Proper metal roof installation.
William Dempsey| Posted inGBA Pro Helpon
We are building a new home in Southeastern Pennsylvania. We are having a Drexel Metal standing seam metal roof installed over plywood sheathing. The attic is unconditioned space with blown in insulation to a value of R-60 or so. Should the metal roofing be installed over strapping. We will have a vented soffit for air circulation.
when discussing with the roofing contractor he is adamant that this is not necessary or appropriate. My thought is that the metal should be separated from the plywood.
Thanks,
William
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Replies
Not sure which product you are installing, but I just cruised the Drexel site for a couple of minutes and found instructions for one snap-lock product indicating it could be installed over plywood. I would download their manual and follow it exactly, and if you have any questions, call them directly.
You need to use a synthetic underlayment between the metal roofing and the plywood or OSB. Also be aware that when you build your roof and sheath it, plywood clips may dimple out as small bumps on the metal. If you want a darker color this will be more visible. Ask your metal roofer about this before you have the issue as you may be unhappy when you see little bumps in the metal from the clips. In order to eliminate this issue, find a way to not have the clips. That could mean setting trusses 16" o.c. or using thicker sheathing like 5/8 or t&g products like Huber Zip...
Joe,
Our code allows 1/2" plywood to span 24" without H clips, and as I recall from discussions with builders down south the IRC does too.
William,
Many types of metal roofing are routinely installed over plywood roof sheathing. All types of roofing need roofing underlayment between the roofing and the roof sheathing.
David Meiland,
Thanks for the information. I followed up at the Drexel web site as you suggested and indeed found they specify the product can be installed over plywood.
Thanks,
William
Joe,
Thanks for the helpful information. We are using GAF Tiger Paw synthetic underpayment beneath the metal roof. As for the plywood clips I appreciate the heads up and will be sure to discuss with the roofer.
Thanks,
William
Martin,
Thanks for the follow up. I failed to mention that we are using a synthetic roofing underlayment. GBA is a very helpful resource for me to ask questions and get helpful feedback in a timely manner. I appreciate the help.
Thanks,
William
This point always gets an argument but don't forget, metal roofing ALWAYS has underside condensation in spring, summer and fall, and you would be surprised at the quantity of water. Sheathing needs protecting from this and if your purlins or strapping is horizontal, flat to the deck, and where the water's (condensation) downhill flow (underside of the panels) is impeded by contact with the metal panels it will pool on the uphill sides of the purlins.
Flitch,
I have installed many metal roofs on 1x4 or 2x4 purlins on asphalt felt on solid roof sheathing. Condensation does indeed occur, but in all cases it evaporates harmlessly before it has a chance to get past the asphalt felt.
I realize that my experience is irrelevant to the situation discussed here -- installing metal roofing on plywood without purlins. But my experience shows that the "pooling" you fear is not necessarily a problem.
Flitch,
Thanks, this is something I will keep in mind.
William
William,
Condensation forms when air moisture laden air meets a cold surface. Installing metal roofing directly on synthetic underlayment on plywood, where all the materials are in contact with one another, means there isn't much air to provide condensation. What little occurs evaporates or makes its way down through the small gaps in the panel's profile.
I will discourage the use of 1/2 sheathing at 24" on center if you are able to afford the up charge to the 5/8 variation. I think you will be happier with the look. Martin, being a roof man at heart may back me up on this particularly if you are using a darker color where you want as solid and straight a roof to set those panels on with as little deflection as possible. If any of you have done any auto body work or have been into fancy cars, you know that what is underneath is important, especially in a black or dark colored car if you will think of it that way. A standing seam metal roof is not only functional but beautiful and you don't want any unhappy surprises or disappointments in the way the roof sets up on that building for all the world to see.
Joe,
Thanks and as it so happens the sheathing is 5/8" cdx 24" on center. I appreciate your advice.
William
乔和其他人——为什么是合成地毯衬specified ? There are many older metal roofs installed decades before synthetics became available. Is there some big advantage to these newer materials when using metal roofing?
Kevin,
Synthetic roofing underlayment is tougher than asphalt felt. It holds up better to job-site abuse. Because of the fact that metal roofs can get some condensation on the underside of the metal, it's more important to avoid holes in the underlayment with metal roofing than with asphalt shingles.
That said, asphalt felt can work fine. I like to install #30 asphalt felt, not #15 asphalt felt, under metal roofing. Install it with care, and don't put random holes in it.
Kevin,
Some manufacturers specify synthetic underlayments when installing directly on a substrate. Building paper can get too hot and adhere to the underside of the metal panels causing it to deteriorate.
THANKS,Martin and Malcolm.