Bushfire protection starts with good design.
NASH’s view is that homes in bushfire-prone areas which:
• resist ignition
• limit propagation
• add no fuel to a fire
• remain strong and secure with minimum maintenance
represent better value for their owners and the community.
You should read this if you are:
• building or extending a home in a bushfire-prone area
• renovating or repairing a home and wish to make it more bushfire resistant.
Protecting assets through good design.
When building, extending, renovating or repairing a home which could be subject to bushfire attack, what should legally and sensibly be done to minimise the possibility of danger, damage or destruction?
Architectural design, detailing of key areas and selection of materials all play a part in the overall bushfire resistance of the finished dwelling. Also important are the development controls which determine the siting of new homes in relation to potential bushfire hazards and the level of community resources devoted to fuel reduction and firefighting - information on these topics is available in the wider community.
This information is intended as a guide to the principles of bushfire resistant design with an emphasis on the application of steel and similar fire-resistant materials. It is not intended to be a single source of expert information. References are provided to more detailed information on specific topics. If you live in an area where bushfires are common, your local state and community fire authorities will be able to assist you with bushfire preparedness.
The bush came first, and the built environment in Australia has encroached on bushland as communities expand to house and employ more people on affordable land. Inevitably, at the urban fringe a certain percentage of homes remain at some risk regardless of the development controls put in place to manage construction in bushfire-prone areas.
By concentrating on design and materials selection, NASH aims to complement bushfire suppression and land use planning. In this way, balance is maintained with the community’s efforts to reduce the personal and property losses associated with bushfires.
Burning trees vs well-planned housing
Contemporary discussion of bushfire construction has tended to focus on the fire itself: Vegetation types, fuel loadings and firefighting strategies. It’s as if humans have a right to live wherever and however we like, irrespective of the natural reality around us. The “fault” lies with the burning trees, and if we can somehow fix or manage that we don’t need to worry about special building practices.
The reality is that both of these things are important, but building siting and design is simply not as newsworthy as a bushfire and the science is frequently lost in the story (see Ref 5, pages 72-73). When official enquiries are held into serious bushfire events, much less time is spent examining the building details compared with how the forest was managed, how the fire started and how it was fought. Fire management is essential community work, but it does not create less vulnerable buildings.
The bushfire event
Dr Caird Ramsay and Lisle Rudolph in their CSIRO publication Landscape and Building Design for Bushfire Areas (Ref 4, page 3) describe the preconditions for bushfire in this way:
“When concentrations of flammable vegetation, high temperature, low humidity and very strong wind all occur together, they set the stage for a serious and destructive bushfire.”
Once the preconditions are present, natural events such as lightning or man-made events – accidental, negligent or malicious – such as power line damage, escape of controlled fires, discarded cigarettes, machinery and arson are all that are required to trigger a bushfire event.
How big the bushfire becomes depends on how much fuel is present in the bush, the persistence of the weather conditions, the efforts of the firefighters and the preparedness of the affected community.
What harm to life and property will result from the event also depends on other factors which are much more within the control of homeowners, designers and builders.
Communities Taking Control
Independently of the bushfire itself, communities and their individual members can influence the destructiveness of the bushfire event in at least four ways:
Development Controls: Where the community allows buildings to be built
Building Regulations: The designs and materials to be used in construction
Maintenance: The standard of maintenance recommended and practised to keep the design measures working
Housekeeping: The state of preparedness in the days preceding the actual bushfire event.
There is one other factor: the decision to stay or leave the prepared home as bushfire approaches. Forced evacuations are now unusual in Australian bushfire management, and increasingly the well-sited, well-designed, well-maintained and well-prepared home is seen as a safe refuge for able-bodied homeowners (see Ref 5, pages 71, 214).
How Bushfires Attack
The mechanisms of bushfire attack on houses and the way types of house respond are well understood and documented. Australia’s CSIRO has undertaken detailed and extensive analyses of bushfire events over many decades. This research has been used to support national, state and local planning policies and standards aimed at improving the survival of buildings subject to bushfire attack.
• burning embers
• heat radiation
• flame contact.
Not all fires involve all three modes of attack on every house, and different materials are affected in different ways by each mode. Each mode needs to be understood for its effects to be determined and for effective defensive measures to be devised and implemented.
