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rockies
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# Posted: 9 Aug 2016 08:41pm
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I was reading a couple of articles on designing cabins to survive a wildfire and was interested in their suggestions for how to make cabins more fire resistant.
One of the main causes for cabins burning down turns out to be "burning from within". In other words the cabin didn't burn because a burning tree or hot embers fell on the roof, it burned because the fire raced across the ground and underneath the deck or cabin and came up inside. The cabin should be skirted in a fire proof material to prevent flames from going underneath the building and deck. I also mention in another thread on this forum some products that you can spray onto the deck timbers and underneath the cabin that help make the surfaces more fire resistant.
Another concern was the roof. I didn't realize that hot embers can still ignite the wooden roof deck underneath a metal roof. There are now membranes that can adhere to the wood deck that provide additional fire resistance. You can also apply these products to the walls underneath the siding ( which should preferably be cement based or metal).
One major area mentioned of how fire gains entry into a cabin is the eave venting. I know there have been many discussions on the forum about how to properly vent a roof. Usually this is for a gable roof. Apparently once fire becomes established against the side of your cabin it races up the exterior wall and straight through the vents into the roof assembly or loft. At a minimum the underside of the eaves should be covered in a cement based product (including the fascia board) and metal vent covers attached that can be closed should a fire be nearby.
Of course, if you're not at the cabin when a wildfire starts you can't close those vent covers so in almost all cases an unvented roof design is recommended.
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MtnDon
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# Posted: 9 Aug 2016 10:43pm
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There are vents that have been available for a few years now that meet the requirements of the strict rules that are in effect in much of California and some other areas with great wildfire danger and large population density.
Some of them need replacing after they are exposed to the heat of a wildfire. One I have seen here has a honeycomb structure. Air passes through the honeycomb normally. Exposed to high fire temperatures the intumescent paint that the honey comb is coated with, expands and blocks the air flow.
I've also seen some that are metal, have a fusible link which melts and the vent slams shut.
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bldginsp
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# Posted: 10 Aug 2016 02:38pm
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I used a different type of fire vent, with interlocked metal channels that make a baffle which, supposedly, prevents entry of embers or water in high wind.
https://www.brandguardvents.com
Location of eave vents is important. In the usual location at the bird blocks, embers get blown in when fire wind blows embers against the house. The embers and air go up the wall and get trapped under the eave, creating pressure the forces the embers into the vent to the attic. One solution to this is an eave-less building, but then you have no water drip away from the walls.
One solution is to put the vents on the outer edge of the eave, if you have a soffit air space where air can pass (see photo)
Another proposed solution is to put the vent on the roof just above the bird block line. But this requires cutting the plywood there, which compromises shear transfer from walls to roof diaphragm.
Note vents in edge of eave
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rockies
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# Posted: 10 Aug 2016 07:16pm - Edited by: rockies
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Bldginsp, as you mentioned the area under the eaves is a critical location for fire.. In addition to embers blowing into the vent the more dangerous force is the pressure difference between the inside of the cabin and the outside during a wildfire. Fire can quite literally be sucked into the cabin through the vents.
In reading the roof design recommendations on the "Green Building Advisor" website they always recommend having the least number of holes cut through or into a roof and that's why I prefer the unvented roof designs. No holes at the eaves, no holes at the ridge or gable for venting.
Whether or not the roof is vented or unvented the underside of the eaves and the fascia board should be covered in a fire proof material. Hardie cement boards are good or perhaps a metal fascia and eave cover with a fire membrane underneath.
I was also wondering, has anyone installed metal "L" or "U" shaped channels to the outside or their window and door frames to hold metal fire screens in case they had to evacuate the cabin? I always see people on the news nailing plywood over their windows and doors in case of a hurricane but I wonder if a system of permanent channels could be installed so that the fire screens could be easily installed and removed in case of a wildfire?
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bldginsp
Member
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# Posted: 10 Aug 2016 08:24pm
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I've contemplated shutters made of metal roofing
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toyota_mdt_tech
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# Posted: 10 Aug 2016 09:02pm - Edited by: toyota_mdt_tech
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Gable end vents can get sparks blown into the attic and a fire starts in the dry wood in the attic. Also, any trim that creates a ledge such as a belt moulding between a lower area of siding and upper AREA, Usually installed to his Z flashing. Take the belt molding and cut the top at a 45 degree angle and if you use T1-11, caulk the gaps in the siding. Also, a porch, store nothing under it and a wooden porch, trim it with cement 4" trim, this is where the porch meets the siding. Spray the deck with stuff like "Flame Stop" or "No Burn"
http://www.flamestop.com/cart/fire-retardant-spray-for-wood.html
http://www.noburn.com/products/fire-retardant-coatings
And as mentioned, a full footing/stemwall is the best thing one can do. So glad I did it with mine.
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rvlover
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# Posted: 16 Mar 2017 03:44pm
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I used a product called Flamex PF-2 on my deck 2 years ago and it still flame retardant. It was less expensive than the other brands and is even lab tested for exterior use. I also bought their interior product from them to treat our RV awning in case of flying embers.
https://natfire.com
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Cowracer
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# Posted: 17 Mar 2017 09:31am
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My company makes this. Not at my plant, but I have been to the plant that makes it. Good stuff.
URL
Tim
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