As a bushfire approaches a dwelling, or anything else in its path, the dwelling will be subject to the following threats:
Wind-borne burning embers – these may be carried up to 5 km ahead of, or fall up to 1 km behind, the fire front (in extreme cases embers may travel 35 km). These embers may cause spot fires well ahead of the fire front or cause fresh fires to break out after the threat has seemingly passed.
High winds – the velocity of winds associated with bush fires may be greater than that for which houses in the same locality but not subject to bushfire attack have to be designed. Wind behind a bushfire creates a moving fire front that consumes fuel in its path. Fire fronts most often travel at about 5 km/h, but can move at up to 30 km/h depending on terrain, wind and fuel availability. In large fires, the thermal energy can create its own weather phenomena, eg flaming whirlwinds in crowns of trees and rolling clouds of fire. Where a bushfire front comes up to or passes a dwelling, these local weather phenomena within the fire may result in reversal of wind direction and increase the magnitude of the wind forces to which the dwelling is subjected.
Wind-borne debris – the high winds can tear off branches from trees or sheet metal coverings from roofs and these then may become missiles that impact on adjacent dwellings.
Heat – the fire heats the air above it; this heated air is then frequently blown ahead of the fire front and dries out and preheats potential fuel sources and may in some cases raise the temperature above the ignition temperature5. When the fire is close, external surfaces of the dwelling will be subject to radiation that also may raise the surface above the material’s ignition temperature.
Direct flame – if the dwelling is close to the bush (or if adjacent trees, shrubs or combustible material ignites), the external walls and roof may be subject to attack from flames. Further, the local weather phenomena described above may mean that buildings some distance away from the fire front may also be subject to direct attack from flame.
Property Bloom recently found a development site across the road from a site where we’d just completed a dual occupancy project.
On one side of the road where we’d just built the land was NOT bushfire prone. On the other it was a different matter and the land was rated bushfire prone.
So what exactly does this mean and how will it impact on development costs?
Bush fire prone land maps are prepared by local councils across the State of NSW and are certified by the Commissioner of the NSW Rural Fire Service (RFS). For more info go to:
Whilst we want to build dwellings that are safe, what exactly do we need to add to our dwellings when they are in a bush fire zone to meet requirements?
Firstly, let’s define Bushfire Prone…a Bushfire-Prone Area is an area that can support a bushfire or is likely to be subject to bushfire attack. In general, a bushfire-prone area is an area occurring within 100m of a high or medium bushfire hazard.
New development on areas identified as bush fire prone are subject to the development and planning controls of ‘Planning for Bush Fire Protection 2006’ and must be designed to improve the survivability of the development and the occupants that are exposed to a bush fire hazard.
For our development, to meet the requirements we need to add aluminium fly screens to all windows and doors in lieu of standard mesh made of woven fibreglass yarn coated with PVC.
Tiled roofs must be fully sarked – sarking is an impervious barrier comprising of foils and films which is used to prevent water from penetrating the building envelope beyond the exterior roof and it’s a requirement in some bushfire prone areas.
Metal fascia and gutter guards were also required to stop leaves from building up and becoming a potential hazard. During a bushfire, sparks can travel many kilometres in front of a main fire front and can ignite dried leaves and rubbish in your gutters. Another advantage of adding these guards is to keep birds and rodents out of the gutters and roof as they can cause damage and spread lice.
Spark proof garage door seals which involves an aluminium holder fastened to the door which closes any gaps to the entry. It’s flame retardant rubber compound slides into the holder and rests above the lintel to provide a barrier to help delay the onset of fire.
Metal covers for the weepholes to the brick vents. Sub-floor vents are screened to allow free air movement, but prevent spark or ember ingress. The non-combustible screen is covered with a corrosion-resistant metal spark guard.
This has impacted on the build costs for this particular dual occupancy project by about $10,000.
What we will also consider is that mortar joint finishes Ironed or Flush joints have been found to perform better than raked joints when subjected to fire but this is not mandatory.
BAL Risk Assessment – Granny flats
The NSW Rural Fire Service regulations require a Bushfire Attack Level (BAL) for a granny flat project. If the BAL is 40 or the highest risk, Flame Zone, approval must be sought by a DA with the local Council.
If the BAL is low, there is no need to do anything except pay for a Bushfire Assessment from a qualified assessor as long as the property is Complying.
If the BAL is 15 to 25, there will be requirements for protection measures. This may include toughened glass, aluminium flyscreens and gutter guards.
Generally, Property Bloom does not look for granny flats sites within Bushfire Prone areas unless a water hydrant is nearby as it can add costs to the construction and this strategy is all about low costs and high yields.
It’s important to understand that whilst building and developing property in bushfire prone areas can be very desirable due to the views and peaceful environment a bush setting brings. But it will add to the build costs and these additional costs may not always be seen to add value. You should use these upgrades as selling points as in effect what you have done is in included a ‘bush fire protection system’ for the home, much like a security system will add value, so should the bushfire protection system. So be sure to point this out to potential buyers and valuers.
In the end, the location of our dual occupancy project – a premium area close to all facilities – outweighed the additional costs for the bushfire requirements and we looked at this as an ‘opportunity cost’ to build in this leafy, green area.