Ember attack is the most prolonged and persistent mode of attack, commencing before the fire front and persisting for several hours afterwards. Radiant heat is at dangerous levels for perhaps 5-10 minutes before and after the flame front passes. Flame contact may impinge on the building for just a few minutes. Strong winds associated with fires exacerbate the effects of all three.
How Buildings Respond
How Buildings Ignite
A building element ignites when it is exposed to a source of flame or heat and reaches its ignition point. Commonly the source of flame is provided by embers and burning debris. If this burning material breaches the building envelope, or ignites adjacent litter or kindling, building ignition can occur. Houses are not consumed by bushfires, but by house-fires started by the bushfire.
Kindling and embers tend to build up in the same places, driven by wind in the minutes, hours or days preceding the bushfire. Even before the peak radiation level, the combined effects of radiant heat and hot dry winds act to dry out moist materials and make them easier to ignite.
At peak levels, radiation can crack and distort windows, doors and cladding materials, allowing breaches of the building envelope and ember attack on flammable contents. Flame contact can cause building ignition when exposed materials, dried and prepared by sustained wind, ember and radiation attack, are contacted directly by flames.
Roof structure, eaves, verandahs and sub-floor spaces are the most vulnerable, and will benefit most from good design and material selection.
In addition to flames from the bushfire itself, flame contact may occur from secondary fires in stored materials, outbuildings, adjoining properties, etc. For example, embers might attack and ignite litter driven against a woodpile, igniting the woodpile causing a large fire which then spreads to an outbuilding and eventually to the main house. Once firmly alight, this house itself becomes a source of fire for adjoining houses even if they have survived the primary attack from the bushfire.
House-to-house spread has been identified by researchers as a “significant cause of property damage during bushfires” (ref 8, page 3). A house may survive the onslaught of the bushfire itself, but remain vulnerable to attack from fiercely burning adjacent houses.
According to experienced firefighters and researchers, attended houses have a dramatically higher survival rate than vacant houses. Well-prepared and able-bodied occupants armed with relatively small quantities of water can fight spot fires before they become major blazes. Even where a room has been breached via a broken window or door, effective firefighting may still be possible once the fire front has passed provided an adequate water supply is available. The aggregate amount of burning material in any one street or suburb will have some effect on the success of post-bushfire firefighting efforts. The less flammable material, the less to ignite, burn and spread and the less to extinguish.
All building components should be fit to resist the conditions of service to which they will be exposed. The simple reality is that houses are not consumed by bushfires, but by housefires started by the bushfire. If the house does not ignite, inside or out, it will not be destroyed by fire. It follows that we should do everything technically possible and economically justifiable to prevent ignition of any part of the house or its contents from which a fire could propagate. It all comes down to good design.
Building design elements and details which encourage wind-driven debris build-up should be avoided. Remember that if the house does not ignite, it will not be destroyed by fire. It sounds simple, and it is. It’s your house – or your client’s house – so follow these general principles:
• Keep embers out of the dwelling and its structure by blocking, screening or shielding
openings, voids and build-up points.
• Use non-combustible materials for any permanent part of the dwelling that embers
could come into sustained contact.
• Use high-quality, durable exterior materials to ensure long term strength and fire
resistance with minimum maintenance.
If you can’t prevent ignition, try to keep the fire localised. Building elements adjacent to those which may ignite can be made from non-combustible materials, limiting propagation of any fire. A small fire is easier to extinguish and puts other structures – including your neighbour’s house – at less risk. If there is any possibility of a spreading fire, the design features of the house should make fighting the fire as easy as possible. For example, sub-floor spaces, if not fully screened should be completely accessible to allow rapid extinguishing of spot fires.
If propagation is going to occur, the less fuel it has available the smaller the fire will be and the sooner it will burn itself out. Fuel reduction is the primary strategy in forests to reduce the incidence and severity of fires. Fuel reduction around your property in its asset protection zone or “APZ” (Ref 6) reduces radiant heat on the house and spot fires, and assists firefighting. Fuel reduction through the selection of non-combustible materials for the house itself minimises the contribution of those materials to any fire which develops.
