Fire-resistant roofing - Composition Roofing enough? Use DensDeck with it?
We are building in central Washington state in a wildland urban interface area, and want to use a Fire-resistant roofing, even though we are not required to have a "Class A" roof or anything particular. We have a mix of 5/12 and 9/12 roof slopes, and some valleys in the 9/12 sections. We are leaning towards composition roofing rather than steel based on cost, but we are wondering about the adequacy of composition roofing's usual Class A fire rating alone compared to using steel, or possibly adding a non-combustible underlayment (DensDeck? or any alternate) in order to get very good overall fire resistance in terms of both ignition, flame spread and resistance to burn-through.
Is composition roofing really about equal to steel roofing in terms of overall fire resistance? Are composition roofing's Class A ratings really stand-alone ratings that don't need an added non-combustible underlayment? Does adding DensDeck under the roofing do worthwhile things to overall fire resistance if the roofing material is already Class A rated? How does the use of an ice-and-water barrier membrane in lieu of the normal roofing paper affect fire resistance, since it seems to be a more combustible product? Are there alternatives to DensDeck?
Thanks very much for your question.
An asphalt compostion shingle is a stand alone Class A roof covering (assumed assembly is sheathing and roofing felt, then the asphalt comp covering). You may have looked at roof rating information on the eXtension website, but in case not, the link is ...
Another site with information regarding the fire rating of roof coverings is ...
Depending on the type of asphalt composition shingles you choose, you may get a longer service life from a steel roof, but as you point out, the cost of the asphalt comp is lower. Both have a Class A fire rating.
An alternative to Densdeck would be a fire rated roll roofing product (some information about this product on the Firecenter webpage).
Densdeck also has a Class A fire rating, but including it in an assembly that already has a Class A fire resistance rating is redundant, particularly if you choose an asphalt comp covering. If you choose a covering that creates / allows for spaces between the roof covering and roof deck, roll roofing could be used to provide additional protection for embers that might get between the covering and roof deck.
If you choose an asphalt comp shingle covering, and your home has valleys, you could use a roll roofing product in the valley, as an underlayment for the roof shingles. This would only make sense if you used a metal product in the valley. If you weave the shingles at the valley, no need for the additional underlayment.
I have to look into your ice and water shield question. Please send me an email, and I will respond to you directly after I gather some additional information.
Thanks. I hope this information has been helpful.