building in bushfire-prone areas -...

Building in bushfire-proneareas

Updated 2009 Edition

While the contents of this publication arebelieved to be accurate and complete, theinformation given is intended for generalguidance and does not replace the services of professional advisers on specific projects. Local or state regulations may require variation fromthepractices and recommendationscontained in this publication. ThinkBrick Australia disclaims any liabilitywhatsoever regarding the contents ofthis publication.

This publication, its contents and format are copyright © July 2009 of Think Brick Australia and may not be reproduced, copied or stored in anymedium without prior, written authorisation from Think BrickAustralia. ABN 30 003 873 309.

Cover: As well as utilising the naturalfire protection afforded by brickwork,this attractive house has double-glazedwindows to provide additional heatresistance to these vulnerable buildingelements, brass mesh shielded vents toexclude sparks, valley and gutter guardsto reject leaves, and stainless steel fittings to the decking. Photography:Roger du Buisson.

PO Box 275, St. Leonards NSW 1590

Suite 7.01, Level 7, 154 Pacific Highway St. Leonards NSW 2065 Australia

Telephone +61 2 8448 5500 Fax +61 2 9411 3801

ABN 30003873309

www.thinkbrick.com.au

3 The bushfire hazard

4 The nature of bushfires

7 Building in brick in bushfire prone areas

8 How much bushfire protection?

8 Planning and design

9 Construction Details

17 Further reading

17 References

Photo: Bushfire CRC

This publication discusses thedetailing and construction of brickhousing – Australia’s most popularform of construction – to cover thespecific provisions and practicesthat will increase housing safety inbushfire-prone areas. Although thefocus is on brick housing, many ofthe suggestions may also be applicable to other constructionforms.

In the temperate areas ofAustralia, wet winter and spring weather is often followed by a hotdry summer. Vigorous growth ofvegetation, some of which containflammable oils, results in a build-up of fuel load. The actions of hot winds from the arid interiorand very low levels of humiditythen create potentially dangerousfire conditions over large areas ofthe most densely-populated partof the continent.

In this situation summer bushfiresare a fact of Australian life. Fromtime to time fires occur that are ofsuch intensity and cause such lossof life and property as to generatewidespread public interest inmeasures that could be taken toreduce both the hazard and thedamage.

Research and investigation aftermajor fires in New South Wales,Victoria, South Australia andTasmania between 1945 and 1985

1,2

has resulted in a considerable bodyof information on the problemsposed by bushfires. One significantoutcome was the 1991 publicationof Australian Standard AS 3959Construction of Buildings in Bushfire-Prone Areas. This standard wasrevised in 1999 and 2009 and isbeing further reviewed followingthe devastating fires that occurredin Victoria in February 2009.

The provision of refuges with ahigher level of protection againstbushfire attack is being consideredas part of this on-going review.A number of other publications arealso available on the nature ofbushfires, fire prevention, fire fighting, and site selection

3and

management4.

This publication does not detailthese matters but concentrates on planning, design, materials selection and building details forbrick houses in bushfire-proneareas.

The general philosophy is thatgood design, careful constructionand sensible maintenance will produce a house that not only provides protection from normalweather and storms, but also frombushfires. Any extra costs arisingfrom measures taken to improvethe chances of a brick house (andits contents) surviving a bushfireare small when compared to thecost of replacing a house and contents that have been destroyed,even after allowing for an insurance payout.

This is consistent with studies thathave shown that houses that areattended during bushfires have significantly better chances of survival than vacant houses,

5, 6, 7

and that fewer people die in houses than in the open or in vehicles.

The bushfire hazard

Building in bushfire-prone areas / 3

“Good design, careful

construction and sensible

maintenance will produce

a house that not only

provides protection from

normal weather and

storms, but also from

bushfires”

Contributing factorsThe intensity of a bushfire willdepend on:

• The fuel load, its type and distribution;

• Moisture content of the fuel;

• Wind velocity and its direction;

• Topography; and

• Ambient temperature andhumidity.

Of these factors, fuel load is themost important, particularlyground-level fuel such as leaves,grass and twigs up to 6 mm diameter. In practical terms fire intensity effectively increases asthe square of the fuel load: doubling the load quadruples theintensity and halving the loadquarters the intensity.

