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Bushfire behaviour

Fire attack mechanisms - an introduction

How does fire attack your home and what can you do about it?


A checklist of what actions a house might be faced with - large fire front, ember attack, adjacent buildings, adjacent combustible objects, surface fire, tree and large branch strike, wind and wind-blown debris, smoke, and what considerations we might use that checklist of mechanisms that could attack a house to really deeply consider how our house and surroundings might respond.


Fire attack mechanisms

What actions can you take to reduce fire attack risks?


Well over 80% of all the houses that are lost in these bushfires is actually lost without any of that large fire front action reaching the house.

There's always embers, there's always surface fires acting around your property, there's always wind and wind-blown debris context that either is intense enough to be a direct action in itself or simply adds an important additional context to everything else and smoke exposure is ubiquitous to all fires.

So ember attack, surface fire, adjacent buildings, and adjacent combustible objects are all the factors that actually end up playing out and being most responsible for loss in fire.

Accumulation of new dry debris happens extensively during a fire event. Unfortunately those freshly cleaned gutters end up filling up again during the fire event.

Preventing debris accumulation by design, like not having gutters or having non-combustible gutter guards that just don't allow debris accumulation, or having decks that aren't ignitable by debris accumulation, are the types of strategies I hope people consider when they're considering how to best upgrade their houses.

Embers. It is a real question of detail and understanding about "can a 2mm object get through and into either a building cavity or into the occupiable space of your home," is really the only way to unpack it and spend a lot of time understanding where all these features are.

Embers and fine debris that is reading to burn come like a sandstorm and they build up against things and so, in the end, you've got lots of small flame sources forming where event debris can access or build up, that's actually what's going to ignite your window frame, or your mulch, or whatever it happens to be.

House-to-house ignition. When houses are closer than 12m from each other your chances are elevated because you don't have to just get one house through the fire, you've got to get two or more houses through that fire, so it is a real question of a discussion at the streetscape level.


How a house is attacked - an introduction

What is it like to experience a bushfire from the house's perspective?


Our research team is a little unique in that we study bushfires from the perspective of the house. So, what is it like to experience a bushfire from the house's perspective?

If we list these processes out, this is a fairly good profile of what we're up against: ember attack, debris accumulation, surface fire, consequential fire, flame front contact, radiant heat, wind action, tree strike.


How a house is attacked

We know how houses are attacked by a bushfire.

If you take action to prevent ember entry then more houses and people will survive.


Our research team is a little unique in that we study bushfires from the perspective of the house. So, what is it like to experience a bushfire from the house's perspective?

If we list these processes out, this is a fairly good profile of what we're up against: ember attack, debris accumulation, surface fire, consequential fire, flame front contact, radiant heat, wind action, tree strike.

Ember attack processes are obviously very important for house survival, but they are actually absolutely critical for occupant survival.


Embers

Embers are an absolute certainty in a bushfire.

If you know what to do and take action you can reduce the risk of your home being destroyed.


The only absolute certainty is there'll be embers there when there's a bushfire.

It comes down to how many and how big the flame sources are that result from these ember storms.

Ember attack is everything fine that's become airborne that's essentially combustible. So leaves, twigs that are either not burnt yet or are burning and flaming while they fly, or they're glowing while they fly.

They turn up and lodge in and get through gaps and build up in corners and crevices and are continually fanned by the wind, and just create prolific combustible debris build up and localized flame sources on everything external, particularly where there's a complex corner, like everywhere where leaves blow and accumulate in a normal windy day is where these embers and small flame sources build up.

The larger the gaps the more embers and debris get through, and the more large embers get through the probability of ignition happening in that cavity increases.

It's almost impossible to imagine that we could eliminate ember attack at its source by clearing enough vegetation out to many hundreds of meters.

However, solving it from a house design perspective is relatively straightforward.

A house is made up of many gaps, and we need to be really diligent in reviewing where all those gaps may be that embers could enter, and consider what might happen if ignition occurs on the other side of those gaps.

In terms of finding gaps and understanding where they go, it's really a detective process of looking and looking and looking at your house to understand where they are and resolve them through whatever means you can.

It's really an attitude and an approach that you just have to come to terms with, observe very carefully, and look at deliberate vents that may not have small enough apertures. The 2mm rule is absolutely relevant to every gap around your entire house.


Tree strike, smoke

Large trees and branches can strike your house or block your path.

What should you know and what can you do?


Retention of tall canopy trees do a lot to help moderate the potential risk of direct wind loads acting on your house that is so severe that it could compromise the integrity of your house, or roof, in a fire event.

Tree and large branch strike in proximity of house is obviously a really major challenge and issue. It also is a major question around reliability of egress, either off your property and through the local neighbourhood to a place of relative safety.


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