The majority of houses destroyed in bushfires survive the passage of the fire front. Wind-borne burning debris causes the most damage.
Direct flame contact and radiant heat can ignite and destroy houses, but they are only significant during the few minutes it takes the fire front to pass.
But houses can be hit by showers of burning debris for hours before and after the fire front passes. Strong winds are a danger, as was evident during the January 18 firestorm.
These facts need to be considered when designing a house.
Burning debris carried on the high winds can ignite houses in several ways. It can pile up against timber used at or near ground level such as stumps, posts, sub-floor enclosures, steps, door and window frames. It can accumulate on timber used for decks, verandahs and pergolas. It can lodge in gaps in and around timber and other combustible materials used for exterior wall cladding, and door and window frames. And burning debris can enter the house through windows broken by heat or flying debris.
The steeper the slope, the faster the fire travels. In the ACT the majority of winds driving bushfires are westerlies, or north westerlies.
The main strategy in protecting a house is to prevent external ignitions, and stop the fire getting inside. Design and material choices can help.
A simple roof design can lessen the risk of bushfire damage and is the easiest to protect. Roofs with intersecting planes and valleys may form dead air pockets where debris and embers collect. Steel roofing is the safest. However, gaps must be sealed or protected to stop ember or spark penetration. Roofing must withstand strong winds and resist dislodgement to prevent the entry of embers.
Roof valleys, dormer windows and skylights provide opportunities for embers to lodge on a roof.
Tiled roofs should be fully sarked and fixed.
Eliminate gutters. If fitted, use metal gutter guards to stop leaves collecting. Wood shingles and bituminous materials are most vulnerable.
Concrete slab-on-ground floors stop embers getting under the house. Where unable to build flat on the ground, use a suspended concrete floor with non flammable support. Build into a slope rather than support the house over the slope. Any space between raised floors and the ground should be enclosed and sealed. Any timber used in raised floors and flooring supports should be treated to fire resistant standard.
Brick, stone and block are the best materials for walls.
Timber framed walls should incorporate sarking behind the external cladding and/or fire resistant insulation material. Where timber cladding on framing is used, fire retardant treated timber should be selected.
Windows are one usually fail before the building ignites, providing a direct entry point for embers. Smaller windows are less likely to fracture and collapse. They are preferable to large windows facing the fire. Toughened glass, laminated safety glass and other new glass technology can help. Metal fly screens help. Combustible window frame materials should be avoided.
External security doors with spark proof metal screens that cover the entire door space should be fitted. Also, it is best to fit draft seals to all external doors, and to protect around garage roller doors.
Vents, eaves and soffits should be protected with spark guards. Open eaves that expose rafters are vulnerable – eaves should be enclosed and the gaps between rafters closed. Soffit lining closes the eaves – a horizontal soffit lining of fire retardant material should be used, and vents should not be put in the soffit lining.
Chimneys, roof ventilators and wall vents designed to prevent the accumulation of water vapour should be screened with fine metal mesh to prevent ember entry.
Decks and balconies should be built with fire resistant material as they can be a key ember attack point. Avoid light decorative timber work such as lattice and trellis panels. Where timber is used it should be dense hardwood smooth sawn.
For further information contact the ACT Planning and Land Authority on
6207 1923 or visit www.actpla.act.gov.au