The recent post ‘Bushfire Hazard Assessment FAQ’ explains the associated terminology. Here I talk about the implications the Bushfire Hazard Assessment had on the design of 60k House.
Get an expert in early
Before you make any decisions about where exactly to site your house it is first advisable to engage a bushfire hazard assessor. This is a relatively new requirement arisen from the ashes of recent bushfire disasters.
It was obvious I would need a Bushfire Hazard Assessment – my block is half covered by trees and there is bush nearby. Given my understanding of the site conditions from Site Analysis and following the initial Site Planning ideas I knew roughly where I wanted to site 60k House. What I didn’t know was what Bushfire Attack Level the building would have and how much clearing around the house site I would need to do to establish a Hazard Management Area.
BAL is determined by several factors, including the vegetation type, the slope of the ground and the Hazard Management Area (also referred to as Defendable Space). I’m a greenie – I like trees. There’s not much point having a bush block if you cut down all of the trees – then it’s no longer a bush block! When I first considered building a shack at my site the idea was to build in the clearing amongst some large trees. There were no bushfire requirements back then – this was before the devastating Black Saturday fires in Victoria and dramatic fires in Tasmania in 2013. The previous owner of my block had ‘managed’ the land by regularly burning it – this was effective in keeping the understorey clear but also burnt the hollows in the trunks of the trees making them unhealthy and unsafe. After hosting my first block party the crown of one of the large blue gums fell out and was left hanging from the top of the tree. This ‘widow maker’ was dangerous and it became apparent I didn’t want to build too close to something that could come crashing down through the roof in the middle of the night.
Finding a balance
It’s a pure coincidence that ‘BAL’ forms the start of ‘balance’. I commissioned a preliminary Bushfire Hazard Management Plan to determine what the BAL and HMA would be for 60k House. I was given two options: clear the minimum amount and have BAL 19 or make a much larger clearing to achieve BAL 12.5. You don’t often get options. The construction requirements for BAL 12.5 and BAL 19 are nearly the same.
Because I wanted to minimise the felling of large trees on the site I opted for the minimal clearing and higher BAL rating. To achieve BAL 19 I would still need to cut down a couple of large trees and maintain a decent amount of cleared area around the building. The HMA is determined by a combination of the vegetation and the slope – this means each direction has a different distance that needs to be cleared. Because the site is a north facing slope it needs to be cleared 34m to the north as fire will travel faster uphill; only 23m clearing is needed to the south, and 13m to both the west and east (mostly because of the type of vegetation).
That’s a plan
The BAL and HMA forms the basis of a Bushfire Hazard Management Plan. A BHMP also includes other requirements such as suitable vehicle access for fire-fighting trucks and onsite water supply – usually in the form of an accessible dam or 10,000L fire-fighting water tank. The designated water tank cannot be used for other purposes (such as feeding your house), has to be located at least 6m away from any building (in case that catches fire) and needs to be fitted with special fire-truck friendly fittings for ease of connection to pipes and pumps. Most Councils (local planning authorities) require at least a preliminary BHMP be submitted at the planning stage – some require a fully endorsed BHMP. I will talk more about this when I discuss the approvals process.
What did the BAL rating mean for the design of 60k House? The main points were the cladding, the doors and the windows. For BAL 19 the critical dimension is parts of the building within 400mm of the ground – the area most likely to be under attack from a bush or grass fire. There also needs to be no gaps or holes in the building more than 3mm – to prevent embers blowing in.
The cladding (or ‘External Walls’ in the table) within 400mm of the ground needs to be of non-combustible material (roofing iron, etc), 6mm fibre cement sheet or bushfire resistant timber. Each timber has a listed density – only certain timbers are deemed dense enough to withstand a bushfire. All of the lower parts of 60k House will be either clad in timber weatherboards milled from blue gum or stringy bark felled on site, or a door. 6mm cement sheet is used for the cladding either side of the windows and 4.5mm cement sheet for the cladding above the window line.
All external doors need to be either 5mm toughened glass or 35mm solid timber. Writing this post has made me realise I have stuffed up the entry door! The timber to the lower section would not have been thick enough. The door has already been manufactured (but not collected) – I’m currently working around how to modify this door and use it elsewhere… Interestingly doors do not need to have flyscreens (or bushfire shutters), but windows do. The reasoning behind this logic is that if a bushfire comes racing towards your house and you decide to evacuate, chances are you will close the door behind you, but you may forget to close a window. An open door or window will allow embers (sparks) to enter the house and set fire to your plush soft furnishings like curtains or shag pile rug.
The design of the windows has been a major headache in the overall design for 60k House – I am still trying to resolve the windows for the bathroom. I will talk more about all of the windows in a later post. The main point is that ALL operable windows (windows that open) need to have either bushfire shutters fitted externally, or metal mesh fly screens fitted internally. This severely limits the type of window openings you can have.
Is a home amongst the gum trees a thing of the past?
Yeah, I think it is. Designing a new home on a bushfire prone site is much more difficult and limiting than in an urban environment. Except bushfire regulations, like most building regulations, aren’t applied retrospectively. You will still be able to find and buy existing homes nestled among trees in the bush, but don’t expect to easily be building one yourself.
Costs: Bushfire Hazard Management Plan – $600
Disclaimer: Any advice contained within this blog is of a general nature only and cannot be relied upon. Details provided are in good faith and relate specifically to this project. Any author will not be held responsible for advice or information presented.