Background
• Australia has always had bushfires
• Their severity and frequency is increasing
• Urban sprawl pushes people to build in bushfireprone areas
• Qn: Should houses be built in bushfire-prone
areas? If so, what types of construction should be
permitted?
The Critical Issues
• Criteria for building in bushfire-prone areas:
– Bushfire-resistance
– Environmental sustainability
– Affordability
• These criteria often conflict
• Aims of study:
– Identify conflicts between criteria
– Provide guidance to builders by quantifying various
material combinations
Parameters that increase the
destructiveness of fires
1.
2.
3.
4.
5.
6.
7.
Temperature > 30 C
Wind > 20 km/h
Dry air (relative humidity < 30%)
Unstable upper atmosphere
Terrain
Vegetation
Building features
Parameters 1-4 are not controllable by individuals
My research focussed on Parameters 5-7
Building Features Evaluated
• Roof:
– Clay-tiled
– Concrete-tiled
– Steel
– Living
• Exterior Walls:
– Brick Veneer
– Double Brick
– Hollow Concrete
block
– Steel
– Rammed Earth
Assessment Criteria
1. Bushfire-resistance
• Non-combustible materials against AS39592009
2. Environmental sustainability
• Embodied Energy and Embodied Carbon
3. Affordability
• Construction costs
Hilly Terrain
1. Bushfire-resistance
–
–
–
Build on a flat block at base of a hill
Avoid building on north-facing slopes
Avoid downslopes under vegetation
2. Environmental-sustainability
–
–
Flat block at base of a hill has minimal environmental
impact
Avoiding north-facing slopes conflicts with passive solar
design, increasing emissions during the use phase
3. Costs
–
–
Flat block at base of a hill reduces construction costs
Avoiding north-facing slopes increases running costs
Vegetation
Forests - greatest risk; Grassland - lowest risk
1. Bushfire-resistance
– Maximise distance between vegetation and house
(Clearing allowed: Trees 10 m from house, ground fuel 30 m)
2. Environmental sustainability
– Avoiding or clearing trees foregoes benefits of passive solar shading
– Clearing destroys ecosystems, leads to erosion, salinity, invasive
weeds and raises CO2 levels
3. Costs
– Loss of shading may increase energy costs by 30%
– Tree clearing is costly
House shape
Simple, rectangular, single-storey
1. Bushfire-resistance
– Fewer ember entry points
– Less surface exposed to radiant heat and flames
2. Environmental sustainability
– Rectangular shape assists cross-ventilation
– Cross-ventilation in a single-storey house is less effective
than double-storey
3. Costs
– Reduces architectural, construction and maintenance costs
Roofs
Simple, steep, sloping away from house
(Boundary: Materials, support system, battens and hardware)
Exterior walls
(Boundary: Materials, binding medium, support system; battens,
exterior render, interior plaster and paint where required)
Roof and exterior wall combinations
2
1
Eighteen Government-funded designs
Roof
•
•
•
•
Many ember entry points
Embers roll down onto the house
Timber frame
Metal cladding
Exterior Walls
• Many ember entry points
• Timber frame
• Compressed fibre cement cladding
Other
• Option of concrete stumps with
timber joists flooring
• Compressed fibre cement sheet
decking
Eighteen Government-funded designs
Design
• 65% of house is protected from weather
• Entertainment area reduces spread of fires
Roof
• Living roof does not fuel fire
Exterior Walls
• Hardwood
Building Standard Regulations
• AS3959-2009 modelled on temperatures 400 C lower
than Black Saturday’s
• Black Saturday’s temperature was 127 C lower than
possible in bushfires
• Test standard AS1530.8.2 does not include effects of wind
Conclusions
• Houses built to current building standards are unlikely to
survive fires of even lower intensity than Black Saturday
• AS3959-2009 and AS1530.8.2 standards are inadequate
• A living roof on rammed earth walls is the best
combination for fire-protection, low embodied carbon, at
moderate costs
Conclusions cont.
• Second best choice is a living roof on double brick walls,
at 21% less cost
• Fire-risk, embodied carbon and costs can be reduced by
building:
– On a flat block at the base of a slope, away from
dense forest and downslopes under vegetation
– A rectangular single-storey house
Questions