Background

• Australia has always had bushfires• Their severity and frequency is increasing• Urban sprawl pushes people to build in bushfire-

prone 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. Temperature > 30 C2. Wind > 20 km/h3. Dry air (relative humidity < 30%)4. Unstable upper atmosphere5. Terrain6. Vegetation7. 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 AS3959-

2009

2. Environmental sustainability• Embodied Energy and Embodied Carbon

3. Affordability• Construction costs

Hilly Terrain1. 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

VegetationForests - 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 shapeSimple, 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

RoofsSimple, 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

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

Eighteen Government-funded designs

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