Modes of attackThere are four “weapons” withwhich bushfires attack buildings.The first, burning brands andsparks, often have their effectbefore and after the passage of the fire-front. “Burning embers blownahead of a bushfire are the mostcommon cause of bushfires igniting houses and causing damage,” states an NSW Rural FireService report

8. The other three are

associated with the passage of thefire-front. Although they operatefor only a short period they mayopen serious breaches in the outerfabric of the building, throughwhich flame, sparks and smokemay enter.

Burning brands and sparksFor some time, perhaps up to thirty minutes before the fire frontarrives, a building can be showeredwith sparks and burning fragments carried up in the plumeof hot gases above the fire. In thestrong winds that accompanybushfires these burning brandsand sparks can travel considerabledistances but are at their mostsevere up to 500 metres from thefire-front, where direct spotting byburning bark or twigs can occur.

Attack by burning fragments andembers can continue after the fire-front has passed, so the periodof exposure to this source of danger is to be measured in hoursrather than minutes. There arenumerous reports of unattendedhouses burning down severalhours after the fire-front haspassed as a result of earlier entryof burning materials.

A house should be located beyond the potential direct spotting distance of Eucalypt forests

The nature of bushfires

4 / Building in bushfire-prone areas

High wind speedsLarge fires generate their ownwind and speeds may exceed 150 km/h (42 m/s) near the fire-front. Such speeds will not bemaintained for very long, but thepotential for serious damage isgreat enough to warrant specialattention to roof attachment andthe protection of windows, both of which are vulnerable to wind damage.

High levels of radiationAt the fire-front, the flames couldemit radiation levels of 150 kW/m

2.

This may rise to more than 200kW/m

2for short periods if the fuel

load is high. The intensity of radiation received at the buildingdepends upon the distance to and the size of the flame front.AS 3959–2009 provides construction requirements corresponding to radiation exposure thresholds (BushfireAttack Level or BAL) ranging fromlow to 40 kW/m2. The flame zoneis considered to have exposure inexcess of 40 kW/m2. Radiationintensity could readily reach levelssufficient to fracture glass andignite timber in the absence ofsparks or brands but the combination of high levels of radiation, burning brands andsparks constitutes a major weapon of the bushfire attack.

Flame contactDirect flame contact with exteriorparts of the building, particularlyprojections such as eaves,verandahs or pergolas, may occurif there are fuel sources such asvegetation or wood heaps close tothe house.


Photo: Country Fire Authority

During manufacture clay bricks arefired at temperatures as high as orhigher than anything reached evenat the heart of a large bushfire andare in the kiln for much longerthan it takes for the front of abushfire to pass a given point. Itfollows that the bricks themselvesare incombustible.

The CSIRO survey of houses in theOtway Ranges in Victoria thatwere exposed to the 1983 AshWednesday fires

6, showed that

masonry walls significantly reducethe chances of a house beingdestroyed by bushfire. The use ofbricks confers a number of otherplanning and design advantages.It has been shown in the Manual 3The Full Brick Manual (see FurtherReading), that full-brick construction is thermally comfortable and energy efficient,while at the same time minimisingnoise problems and maintenancecosts.

Flush Weatherstruck Ironed Raked

Building in brick in bushfire prone areas

So far as brickwork constructionmethods are concerned, the detailsgiven in Manual 9 Detailing of ClayMasonry Walls and Manual 10Construction Guidelines for ClayMasonry should be followed (see Further Reading). For bushfire-prone areas it is advisable not torake back the mortar joints, but touse a flush ironed or weatherstruckjoint. These joint types are lessaffected by heat than raked jointsand more easily re-pointed shouldextreme exposure to heat causedeterioration of the mortar surface.

It is accepted that bricks and themortar are not combustible andthat well-designed and constructedbrick walls have an inherently highlevel of fire resistance. Howevermany other structural components(and furnishings) may burn and lead to the destruction ofeverything except the brick walls.The rest of this publication givesdetails of design principles and construction practices that improvethe survival chances of a brick houseexposed to bushfires.

