Why two layers of 1/2″ plywood over a slab with 1″ foam on it compared to one layer 3/4″
ScottVeatch
| Posted in General Questions on
Why two layers of 1/2″ plywood over a slab with 1″ foam on it compared to one layer of 3/4″. I am having trouble sourcing the 1/2″ advantec but 3/4″ is readily available!
If double 1/2; really is that much greater is it better uning standard CDX ply compared to the one layer of 3/4″ with Advantec benetfits?
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My understanding is to mitigate any bowing in the plywood. Two layers overlapping all the seams ensures each end is screwed to the middle of the other layer, locking everything together and flattening it out.
You can use a single layer of 3/4" as long as the tongue-and-grooves engage to keep the panels aligned, and you screw through the foam into the concrete below. Two layers of 1/2" (or 5/8", or 3/4") are self-aligning and can float over the foam without fastening to the concrete. https://www.finehomebuilding.com/2005/03/01/the-no-mold-finished-basement
The idea with a double layer is you offset all the seams, screw the two layers together and you end up with what is effectively a single piece for the entire floor. It's heavy enough that it won't move so you don't have to fasten it to the concrete.
I hadn't heard of this. I was planning on trying to cheap out a use a single 7/16 layer on my slab under my 3/4 hardwoods. Now I might go do some more research
Eric_U,
The other advantage of two layers is its ability to spread loads.
I've used a single layer of 1/2" plywood over foam in a backyard office. The foam has adequate strength in terms of PSI values, but doesn't do well with point loads, so when you screw down the seams, you get noticeable compression at the edges.
Probably the biggest reason to use two layers over a slab, is not having to Tapcon the whole floor, which is an awful job.
I was thinking 1" nails plus a polyurethane adhesive, that way the nails wouldn't hit the slab. I guess that might not be strong enough?
If you use an adhesive nails are usually just important before the adhesive has set. Once it’s set it will provide all the strength. I would probably just use standard liquid nails or sim. Vs polyurethane
I'm thinking nails would bounce all over the place if you tried to drive them into plywood over foam. A nail gun might work but I don't know of one that would drive such short nails. Maybe a roofing nailer?
Much the same as laying a plywood underlay on subfloor: The adhesive does the work. You typically use an air-stapler to keep the two layers tight while it dries.
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I would not use 1/2" CDX.
For best results you want something t&g, which means 5/8" minimum. OSB also tends to be much flatter and stays flat (does swell a bit) if it gets wet.