Flat-Roof Assembly for High-Wind Area
woobagoobaa
| Posted in General Questions on
Hoping someone can point me to a reference or describe general best practices for a flat roof assembly exposed to high wind/rain locales. My main question is should it be at all possible that wind can peel back the outer membrane from a properly constructed flat roof? Should there be a WRB between the sheathing and the outer membrane? I am more familiar with residential roofs … my own has a fully adhered WRB under the shingles. Thanks a bunch.
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WG,
FWIW, I have 1:12 roofs with pvc fabric membrane mechanically fastened, not glued to the deck. The seams and edges are heat fused to special metal forms that are screwed down. A full roofing membrane was adhered to the nail base first. Two layers of fiberglass mat reside above that and under the membrane for fire protection. This of course prevents the membrane from being a glue down. I believe EPDM and TPO are both glue down membranes. I would look at application instructions to get a handle on appropriate substrates for either system.
I can say from experience that (between fastener rows) certain parts of the roof membrane lift up when the wind comes at certain angles. I will also note that even my standing seam will rumble and visibly flex when the winds get near 50. The roof has been on 6 years and the winds come every spring, so I guess the mechanical fasteners are doing okay. I will observe that the size of the screws used is daunting. Definitely going to leave a mark through the decking membrane.
That said, you might want to consider what will be required to replace the membrane in the future. Mine has a lifetime warranty, but TPO and EPDM may not offer that. I can peel mine off easily enough if need be, unglue-ing a membrane and dealing with the old surface underneath may require a fresh decking with new ply or OSB. I don't think you can over-roof any of the materials.
Pictures of large commercial building roofs damaged by tornados or hurricanes show membranes can be peeled off, but it is not clear what types they were or if sloppy installation was revealed. Of course, 120mph winds might overcome any roof.
Consider a liquid polyurethane roof membrane.
It gets rolled on and is adhered to the deck.