“Masonry walls

significantly reduce the

chances of a house being

destroyed by bushfire”


There are many ways, both activeand passive, for improving the survival prospects of houses inbushfire prone areas. Which onesare used and how much money isspent on them will depend on individual judgement of the hazardlevel that exists and the level of riskthe property owner is prepared toaccept (after laying off some of therisk by means of fire insurance).

In making such a judgement itshould be recognised that althoughinsurance can offset some (butrarely all) of the financial loss associated with the destruction of

a home in a bushfire, it cannot compensate for the disruption andthe trauma of losing personal possessions and being left homeless.

It is not possible to give a simple setof answers that apply to all houses.This publication describes the designand construction procedures that are appropriate tothe general level of bushfire hazard,and to indicate the level of protectionwhich would be adopted by a prudent owner who is prepared tomaintain an average level of propertymanagement.

If the local building authorityrequires it, the methodology of AS 3959 must be followed. Thisinvolves a detailed assessment of thebuilding site for Bushfire Attack Level(BAL) on the basis of climate, slope ofground and vegetation. AS 3959 setsout construction requirements foreach of the six levels of BAL. It shouldbe noted that the building coderequirements of AS 3959 are a minimum requirement, which balances safety and cost, and thatbuilding owners can specify a higherlevel of protection.

How much bushfire protection?

Questions related to site selectionand layout are well treated inother publications

3and will not be

repeated here. So far as the shapeand layout of the house are concerned, a simple rectangularfloor plan, covered by a low-pitched roof having no re-entrantcorners or valleys, is to be preferred. For reasons related tothermal performance it is preferable to have the long axis ofthe house running east-west if possible.

Planning and design

Photo: Bushfire CRC


General

Field evidence shows that themost vulnerable parts of a buildingare:

• The ground/wall junction wherecombustible material can accumulate. Timber posts,cladding and battens,particularly rough sawn timberand sections which may split orcrack, are all sources of risk;

• All penetrations of the walls.Doors, windows and vents allprovide gaps and openings wheresparks or embers may enter thebuilding; and

• Roof penetrations and eaves.Skylights, vents and the eavesprovide points-of-entry to theroof space, where the start of afire is most difficult to detect and fire fighting measures most difficult to apply.

Protection againstspark entry

There is ample evidence from fieldstudies that the entry of sparks orfire brands, particularly into roofspaces and under suspendedfloors, is the most frequent causeof ignition of houses by bushfire.Carefully fixed corrosion resistantsteel or bronze mesh with a maximum aperture size of 2 mmprovides cheap and effective protection for vents and openingsinto sub-floor, wall or roof spaces.Such screening should be considered the irreducible minimum fire protection for allhouses in bushfire-prone areas.

Construction Details


Footings and floors

If slab-on-ground construction forthe footings and floors is used:

• Sparks and embers cannot enterand the material of the floor isnon combustible;

• Flammable materials cannotaccumulate or be stored underthe house; and

• The profile of the building is lowered, reducing wind loadings.

On sloping sites, where practical,it is advantageous to set the houseinto the slope on a slab-on-groundsystem

3. On steep sites where a

suspended floor is necessary thestructural members (beams, piersand columns) supporting the floorshould be built from non-combustible materials. In somecases it might be advisable thatthe floor and its supports be offire-resistant construction.

Suspended timber floors at groundlevel require careful attention.They must be ventilated if they areto be durable, but all vents mustbe protected by mesh (as describedin the previous section). Thesescreened vents must be protectedfrom accidental damage or damage by animals and regularlyinspected to ensure that theyremain effective.

Balconies, decks andverandahs

Suspended timber decks are difficult to protect from fire. Inbushfire hazard areas high-sethouses and suspended featuressuch as timber balconies and decksare best avoided. If they are used,particular attention should be paidto reduction of fuel loads aroundthe building, to minimise radiationand flame contact.


External walls

CladdingBrick walling, of either cavity orveneer construction, is generally tobe preferred in bushfire areas sinceit withstands the heat radiationand significantly reduces the likelihood of fire starting along theotherwise vulnerable ground/walljunction. No special methods areneeded in building brick walls forhouses in fire-prone areas. As previously stated, flush, ironed orweather-struck joints should beused.

