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I'm doing renovations on my 60's house in the southern California desert and I need to make penetrations for plumbing vent pipe and a bathroom fan roof cap in a low slope spray foam roof but I haven't been able to find any information on the procedure. I'm not sure what's under the foam but my guess is rolled asphalt. The foam is as little as 1" and as much as 2" thick.

For the roof cap can I assume I'd cut the foam about an inch larger than the roof cap, caulk underneath and screw it down, then make the patch with something like Henry's white elastomeric roof patch, mating the patch with fabric tape, then a few coats of elastomeric roof paint.

Can I use Great Stuff to build up the foam? What kind of caulk under the roof cap?

g-ulrich
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    Quite sure most spray foams available to DIYers do not do well outside in the sun, especially on a roof. Your idea for patching the vent sounds good, but think a local roofer can give better ideas of what that foam is. – crip659 Feb 09 '23 at 18:16
  • Pretty sure the stock spray foam roof approach involves a coating on top for the sunlight exposure. Unfortunately, they are "weird" in this area, meaning you can't find someone who does them without importing them several hundred expensive miles... – Ecnerwal Feb 09 '23 at 19:24
  • Yeah that’s exactly right. The foam is coated with elastomeric paint. Exposed foam will break down in the sun. They’re really popular in this area because they’re supposedly a 50 year roof with an R value and the white color is great in the desert. – g-ulrich Feb 11 '23 at 03:59

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