leave the slab exposed to water, which may collect at the bottom of the cavity. This couldcause dampness in the home.

a) The dpm should be dressed up the side and over the slab (see sketch A). The dpm should be pulled taut so that itdoes not bulge and tend to form a ledge on which mortar could build up.

b) A separate dpc is not required between the top of the slab and the inner leaf masonry where the dpm has beendressed up the side and over the slab.

QuestionIs toothing of brickwork acceptable when building the walls up to previously raised corners?

AnswerBS 8000 Part 3 ‘Workmanship on building sites – Code of practice for masonry’ refers to raking back corners whichavoids the need to tooth.

It is difficult to make solid, strong and rain resistant joints and achieve good appearance when building onto toothedbrickwork.

Where new walls are being constructed corners should be raked back to avoid toothing.

Sketch A Sketch B

QuestionAre log retaining walls acceptable for the support of gardens and paved areas?

AnswerInadequately designed or constructed retaining walls may cause movement of the garden or paved areas. Paved areasmay suffer damage.

Untreated logs may lack durability.

Where log retaining walls give support to paved areas or retain more than 1.0m of soil, they should be a preservative treatedproprietary interlocking system, designed and installed in accordance with the manufacturer’s recommendations.

NB.Log or timber retaining walls are not acceptable for use where the wall serves a critical function such as providingsupport to the structure of buildings or drainage. See latest edition of Chapter 9.2 ‘Drives, paths and landscaping’.

Toothing of brickwork

Log retaining walls