Zone 5 metal roof
I’m building a small carriage house in Zone 5 Colorado and have a roofing question. I’m planning a simple gable design 4 in 12 sloped metal roof (peak oriented east-west for later solar install on south face). Conditioned attic with closed cell spray foam under the roof sheathing for the first 2.5″ then open cell to achieve R-38 total (that’s the prescriptive roof insulation requirement here and after doing a fair amount of BeOPT modelling i couldn’t justify taking it higher).
In conversation with some local GCs and roofers, all are saying with the heat of a metal roof that a peel and stick membrane is necessary on top of the roof sheathing. My understanding is that’s absolutely wrong, with the right reflectance and emittance a metal roof will be significantly cooler. Further, with the closed cell against the underside I’m not too keen on a vapor closed barrier on top of the sheathing (with the vaopr closed foam underneath), meaning if water does find a way in to the roof sheathing it doesn’t have a good path to dry out. What would the recommended underlayment be in this scenario?
Also, I’m looking for cost effective metal roofing. Martin had suggested through-fastened metal, but wouldn’t that lead to leakage at the fasteners with expansion and contraction of hot/cold cycles?
Thanks in advance
Tim
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Replies
Tim,
A properly installed through-fastened metal roof won't leak. Anyone who tells you that this type of roof will leak doesn't have enough experience to trust.
A standing-seam metal roof allows no outward drying, so the vapor permeance of the roofing underlayment is irrelevant.
If you want to design a through-fastened metal roof to dry to the exterior, you can -- one approach is to install the roofing on 1x4 or 2x5 purlins, installed above the roofing underlayment. Talk to your roofer or the roofing manufacturer if you think this is an approach you want to take.
If you are planning a roof assembly that allows for outward drying, you can specify a vapor-permeable roofing underlayment if you want (or you can use asphalt felt, which allows outward drying). If you are worried about high temperatures, specify a high-temperature roofing underlayment (but be aware that this type of roofing underlayment is vapor-impermeable).
You don't need peel-and-stick underlayment, but peel-and-stick underlayments are one option.
Thanks for the response, Martin.
My preference is through-fastened rather than standing seam for the cost benefit. I'd assumed if it was corrugate or a similar shaped profile it would allow for some air/moisture movement underneath (but also thought purlins were a wise choice to increase the drying capability). I have received some pushback that purlins would likely lead to waviness in the metal, but I'm sure that depends on the gauge of the metal and gap between purlins.
Do you think I'm being overly cautious in wanting a vapor open underlayment on top of the sheathing so the sheathing has a drying path in the case of a leak? Would you just go with 30# asphalt felt and call that good?
I'm not concerned about high temperature. i think that's a myth that proper reflectivity and emissivity choices dispel.
Tim: review comments from this very experienced builder:
https://www.youtube.com/watch?v=v0w21Bg1HJI
Keep in mind that anything that isn't 100% airtight (a metal roof isn't close) will move moisture. Even more so when exposed to wind pressures and temperature extremes. An air channel makes it consistent. Experience with walls (perhaps relevant, perhaps not) shows that as little as 1/32" is effective (see bsi-111-double-play).
About 5/16":
https://www.dorken.com/en/our-products/products/commercial/delta-trela.php
Thanks for the link, Jon. I've seen a lot of Matt's posts and found them to be informative. I'll check it out!
Tim,
Check the installation instructions of whatever metal roof you choose. None of the ones I use allow installation over mesh substrates.
On the larger issue: It's an open question whether you get any useful drying by installing a very small air-space with no dedicated drying path. There has been a lot of discussion on the topic here on GBA, but so far no evidence they are effective.
We put a screw-down roof on the garage and then went to a snap seam Fabral roof for the house. There are very similar profiles from different manufacturers. I bought the tools to do some of the required bending at eaves and rake as my "roofer" didn't have them. I installed over a synthetic underlayment directly over zip sheathing. It's a nice looking roof that will last longer than a screw-down roof, but the extra cost for materials and labor may not be worth it (maybe $3/sf). I can repair a screw-down roof myself but I'll be in trouble if this roof gets damaged. I took great care to have zero roof penetrations.