Underlayment for corrugated steel roof on SIPs
My question is what would be a recommended underlayment/membrane for a corrugated steel roof attached to SIPs. The house is in the Central Valley of California (Hot/Dry).
I’m building a straw bale house (modified post & beam). SIP panels over trusses/rafters form the structural part of the roof system. Corrugated steel panels (galvanized silver finish) will be used for the the exterior roof cladding. The roof has a pitch of 6:12 with a smaller “dormer” section at 4.5:12.
I am currently planning to screw the corrugated panels directly to the SIP panels with a breathable membrane as a roof underlayment. The breathable membrane is meant to allow breathing/drying of the SIP sheathing from unintended moisture that emanates from the interior or leaks from the exterior. I’m assuming the corrugations in the metal roof will act as a ventilation and drainage channel, as well as reducing heat transfer to the SIPs.
The corrugated roofing cannot be installed until the “dormer” windows are installed, and the walls in this area are plastered. Windows and plastering are delayed until completion of the straw bale infill (progressing slowly). For this reason I’d also like a durable membrane/underlayment, which can take months of direct exposure to summer weather, and stand up to plastering crew traffic (3 coats).
Asking again, what would be a suitable breathable roofing membrane underlayment, or what would be a better approach?
It seems the SIP manufacturers and the roofing product manufacturers are either not willing to make a recommendation, or recommend something intended for standing seam roofs or more challenging climate scenarios.
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