I am re-roofing a 28′ x 32′ unattached, unheated for the time being garage. The current 7/16″ plywood on the roof is wavy. I am going to re-nail plywood and then go over top plywood with 7/16″ OSB. Is there going to be a problem with condensation or moisture between the OSB and plywood? Is it a bad idea to lay the OSB over the plywood?
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I don't see a problem with moisture.
But you should figure out why the ply is wavy. If it is nearly falling apart from rot or delamination, it should be replaced. It would not get any better and soft spots could eventually impact on the OSB.
For instance, if it compresses, the OSB will be floating proud and some of the nail;s holding the OSB would then show proud above its surface. Those nail heads would wear out the shingles from the back side.
It is a judgement call, and since I can't see the condition of the ply from here...my judgement is not worth much
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Piffin had good answers.
Moisture would only enter into the picture if the garage were heated and/or being lived in. Heat sources can add moisture, as can cooking, taking showers and so on, but that's seldom done in a gargage (although the owners of my house lived in the garage while building the house).
well now, suppose the previous owners washed the car INSIDE the garage?
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Yeah, that could cause problems--or doing laundry. My in-laws have their washer and dryer in their garage in Fla. Up here, my idiot cousins put a hose out onto a screened porch at out hunting cabin and took showers there last summer when they visited the cabin on a vacation. Fortunately the floor is concrete. It's sort of my fault because I convinced my dad to yank out the old shower with the rusted-through pan that tipped and wobbled when you stood in it because the drain was installed too high (and, as it turned out was cross-threaded, not plumb and leaked slightly), then we never got around to putting the new shower in. (Though it's up there, still sitting in the box and I built a support for the new pan and have measured the drain and it just needs to be glued in, but Dad is reluctant to actually do it or let me do it.
LOL, one of those Dads huh?
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