Understanding how bushfire behaves and destroys houses is important when planning, designing and selecting suitable plants for a garden. There are three major factors that influence bushfire behaviour: topography, weather conditions and vegetation.
TOPOGRAPHY (OR SLOPE)
Fire burns faster uphill. As the slope increases so does the speed of the fire and its intensity.
Flames and radiant heat preheat the vegetation ahead of the fire. This dries it out, making it easier to burn.
Hot, dry and windy days provide ideal conditions for a bushfire. In summer, these are common weather conditions that increase the flammability of vegetation.
Low humidity and high temperatures, which are fuelled by hot winds, dry out vegetation, allowing it to readily ignite.
Plants are the primary source of fuel for a bushfire.
The amount of fuel available to a bushfire and where the fuel is located can directly impact on house survival. Understanding how vegetation influences fire behaviour is important when planning a garden.
Within a property, vegetation management and the placement of other flammable objects around the house can determine the amount of fuel available to a bushfire.
The amount, type (flammability) and arrangement of vegetation affects how easily a bushfire will spread throughout a garden.
Fine fuels such as leaf litter readily dry out, ignite and can be carried as embers. Shrubs, vines and other elevated fuel can act as ladder fuels, allowing fire to climb into the canopies of trees, significantly increasing bushfire intensity.
Breaking up the continuity of the vegetation can limit the spread of fire within the garden.
Remember there are no ‘fire proof’ plants. All plants can burn under the right conditions – typically in extreme fire weather following extended drought.
See Section 4 for more information about how to minimise bushfire risk through garden design.
Leaf litter and dead plant material on and around houses and gardens can be cleared to reduce the risk of them catching fire or becoming burning embers.
HOW BUSHFIRE DESTROYS HOUSES
House survival is influenced by many interacting factors. The four main ways houses are destroyed during a bushfire are:
Ember attack is the most common way houses catch fire during a bushfire. Ember attack occurs when small burning twigs, leaves and bark are carried by the wind, landing in and around houses and their gardens.
If they land on or near flammable materials, such as leaf litter and dead plant matter, they can develop into spot fires. Embers can also ignite a house if they land on or near vulnerable parts of the building.
Radiant heat is the heat created from combustion during a bushfire. It can:
ignite surfaces without direct flame contact or ember attack. This is due to the heat being received from the fire
dry out vegetation ahead of the bushfire so that it burns more readily
crack and break windows, allowing embers to enter a building
distort and melt materials such as plastic.
Flame contact occurs when flames touch a house. Any burning vegetation can directly ignite a house if it is planted too close.
Wind can be very destructive to houses in a bushfire because it:
can cause trees to fall onto buildings
can break windows
can loosen roof tiles
can blow roofs off houses under severe conditions.
Before designing a garden, there are a number of factors to consider. Reducing bushfire risk to any house is most effective when considered early in the planning process.
Think strategically about where the house is located and how the garden around it is designed. That way, it is possible to achieve multiple outcomes – bushfire safety considerations are incorporated but are not the only function of the garden.
Find out what building and planning regulations apply to the property. Visit land.vic.gov.au or talk to the local council. Depending on the bushfire risk, these regulations may influence:
where the house can be built
the construction level required
how to manage the vegetation within the property.
Information in this section is based on the bushfire protection requirements for building in high bushfire risk areas.
The requirements are fully set out in Planning Schemes at Clause 52.47 Bushfire Protection: Planning Requirements (see Further Resources) but are summarised below.
Understanding how these factors influence bushfire can avoid unnecessarily increasing the risk within properties.
Siting and design
One of the most effective ways to reduce bushfire risk is the appropriate location of a house within a property. Features of the topography can be used to help minimise bushfire spreading into and within the property. Houses should be located away from unmanaged vegetation, steep slopes, saddles or narrow ridge tops. They should ideally be located close to public roads and accessways.
Look at the landscape in and around the property:
What is the bushfire risk from the surrounding area?
Is there existing vegetation within or close to the property that will pose a significant bushfire hazard?
Anywhere that embers can lodge or enter a house can start a fire.
There are areas of a house that contribute more to overall bushfire risk than others. These include decks, windows, doors and roof areas. Complex designs that may create nooks and crannies allow dead plant material and embers to drop and accumulate.
Defendable space is an area of land around a building where vegetation is modified and managed to reduce the effects of flame contact and radiant heat associated with bushfire. It breaks up continuity and reduces the amount of fuel available to a bushfire.
It is one of the most important aspects of preparing properties for bushfire. This is because defendable space separates the bushfire hazard and the house. The greater the separation from the bushfire hazard, the lower the risk.
Defendable space can prevent direct flame contact and minimise the effects of radiant heat on the house. This reduces the risk of house loss during a bushfire, regardless of active defence.
comprises an inner and outer zone with different vegetation management requirements
needs careful garden design that considers the location of all flammable objects
requires regular maintenance that should be included as part of every Bushfire Survival Plan.
Requirements for defendable space will vary depending on the type of development and the level of bushfire risk to a property. Section 4 provides further detail about defendable space requirements.
The way a building is constructed can help reduce the risk of house loss via radiant heat and ember attack.
Construction standards are linked to defendable space. The greater the area of defendable space, the lower the construction requirement under Australian Standard AS3959-2009: Construction of Buildings in Bushfire-prone Areas.
A bushfire site assessment is required to determine the construction standard that will apply to any house. Details for undertaking a bushfire site assessment in the Bushfire Management Overlay can be found in Department of Planning and Community Development (DPCD) Practice Note 65: Bushfire Management Overlay and Bushfire Protection: Planning Requirements (see Further Resources).
PROVISION OF SERVICES
Water is essential for firefighting.
The amount and reliability of water is critical for all properties and must be considered in relation to the bushfire risk.
In all areas the water supply must have appropriate pressure, access and fittings. In the Bushfire Management Overlay, a water supply must be provided.
Access is just as important as it provides a way for residents to get out and the fire services to get in. Roads must be capable of accommodating fire trucks and will require specific construction standards, as well as width and clearance, depending on the property.
For minimum water supply and access requirements that apply to a property in the Bushfire Management Overlay, refer to CFA Fire Service Guideline: Land Use Planning 0002: Requirements for water supply and access in a Bushfire Management Overlay (see Further Resources).
Book a free Home Bushfire Advice Visit for help assessing bushfire risk at a particular property.
To book an appointment, complete the online form on the CFA website. One of CFA’s trained Fire Safety Officers will provide tailored advice, delivered on the property.
Self assessment of bushfire risk can be undertaken by using the Online Household Bushfire Self Assessment Tool at cfa.vic.gov.au
Once the layout of the property is decided there are some decisions to make about what type of garden will be planted.
Gardening is a personal activity and when planning any garden there are many considerations apart from bushfire.
While this publication focuses on gardening to reduce bushfire risk, any garden must meet the needs of those that are using and maintaining it. If a garden suits the needs of residents it is more likely to be maintained from year to year.
There are many different styles of gardening. Some focus on native vegetation, productive or water-sensitive design.
Whatever style is chosen the garden must be appropriate to the local area. Seek advice from the local council (see Further Resources) about species that are suited to a particular location. This will help to avoid planting environmental weeds or invasive plants.
Consider bushfire risk early in the garden planning process. By incorporating the design principles in Section 4, costs can be minimised and bushfire mitigation will complement other functions of the garden.
When planning a garden some things to consider include:
the local growing conditions that may affect plant selection
ongoing maintenance requirements
function and style of the garden.
Think about how the features below are incorporated into the overall garden design as they may reduce the spread of fire by providing separation between the house and bushfire hazards. For example:
pools or water features