In an ideal world, the building exterior or “envelope” should neither break nor ignite when attacked by a bushfire. If the house envelope breaks, for example through an extreme wind gust or impact by a tree limb, the break should preferably not allow penetration of the building envelope and access to flammable components or contents. If the house envelope ignites, for example through a larger than expected build-up of burning debris, the ignited components should preferably be self-extinguishing or be unconnected to other combustible building components. Durable materials which require minimum maintenance help to ensure the house will always be as strong and secure as possible.
Steel Advantages in Bushfire Design
• Excellent early fire hazard properties – you just can’t ignite it (Ref 10).
• Non-combustibility – you can’t burn it, so it doesn’t add fuel to a fire (Ref 9).
• Quality and durability – its qualities are unaffected by time or maintenance.
Steel doesn’t ignite and can’t be made to burn under bushfire conditions, and while you are trying to burn it, it doesn’t give off heat or smoke. By the time a structural steel product has been affected by excessive heat, the building has most probably been lost.
As well as defying ignition and being non-combustible, steel building products offer homeowners and building practitioners:
• Wide design flexibility to easily create optimum architectural forms to resist wind,
debris and ember damage.
• Wide availability so the design you want can be built virtually anywhere in Australia.
• Outstanding durability keeping the design you build in good shape for years and years.
• Low maintenance requirements requiring low cost and effort to keep its bushfire
resistant qualities intact.
• Trade familiarity so you’ll always have access to the skills to modify or extend your
bushfire resistant home
• Reliability so you’ll never need to replace your steel building products because of
technical fashion or new research
An added benefit of steel near bushland – termites don’t eat steel – your bushfire resistant home will never be compromised by termite damage which is frequently more common in bushy environments.
With maximum use of:
Steel materials on the building envelope, combined with:
• better glazing
• durable door and window screens
• effective sub-floor ember screening
• properly sited and well-prepared buildings
The probability of ignition, fire spread and house loss is almost eliminated.
Following the devastating Victorian Black Saturday Bushfires, NASH investigaged different methods of providing a non-combustible, robust and durable bushfire solution. This was put to the test by CSIRO at the NSW Rurual Fire Service Eurobodalla Training Centre on April 16 2010 for more information see the special edition NASH News and video footage click here.
There is no single national design standard for bushfire resistant construction. However, the Building Code of Australia (Ref 1) applies reasonably uniformly in most states with some local variation in development planning and physical construction requirements. The detailed construction requirements for various levels of expected bushfire attack are contained in AS 3959-2009 (Ref 2).
Council building surveyors and building design professionals in all areas should be thoroughly familiar with local requirements. Alternative design approaches, using different combinations of materials, will nearly always be possible. Once the general design concept has been developed using the principles discussed here, the designer will apply the detail requirements of the relevant standards to finalise the design specification.
NASH has developed the NASH Standard - Steel Framed Construction in Bushfire Areas based on research carried out by CSIRO, as detailed in references 12,13 and 14 below. This Standard is referenced in the NCC 2015 and can be purchased from NASH.
References and further bushfire reading
1. BCA 2009 Volume 2, Australian Building Codes Board, 2009.
2. AS 3959-2009 Construction of Buildings in Bushfire-prone Areas; Standards Australia, 2009.
3. Handbook HB 36 Building in Bushfire-prone Areas; Standards Australia, 1993.
4. Caird Ramsay & Lisle Rudolph: Landscape and Building Design for Bushfire Areas; CSIRO Publishing, 2003.
5. Joan Webster: The Complete Bushfire Safety Book; Random House, 2000.
6. Planning for Bushfire Protection; NSW Rural Fire Service and Planning NSW, 2001.
7. External Water Spray Systems to Aid Building Protection from Wildfire - a report by the Fire Protection Association Australia. Email firstname.lastname@example.org
8. Peter Ellis: Statement to the Select Committee of the NSW Parliament on Bushfires; CSIRO FFP, June 2002.
9. AS 1530.1 Methods for Fire Tests on Building Materials, Components and Structures
– Combustibility Test; Standards Australia, 1994.
10. AS 1530.3 Methods for Fire Tests on Building Materials, Components and Structures
– Ignitability, Flame Propagation, Heat Release and Smoke Release; Standards Australia, 1999.
11. The Owner Builder Magazine -153 June/July 2009 Bushfire Special Feature