Vented Overroof in Fire Country Design Question
One area of house design I am always concerned about is fire safety. Recently I posted a question and some links about LP Flameblock Fire Resistant sheathing and then I watched a video from Green Builder Media about Climate Change Responsive Design and their method of putting metal roofing panels over an air space (although they are doing it more for preventing the sun’s heat from radiating through the panels and into the attic).
Green Builder Media (minute 14:55)
I’ve seen this kind of design before, usually with a second layer of sheathing on top of the air-space battens in order to provide a solid walking surface under the metal roofing panels but then again you’re back to having the metal panels resting directly on top of sheathing.
As for fire, there’s the question of protecting the air space between the two sheathing layers from embers. A lot of the products meant to screen over the gap between two sheathing layers (or vent it) are plastic which will melt in a fire. And then there’s still the problem of wildfire heat radiating through the metal roofing panels and setting the top sheathing on fire (if it’s just regular OSB or plywood).
What do you think about a cool roof design like this for optimum fire safety:
1. Standing seam metal roof.
2. High heat resistant underlayment.
3. LP Flameblock sheathing
4. 2 x 4 air battens treated with a fire tumescent paint
5. Peel and stick WRB
6. ZIP sheathing with taped seams
7. Eave and rake air space gap covered with 1/8″ open wire screening
8. Attic and ridge metal vents
I suppose there will have to be some special detail at the ridge so that the attic and the air space gap can both be vented. Any ideas?
Thanks!
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Replies
Hi,
Your metal roof alone does not have a Class A rating. It is a assembly rating, the manufactures have rated assemblies for their products. Typically metal roofs need a gypsum underlayment or fiberglass underlayment that is part of the rated assembly to get the class A rating. The California Building code has approved wildland attic vents, just wire mesh is not approved as ember resistant.
There are some specialty underlayments designed to help with fire. Here's are a couple I found.
https://www.owenscorning.com/en-us/roofing/components/products/titanium-fire-resistant
https://www.gaf.com/en-us/products/versashield
I would incorporate rockwool in your design rather than lp osb. I think it would be sim cost and better thermal/fire resistance. I know there has been some debate on standing seam clips directly on rockwool so you could run furring strips but I think you’d probably be fine with just wide clips on comfortboard 110. ca residential code specifies wui building in r337 and 337.7 relates to roofs. And any vents into attic should be listed on ca state fire Marshall list, Vulcan vent or sim.
“Where the roofing profile has an airspace under the roof covering, installed over a combustible deck, a 72 lb. (32.7kg) cap sheet complying with ASTM D3909 Standard Specification for "Asphalt Rolled Roofing (Glass Felt) Surfaced with Mineral Granules," shall be installed over the roof deck. Bird stops shall be used at the eaves when the profile fits, to prevent debris at the eave. Hip and ridge caps shall be mudded in to prevent intrusion of fire or embers.
Exception: Cap sheet is not required when no less than 1 inch of mineral wool board or other noncombustible material is located between the roofing material and wood framing or deck.
Alternately, a Class A fire rated roof underlayment, tested in accordance with ASTM E108, shall be permitted to be used. If the sheathing consists of exterior fireretardant-treated wood, the underlayment shall not be required to comply with a Class A classification. Bird stops shall be used at the eaves when the profile fits, to prevent debris at the eave. Hip and ridge caps shall be mudded in to prevent intrusion of fire or embers.”