Unvented Low Slope (1:12) Roof Assembly
Hi GBA,
I need help with a DIY remote off grid cabin build. What I originally thought was going to be a renovation has turned into an almost entire rebuild. To move forward I have to get my roof sorted out so I can begin working on the rest of the structure.
I have read just about every article Martin Holladay has written on the topic of roofs along with the often longer comment sections, and thought I felt confident regarding my roof assembly but now I have doubts.
At the moment it is a 1000 sq/f , 1:12, mono-pitch shed roof, with 2×8 rafters, spaced 24″ o.c. with a 3/4 CDX Deck in California Zone 14/US zone 3B. I am located between the foothills of a large mountain range and the High Desert of the Mojave. So we experience some snow/rain never in large quantities but more often it is sunny/HOT/dry.
My plan is an un-ventilated cathedral ceiling (attachment 1 from Martin Holladay’s 5 – “Cathedral ceilings that work”), with a metal roof and minimal roof penetrations, max 2 (wood stove pipe and plumbing vent), supporting a 15 panel solar array.
1. Drywall (With no penetrations)
2. Rockwool R-30 inside 2×8 rafter cavity
3. 2×8 rafter
4. 3/4 CDX
5. 1 layer of 2″ Polyiso or 2 layers of 1″ polyiso staggered taped seams R-13
6. 5/8 CDX
7. Peel and stick underlayment (low slope/high temp)
8. Standing seam metal roof (mechanically seamed, 24 gauge)
Does the peel and stick add enough to justify it, in addition to the taped Polyiso layer, or should I omit it? Should I add an air channel below the metal roof? or is that inadvisable with a low slope roof?
My concerns are using the right underlayment (many manufacturers say their products are not rated for low slope roofs). I would like to use a metal roof because it allows solar to be mounted without roof penetrations, but I would consider EPDM or TPO. Solar mounts and a rubber roof just feel like a bad combo to me and a recipe for disaster on an un-vented roof.
Also I know my Foam is higher R than “required” per table R806.5: CA-14 = R-5 But I am on the edge of CA-16 = R-15. So having 2″ Polyiso for R-13 felt like the best compromise.
Thank you! – Jonsie
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Replies
If you can bump up the roof to 2:12 lot of things become simpler as water on 1:12 moves up hill. Most metal roof manufacturers require peel and stick under the metal at that slope.
You are not in snow country so top venting is not needed. Your proposed unvented assembly will work great in your climate.
If you are in a fire area, make sure to go for a rated roof assembly including the proper underlayment.
The air barrier is best done on the 3/4 roof deck. I would tape the seams of that and connect it to the wall air barrier. No need to tape the foam above it.
Peel and stick under the metal is only needed if your roofing manufacturer requires it. For metal roofs with exterior rigid, I like to put the peel and stick over the lower deck, this serves dual purpose as air barrier (saves taping the deck) plus gets the place dried in before the rigid and metal roof goes on.
Thanks for the reply. Unfortunately the horse has left the barn regarding adjustments to the slope.
Absolutely, regarding fire rating. It is a major concern for us and hence the desire for metal roofing/polyiso to combine with the planned cement board siding we will be installing as well. So far I can't find a peel and stick underlayment rated for both fire and low slope... any suggestions?
Originally I wanted to add my peel and stick to the lower deck but was confused by wording in the code saying the exterior rigid had to be directly against the deck.