By Dr Tracy Wakefield, managing director, Appalachian Log Homes –
LETTER: 11 February 2010 – I read your article , Timber, Complex, Sustainable and Good in Bushfires, with great interest and would like the opportunity to comment on the fire performance of log buildings mentioned by Justin Leonard.
I submitted an article for the Fire Australia Magazine last year (to be republished here soon) showing the results of BAL Flame Zone testing on this type of construction and thought you might be interested in expanding this issue a bit more. There are also several other papers on pilot scale fire testing which was carried out, but they are a bit more technical.
In short, the comment that Justin Leonard made about log walls burning for an hour during a bushfire event begs a comment. During AS1530.8.2 testing in a NATA accredited laboratory we subjected a 3m x 3m log wall to a full 30 minute enclosure fire (flame contact and radiant heat levels up to 100kW/m2).
After removing the sample from the imposed radiant heat of the furnace the panel began to self-extinguish in seconds. After two minutes the flaming was extinguished leaving only small amounts of tiny flames and glowing combustion. Within 15 minutes the heat generated by the test panel was so low you could walk right up to it and hold your hand out within 350mm of the wall without being uncomfortable. This might not be true of a masonary wall that was heated in the same manner.
It is well known that the fire front of a bushfire passes through very quickly (in a matter of 1-3 minutes) and given the experimental work carried out in this case, the concept that it would be difficult to exit a log home after a bushfire passes borders on ridiculous.
If indeed a bushfire can ignite a thick log wall in that short a timeframe, as soon as the bushfire passes through the flaming cannot be sustained and within minutes you can exit. In fact this soon after a bushfire would be too dangerous to exit due to the high (in human terms) residual radiant heat levels from the bushfire itself.
A well designed log home meeting the required bushfire standard for the threat (ie flame zone the windows must be shuttered or able to withstand the flame contact, eaves cannot be combustible, doors must be fire rated or non-combustible) would not be a risk.
I have carried out considerable research in this area, sit on a Standards Committee for Engineers Australia, and would love to help provide information to the public about this.