Flat Wood Roof

[chencfp]

I am wondering if it is practical for flat roof by using wood material (ie, Wood truss or 2x12 with 3/4” plywood deck, then applying waterproof material and shingle. Where I am able to find a specification or standard for this type application? Thanks.

 

[Ron]

I'm not sure where you are located, but generally flat roofs are not allowed anymore by most codes. Usually a slope of at least 1/4-inch per foot is required.

You certainly do not want to put shingles on a flat roof. That just doesn't work. You need a slope of at least 2:12 and I would recommend at least 3:12 for shingles. A shingled system is not a bonded waterproofing system. It is a dampproof system that depends on water shedding capability.

As for designing a roof structure of wood for "flat roofs", you must design for a high volume, low frequency storm event. You must account for potential ponding, and consider that long term creep of the wood might cause additional ponding between drains. When ponding occurs on roofs, progressive deflection occurs. If drainage is poor, the deflection increases and more ponding occurs, leading to a higher potential for catastrophic collapse. This is not a theoretical or academic exercise...I've seen the failures occur.

 

[msquared48]

I agree totally with Ron here. However, I would reccomend a min 4:12 slope for any "shingles", wood or three tab.

Where is your structure to be located?

Mike McCann
MMC Engineering

 

[Einsteim]

Yes, you can design a flat roof out of wood. You issue will be with the roofing material. Shingles cannot be used, you can consider PVC, EPDM, etc. to provide a waterproof system. Shingles are not allowed on less than a 3:12 pitch.

The "flat" roof must drain the water somewhere. Roof drains or sheet drainage to an edge are possibilities. "Flat" still means you need a minimal slope (1/4" on 12") to allow the water to drain to the roof drain or to an edge.

Once you have the roofing figured out, it is practical to use wood for the framing.