Wednesday, December 19, 2012

BAL-FZ Roofing detail

One of the complications which comes with building in such a beautiful spot as we are, is that we have to deal with the very real, very serious threat of very intense bushfire. We're surrounded on all sides by "medium forest", mainly comprising Messmate (Stringy Bark) and Blue Gums standing around 30m tall. This part of the world hasn't seen a bushfire for 35 years or so although the Black Saturday fires started just over there, but then took off in the other direction.

All this means that when it does burn (and it will, inevitably) it's going to be one hell of a fire.

With the experience of the 2009 fires behind it, the Australian building codes were updated to reflect the bushfire risk and the requirements for fire protection are now quite strict, especially in our highest risk classification (BAL-FZ, for Flame Zone). One of these details surrounds the roof construction.

Normally with a steel-roofed timber framed building, battens are fixed to the top of the rafters or trusses, and the roofing sheets are screwed down directly on top of the battens. However, this leaves the potential for burning embers to enter into the roof space through the corrugations, and so this method isn't allowed in a bushfire-prone building. Instead, plywood sheets are fixed to the top of the trusses/rafters, then the battens, then a fibreglass insulating layer and then the roofing sheets. This compressed insulation sandwich prevents any chance of embers entering the roof space.

All of this means I need about 100 sheets of 2400x1200x15mm tongue-and-groove plywood...

Road trip time! :)

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