Attaching the finished plywood on pressure treated plywood??

ashug987June 17, 2008

I am making a shed and for flooring I am using pressure treated plywood and then finishing it with AC grade plywood.

How should I attach the finished plywood on top of the pressure treated subfloor plywood. Since there is a moisture difference, do I need to put some sort of vapor barrier in between? If yes what kind of?

What kind of adhesive I should be using to attach these? Or should I just nail it on the joists locations?

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Why do you think you need PT ply? I believe if you back-painted good fir exterior AB ply, and placed strips of tarpaper on the joists, allowing some air circulation beneath the structure, you'd be better off than double-layering the floor.
If for some compelling reason you must use a second layer, place sleepers (firring strips) between. 2 layers of plywood in contact in a situation that will likely be damp is asking for trouble.

    Bookmark   June 17, 2008 at 8:31PM
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Thanks Casey for the reply.

The another reason is that I want to life the floor higher, so going with the two layer of the plywood.

Also I was told by the HomeDepot guy that I should be using the pressure treated plywood at the bottom (especially in our pacific northwest weather) and if I want a finished look, add another later on top of that.

    Bookmark   June 18, 2008 at 1:22PM
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Using the PT ply in your case is a good idea. I recommend screwing the PT ply to the floor joists with 2&1/2" deck screws) You can add glue i9f you wish.

To add a finish layer, install(1/2" staples) a layer of 15 pound roofing felt---overlapping each row about 4" and install the finish ply---making sure to not place the new sheets exactly over the PT ply.(you will only need to make sure the first new sheet is different---each succeeding sheet will be different automatically)

Then screw the new layer to the PT with 1&1/2" or 2" wood screws(deck screws so the PT ply will not rust them.)

    Bookmark   June 18, 2008 at 6:55PM
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Thanks Handymac for your reply

    Bookmark   June 20, 2008 at 4:10PM
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I wonder if priming and painting the finish ply with a good marine paint, both sides would work as well? Just asking for reference.

    Bookmark   June 22, 2008 at 7:27AM
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Paint will not work as well as the pressure treatment for two reasons. First, insect damage is not prevented by paint and second, paint will only slow moisture migration. Add to those reasons the fact paint has to be renewed periodically to keep protecting---meaning getting to the underside of the flooring is necessary---makes that option less attractive.

I spent two years near Tacoma, Washington and quickly learned that if there is not rain nearly every day, it is unusual.

    Bookmark   June 22, 2008 at 8:41AM
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If there's that much moisture to begin with, a sandwich of two layers of plywood is a guaranteed water trap, which could end up just as bad. It seems to me that I'd throw out the idea of a plywood floor all together and have slats, like 1x4 or 1x6 (or 2x if required for strength) so the moisture can move in either direction, and individual boards can be replaced as needed. It is a shed, right?

    Bookmark   June 22, 2008 at 11:56AM
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