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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 comprises an inner zone and an outer zone.

The Defendable Space Requirements:

An inner zone distance as specified by AS 3959-2009 for a particular BAL, an outer zone developed using the methodology of AS3959-2009 – Appendix B but with the following changes:

a Forest Fire Danger Index (FFDI) of 120 (instead of 100) a flame temperature of 1200Kelvin (instead of 1090Kelvin

a building is located so that it is not exposed to a radiant heat flux more than 10kw/m2, based on he AS3959-2009 Method B approach using an FFDI of 120 and a flame temperature of 1200Kelvin. This standard is considered the most suitable performance requirement because: it is the maximum a fire fighter in personal protective equipment can be exposed to for a short period of time (less than 10 seconds), providing some potential to evacuate or defend buildings occupied by people with special needs, it aligns with the criteria for Neighbourhood Safer Places in Victoria and it provides a conservative approach for determining defendable space for uses at particular risk.

Inner Zone Standard Conditions

• Grass must be no more than 100mm in height

• Leaf litter must be less than 10mm deep

• There must be no elevated fuel on at least 50% of the area. On the remaining 50% the elevated fuel must be at most, sparse, with very little dead material

• Dry shrubs must be isolated in small clumps more than ten metres away from the dwelling

• Trees must not overhang the roofline of the dwelling

Outer Zone Standard Conditions:

• Grass must be no more than 100mm in height

• Leaf litter must be less than 20mm deep

• There must be no elevated fuel on at least 50% of the outer zone area

• Clumps of dry shrubs must be isolated from one another by at least ten metres

These tables must also consider:

• Vegetation type;

• Slope;

• Distance of vegetation from a proposed dwelling; and

• Suitable fire weather conditions to reflect the events of Ash Wednesday (in 1983) and Black Sunday (in 2009)

The use of view factor is contingent on determining flame geometry. As such, the process of

determining radiant heat using view factor method is a staged approach in which initially the

bushfire behaviour equations are used to determine flame geometry, and secondly the use of

appropriate inputs to derive radiant heat from the flame. This overall ‘site assessment methodology

planning purposes is