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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It sure can be frustrating when you can't get straightforward answers from manufacturers. I just recommended this product to another user yesterday and I think it might work for you too.
Pro Clima's Solitex Mento 5000 is rated at 38 perms and can be exposed for 6 month. They have some accessory products that help you create a "temporary roof." And the folks at 475 are very helpful, if you have any technical questions.
Please let us know what you end up using. Here's a link to the product page: https://foursevenfive.com/solitex-mento-5000-connect/
Don't screw the metal panels directly to the SIPs. The recommended method by Foard Panel is 15# felt directly on the top of the SIPs with the inside seams taped with Siga Russin Tape. 2x4 strapping at 24" OC flat on top of the felt, another layer of 5/8" OSB and then your metal friendly underlayment on the 5/8" OSB. I personally like Rhino U20 underlayment. This assembly creates your vented roof and protects the SIPs from the heat of the metal roof also. NYS IRC doesn't require any Ice and Water barrier under Metal roofing panels but verify in the California code if it is required or not.
Thanks for the recommendation. That product is what I had envisioned for my application. I still have questions and will follow up with the 475 folks.
I suspect the method you describe would be overkill for my climate, but I went to Foard’s website to see what they had to say. I reviewed information in the “Foard Guidebook.” While they recommend vented roofs in general, they also have specific recommendations for various roof types. For “Pre-formed, ribbed metal roofing panels" they do not indicate strapping/furring or additional decking as they do for shingle roofs ( "SIP, felt paper, steel roofing panels”). However, they recommend using ONLY conventional roofing felt (vs. permeable membranes). So while Foard Guidebook seems to endorse attaching corrugated steel panels directly to SIPs, they are very clear about not using anything other than roofing felt as an underlayment.
As my original question was what underlayment to use, and hoping to find a better/more durable alternative to felt, I'm back to square 1.
Considering my climate and measures to mitigate humidity indoors, I’m more concerned about exterior sourced moisture; windblown rain, inevitable leakage, and humidity in outside air. I like the fact that with out any furring or purlins, the channels created by the corrugations will allow leaked rain and condensate (e.g on back side of metal roof) to drain by gravity, and to evaporate and vent. I believe that the silver reflective finish will moderate heat transfer, as will the corrugation “vents.”
I will follow up once I make a decision.
I agree with you that strapping can cause as many problems as it claims to solve.
As to underlayments: Permeable membranes and felt generally have very similar characteristics in terms of drying, so there is no reason to choose one over the other as your roof manufacturer has done. Most roofing manufacturers recommend synthetic underlayment rather than felt because the high heat they roof generate can melt the felt causing it to deteriorate quickly.
Are you in a fire zone? If so you might consider using 15 lb felt on the SIP roof, 2x4 furring strips, 5/8 or 3/4 plywood and then a fire resistant product like Versashield under your metal roofing panels.
The heat from a wildfire can radiate right through a metal roof panel and set the OSB layer of a SIP panel on fire.
Bear in mind that if you do decide on a "cool roof" (the extra layer of furring and plywood) you should install special fire vents at the eaves and ridges.