How to make a flat roof fire resistant?
April 23, 2019 11:54 AM   Subscribe

I am putting a new roof on my house. The roof is flat. I want to make it as fire resistant as possible (particularly with respect to falling embers from wildfires). How do I do that? At this point cost is not an issue. Thanks!
posted by persona au gratin to Home & Garden (9 answers total) 1 user marked this as a favorite
How flat is "flat"? I mean, what's the rise/run?
posted by aramaic at 12:25 PM on April 23, 2019

Seems like this is a question you need to ask an actual roofing pro about, especially one that does a lot of flat tops. They’d know better what material/methods are out there and be able to keep things in-code.
posted by Thorzdad at 12:49 PM on April 23, 2019

Best answer: cost not a issue? put a pitch on it & metal roofing
posted by patnok at 1:01 PM on April 23, 2019 [8 favorites]

Best answer: Disclaimer: I am not an engineer, contractor, or tradesman. I do find the stuff they do fascinating. And the articles by the Building Science Corporation are some of the best.

An article on designing fire resistant roofs. This layman's takeaways are metal roofing + flame resistant treated timber and rock wool insulation.

A report on roof design. The final figures in the report cover flat roofs. Spray foam insulation is used, and I know you generally want to spray a flame retardant coating on that material anywhere fire is an issue.
posted by bfranklin at 1:17 PM on April 23, 2019 [4 favorites]

Either metal or stone would be the most fire resistant. Most industrial roofs are of a shallow slope membrane with a light aggregate on top.
posted by nickggully at 2:51 PM on April 23, 2019 [1 favorite]

Best answer: If you are in a very high risk area, consider whether putting in some kind of sprinkler system would be helpful. Obviously you would need a water source (a pool? Pond?) a power source and pump, and failsafes for all of these. But basically any material is a lot les Likely to ignite if it is at least damp.
posted by Northbysomewhatcrazy at 3:49 PM on April 23, 2019

Best answer: Are there regulations in your area that could serve as a guide? Not sure how useful individuals making up their own spec will be.

For example, in Australia, the government specifies the level of fire risk in each area (Bushfire Attack Level) and then specifies how much fire resistance new homes must have to gain compliance. An area with BAL-40 would be exposed to burning embers, and possible direct exposure to flames, and the home must survive a sustained heat flux of 40kW per square meter, which is pretty terrifying. This is an very summarized look at the BAL-40 requirements, the full specifications would be a lot more technical and would up to the contractor to understand and be compliant with. (Australian roofs are sloped at 22 degrees, so pretty much "flat" for some definitions of flat...)
posted by xdvesper at 4:24 PM on April 23, 2019 [2 favorites]

Best answer: The roof is part of a system, there's little point making it fire resistant if the walls are flammable.

Australia is very strict on this when building in fire prone areas and the requirements are governed by AS3959, as noted by xdvesper above, and it describes a comprehensive system, of which roofs are just a part. I may be able to point you at the information contained therein if you send me a memail, the standard costs money to buy. I'll note that a pitch of 22 degrees is not a remotely flat roof and Australian roofs vary in pitch as much as they do in any other country. We have flat roofs and they are covered by the standard.

Some very brief ideas though:
*Use metal sheeting for your roof material.
*Use non-combustible gutter guards
*Close any openings with non-combustible mesh
*Keep your roof CLEAN of leaf litter etc. Hugely important.
*I don't think the pitch of the roof is greatly important here. Flat is fine (in reality it will have some pitch, there's no such thing as a 0 degree pitch. You are hopefully looking at at least 2 degrees. One degree is done but expect some issues unrelated to fire.)

Fire rating a house is expensive and may not work anyway. If you don't have to do it I'd be very careful about making an accurate cost benefit analysis first.
posted by deadwax at 6:53 PM on April 23, 2019

Best answer: (Actual roofing pro here)
There is no such thing as a *flat* roof. It would collapse pretty quick from ponding. Minimum recommended slope these days is 1/4" per foot.
Remember *nothing* is fire proof. You just get varying degrees of fire resistance.

How much extra capacity does your roof structure have? Can you get an engineer to give you some numbers?
If you want to stay with the low slope roof, look at "protected membrane" roofs or maybe extensive green roofs.
We typically rate fire resistance in hours by UL. If you have inadequate slope, you might look at having lightweight insulating concrete (LWIC) poured over the existing roof (yup, old membrane and all) and then adhering or mechanically fastening PVC over it. With metal decking, this is UL rated for 1 hour. UL tests take into consideration the fire coming from either side. I use this system often when re-roofing. You can add extra insulation - or not.

UL also has classification A, B, and C regarding fire resistance. This isn't a matter of hours, but of flame spread. Here's an explanation from a membrane manufacturer. Class C would be someting like wood shingles. (I like their products. Tough stuff.)

Sloped roofs - If you are thinking of going sloped, metal is not a bad choice. Many of the metal roof manufacturers have retrofit metal roof systems - they go up on your sturdy existing structure, and put up metal struts on baseplates. The struts are extended and connected by "horizontal" members to form a slope, metal deck is installed, then the metal roof. You would have to install at least an inch of rigid insulation - could be non-combustible stuff - to get a warranty on the metal roofing. They won't warrant going down straight to the metal deck.
The thing to remember here is all those plumbing vents/dryer exhausts/ etc cannot end up going into an unventilated attic. You either extend those things through the metal roof, or, as I did one time, design in some biggie wall louvers in gables to vent the space.
posted by rudd135 at 7:13 PM on April 23, 2019 [2 favorites]

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