Bumpy plywood – will it cause issue for metal roof
I’m working on a roofing project where I’ll be putting in a new deck with 19/32 plywood. This particular plywood pictured is Plytanium brand that I purchased from Lowes. I bought about 30 sheets of this all from a brand new pallet they brought from out back. Because it was only 10 minutes to closing time when I pulled these I rushed somewhat and with the lighting in the store did not notice the surface finish and bumpiness.
I’m putting a 26ga standing seam metal roof over the top of this (black color), with a peel and stick membrane under it. I’m worried that because of the black color that this bumpiness could show thru in places on the metal roof panels. I’m planning on taking it back to Lowes to exchange for sheets without bumps. As regular wall sheathing or under a shingle roof I don’t think this would matter, but in my case I’m hesitant to use this.
Anyone else ever seen sheets bumpy like this before? I searched the internet and couldn’t find anything like this anywhere. I don’t know what would cause this other than some manufacturing issue as I’ve handled a fair amount of plywood in my life and I’ve never seen this before.
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Replies
fingol,
Not great plywood, but it wouldn't be an issue under metal roof panels. Many underlayments are attached with cap-nails that protrude more than those irregularities, and they don't telegraph through. You may get oil-canning, but I wouldn't attribute it to the plywood.
Thanks, appreciate you sharing! I think I mostly was concerned about the oil-canning effect because of the color being black, the panels do have striations between the ribs so hopefully that would help.
You'll never see that. A 26 ga panel still has plenty of stiffness to bridge minor imperfections like that, especially if the panel has striations between the edge ribs. If it's a snaplock panel, the panel field tends to sit proud of the deck anyway, making a minor airgap.
Thank you, that is reassuring to know. The panels do have striations between the edge ribs, and they are a snaplock seam type. I had wondered that very thing you mention, that because the way the panel attaches to the deck it would have a small air gap that might further nullify any protrusions above the deck from the sheathing finish. But at the same time I was erring on the side of caution because it would suck to finish the whole thing only to find the imperfections showing through, so I thought maybe it would suck less to just take the 30 sheets I've got back to Lowes and exchange them. But it sounds like it's totally unnecessary.
The important bit to prevent oil canning is having your roof deck dead flat and not over constraining the panels. Generally it is best not to pin the panels and always install screws in the middle of the slot to allow for thermal expansion.
Akos,
"it is best not to pin the panels and always install screws in the middle of the slot to allow for thermal expansion."
I do that but I also put one gasketed screw into the panel in the top corner so that it expands and contracts around that point.