It is also advisable to modify theweep holes that are required todrain away water that might collect on the flashings. Open head or perpend joints formingweepholes should be protectedagainst entry of sparks by a pieceof folded corrosion resistant steelor bronze mesh with a maximumaperture size of 2 mm.

DoorsAll external doors should be of thesolid core or fire-resistant type,with no letter slots or other penetrations. Doors should fitclosely to frames all round and beprotected from sparks by an effectively latched tight-fittingscreen door on the outside. Againthe screen should be made fromcorrosion resistant steel or bronzemesh with a maximum aperturesize of 2 mm. Sliding doors shouldbe treated as windows.

Because of its non-flammable qualities, brickwork commonly survives even the fiercestbushfire. However it is essential to give proper attention to other details such as sparkentry and shielding or eliminating vulnerable materials such as glass and plastic.

Photo: Country Fire Authority


WindowsWindows are the most vulnerableparts of the external fabric ofbuildings and are the most difficultto protect. Large glass areasshould be screened from radiationto reduce the chance of glassbreakage and ignition of the building contents. The minimumacceptable protection is close fitting corrosion resistant steel orbronze mesh screening over bothfixed glass and opening sashes.If the window frames and sills areof wood the screens (and fire shutters if provided) should bemounted so as to prevent theaccumulation of burning debris inthe angle between the sill and the frame.

Effective screening is much moreeasily provided for sliding windowsthan for casement or awningtypes, where screens are usuallyfixed on the inside.

Protection of windows against flying debris should be. Hinged orremovable shutters of cellulose-cement or perforated metal probably offer the cheapest solution. They should be close fitting, particularly under theeaves, and designed so as not toprovide horizontal ledges whereburning material could lodge.If removable, shutters should bemanageable in high wind conditions by no more than twopeople, and should be able to bequickly and positively attached to the building.

Even where fire shutters are provided, corrosion-resistant steelor bronze mesh screening is essential. The fact that it is permanently installed means thatit is sure to be in place when it isneeded (which the shutters maynot) and the fact that screeningserves a dual purpose improves itschances of being properly maintained.

Vents in wallsBuilding regulations no longerrequire vents in walls in housesbecause it has been shown thatthey serve no useful purpose andare an unnecessary expense.In bushfire–prone areas imperfectlyscreened wall vents may admitsparks or embers and their useshould be avoided. Special purposevents, as in kitchens and bathrooms, must be well sealed tothe wall to prevent burning material lodging between the walland the unit and be fully screenedagainst spark entry.

RoofsPitch and formThe roof space is one of the mostfire-vulnerable parts of a house andthe larger it is the more it is at risk.From this it follows that there areadvantages in using a low-pitchroof of simple shape and withoutre-entrant angles which couldallow the build-up of debris.

Cathedral ceilings, which eliminatethe roof space, have advantages,but if there is a roof space,convenient access to it is essential.The ceiling access-hole shouldcommand a clear view of the wholeroof space and be large enough toallow easy passage of fire-fightingpersonnel and equipment.


AttachmentAlthough the wind speeds (up to150 km/h or 42 m/sec) associatedwith bushfires are below the lowerend of the cyclonic wind range (198 km/h or 55 m/sec), they arehigh enough to warrant careful attachment of the roof structureto the building. The requirementsof the local authority should be followed.

Cladding, sarking and insulationSteel sheeting or tiles are the mostsuitable cladding materials. Timbershakes or shingles must not beused. Of sheet materials,trough-profile sheeting is preferredbecause there are fewer end-stopping problems at eaves andridges than with corrugatedsheets. Tiled roofs must be fullysarked over the entire roof areaincluding the ridge.

All sarking and insulating materials used in bushfire-proneareas must have a FlammabilityIndex less than 5 when tested inaccordance with the AustralianStandard AS 1530.2 Tests forFlammability of Materials.

Skylights and rooflightsPlastic roof tiles or sheets and plastic domelights are all hazardous in fires and should beavoided in bushfire-prone areas.Thermoplastic sheet in a metalframe may be used for a skylightprovided the diffuser installed atceiling level is made of wired ortoughened glass in a metal frame.

