Bush fire building safety advice

Ralph SmithFire & Emergency Services

Authority

People and Bush Fires

Greatest threat to people is radiant heat

Critical to protect people and buildings from radiant heat and direct flame contact

Most bush fires are started by people

Buildings and Bush Fires

Buildings are affected by three main factors:

1. Burning debris – embers2. Radiant heat3. Direct flame contact

Most homes are lost to ember attack

Limiting Vulnerability FESA and local governments publish

information on fire risk Limiting the fuel quantity around

communities and buildings Limiting extremes in fire behaviour by

the appropriate placement of building – slope

Limiting ember attack through materials and building design

Water supplies

Limiting Vulnerability

Ember attack – Jarrah forest type fuels Spotting distances associated with the

Jarrah forest Fibrous bark main source of firebrand More embers are generated from the

older fuels – around double the number of embers between the 5 year old fuel and the 22 year old fuel

Fuel Age- years since

last fire

Moving fires

Reaching the break

Long Distance spotting

2(~4 t/ha)

5m <10m 0

5(~8 t/ha)

20+m 40-50 m 200

22(~18 t/ha)

50m 50-60 m 360

Limiting Vulnerability - Slope

Fire rates of spread (RoS) increase upslope Ros multiplier

+5° 1.4

+10° 2

+15° 3

+20° 4

Radiant heat and flame contact

By maintaining the building protection zone (BPZ) and hazard separation zone (HSZ) will reduce the potential for radiant heat and direct flame contact

As a general guide when planting shrubs determine the height they will grow and then double it to get the planting distance from the house

Radiant heat and flame contact

Try and maintain a gap of between 10 – 15 metres between trees

Be careful not to place plants that may compromise the integrity of the building – windows can be a weak point and allow embers to enter

IntensitykW/m

Flame Height (m)

RoS Fuel Type

Indirect unlikely to succeed

> 3,000> 8,000

> 8> 5 >1250 m/hr

ForestShrubland & grassland

Direct attack not possible or unlikely to succeed

> 2,000> 2,000> 5,000

> 6> 2 > 2

> 400 m/hr> 1,000 m/hr> 6,500 m/hr

ForestShrubland

Grassland

Machine and tanker attack possible

< 2,000< 2,000< 5,000

< 6< 2 < 2

< 400 m/hr< 1,000 m/hr< 6,500 m/hr

ForestShrubland

Grassland

Hand tool attack possible

< 800< 800< 800

< 140 m/hr< 140 m/hr< 300 m/hr

ForestShrublandGrassland

Readily suppressed

< 300 < 60 m/hr All fuels

BPZ & HSZ

Low Bush Fire Hazard

Include

Areas devoid of scrub vegetation & thinned overstorey

Areas due to climatic conditions do not experience bush fires

Reducing the Hazard

Choose a method that you know you can apply for the long term

It may be:1. Burning2. Slashing3. Parkland clearing

Hazard Separation Zone

To achieve a fire of intensity of around 2000 kW/m requires a fuel load of less than 5 t/ha when the FFDI - 80

This equates to a parkland type of vegetative cover

Hazard Separation Zone

Research undertaken by McArthur from the CSIRO indicates that a fuel load of less than 8 t/ha will not support a crown fire

Building Protection Zone

To achieve a fire of intensity less than 800 kW/m requires a fuel load of around 2 t/ha when the FFDI - 80

This does not equate to a cleared area, but a managed area with appropriate vegetative cover

Building Protection Zone

Have a 2 m gap between trees and houses – no overhanging limbs

Prune the low limbs of trees to 2 m Cut the long grass and dense scrub Rake up the leaves and twigs Plant shrubs singularly Prune the dead material from the

shrubs

Scorch in the crown

Low intensity fire near to the house

Burnt shedTree crowns

House damage – low intensity fire ember attack

How to Assess your Hazard

Determine the vegetation type and class

Determine the slope Determine the distance between

predominant vegetation class and the site

Hazard & AS 3959

Hazard Application of AS 3959

Extreme Do not build

High Level 2

Medium Level 1

Low Standard construction

Hazard Separation Zone

The distance between the predominant vegetation and the building is critical in determining the level of protection required from the:

1. Potential ember attack2. Flame contact3. Radiant heat

HSZ – AS 3959 As the distance between the

predominant vegetation reduces the construction standard increases (+ BPZ)

Forest >100m – no increased construction

Forest 60 – 100m (medium) – level 1 Forest 35 – 60m (high) – level 2 Forest < 35m (extreme) – do not build

Fuel levels

At an FFDI of 80 HSZ – To achieve a fire of intensity

of around 2000 kW/m requires a fuel load of less than 5 t/ha

BPZ - To achieve a fire of intensity less than 800 kW/m requires a fuel load of around 2 t/ha