Straight edge capping

Ensure that ridge is scribed to fit tightly over corrugationsclosing all openings

Flat sheet iron scribed toroof profile attatched tofascia to eliminate gaps


Gutters, barges and eavesIgnition of leaves in roof gutters,followed by the spread of the fireto the roof space, is widely recognised as a source of hazard inbushfires. Keeping roof gutters freefrom leaves requires significantand sustained effort on the part ofthe occupant and is better dealtwith in other ways. Two possibleapproaches are:

• Do without roof gutters and lay arubble drain around the house tocollect and remove stormwater;or

• Use a leaf-proof guttering system, together with non-flammable materials such as cellulose-cement board or metalfor barge boards and eaves linings.

The first alternative looks attractive on paper and works well if rain is not accompanied bywind,which is seldom the case.Also it is impractical where watercollection and storage is important.

If the second alternative is chosenit is essential to seal the eaveseffectively, especially at the eaveslining/wall junction, where significant positive pressure candevelop in windy conditions.Fibreglass or mineral fibre insulation batts on top of theeaves lining have been suggestedas a useful aid to sealing eaves.

The junction of the fascia or bargeboard and the roof claddingis also important. As previouslynoted, trough-profile roofingleaves fewer gaps than corrugatedprofiles and is therefore preferred.

Rubble drain


Fire retardent sarkingover purlins

Metal cladding

Metal cladding

Firestop formed byrockwool insulation

Fire retardent sarking

Rockwool insulation

Rockwool firestop

Exposed rafters

Plasterboard ceiling

Fibre cement

Fibre cement Fibre cement soffit

Masonryfirestop


Vents and openingsAll vents and openings in roofs andeaves must be spark proofed withcorrosion resistant steel or bronzemesh screening. Internal dampersare advisable in chimneys andclose-fitting spark-proof firescreens are essential over fireplace openings.

CeilingsWhether a house is ignited by abushfire, a pan of burning fat onthe stove or a cigarette butt in abed, the fire often spreads fromroom to room through the ceilingand the roof space. It is thereforeimportant to reduce the fire vulnerability of this part of thebuilding as much as possible.

As previously noted, a cathedralceiling, with glass or mineral fibreinsulation between the ceiling lining and roof cladding is a suitable form of construction forhouses in bushfire areas. For suspended ceilings a non-flammable lining material such asplasterboard is essential.Hardboard, softboard or plastic lining materials should never beused as they ignite easily and burnor melt rapidly in the hot conditions prevailing during bushfires. Ceilings should be insulated with glass or mineralfibre batts or loose fill insulationsof non-combustible materialpacked between and over rafters.

Damper


Further readingThe following publications are available from the Think Brick Australia in PDF (Adobe Acrobat) formats (www.thinkbrick.com.au).

Manual 9, Detailing of Clay Masonry Walls

Manual 10, Construction Guidelines for Clay Masonry

Contact Standards Australia (www.standards.com.au) for information on any Australian Standards referenced in this publication.

References1 E. Foster, Bushfire, A.H. & A.W.Reed, Sydney, 1979

2 R.H. Luke & A.E. McArthur,Bushfires in Australia, AGPS,Canberra 1978

3 Ministry for Planning and theEnvironment & Country FireAuthority, Design and SitingGuidelines: Bushfire Protection forRural Houses, Melbourne, 1980,1983

4 Country Fire Services &Department of Environment andPlanning, Building for BushfireSafety, Adelaide, undated

5 A.A. Wilson & I.S. Ferguson, Fightor Flee? – A Case Study of the MtMacedon Bushfire, AustralianForestry, 47,4,2 1984

6 How Bushfires Set Houses Alight:Lessons from Ash Wednesday,Ecos, 43, Autumn 1985

7 G. Caird Ramsey, N.A. McArthur &V.P. Dowling, A Survey of HousesInvolved in the Otway RangesFire, February 16 1983, unpublishedpaper, CSIRO Division of BuildingResearch, Melbourne 1985

8 NSW Rural Fire Service, Bush FireProtection for New and ExistingHouses in Urban Area,Sydney 1997

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