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Attaching the finished plywood on pressure treated plywood??

15 years ago

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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Comments (7)

sombreuil_mongrel
15 years ago
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.
Casey
ashug987
Original Author
15 years ago
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.
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That top pic is beautiful, but I would have several concerns 1. Getting it flat. If your subfloor isnt perfect, then it is likely you will have dips in your final surface. This isnt as noticeable with normal hardwood, but with large sheets, it may be very noticeable. 2. How do you attach it? You dont want movement, not so much side to side, but bowing up in the middle or at the edges. I suppose you could glue it down, like how many of the engineered hardwoods and laminates are installed. 3. Getting the edges smooth where one sheet touches the next. There may be variations in your subfloor and in your plywood thickness, so you will have to sand the edges, and hope that they never bow up. But you have to be careful or you will sand right through the outer layer of the plywood and the core will be exposed. I think if I were going to do this, I would put down solid 3/4" tongue and groove as my subfloor. Then I would put down 3/4" sheets of plywood...maple, birch, oak, cherry...whatever species you choose to go with. These sheets are extremely smooth from the lumber yard, so I dont see any need to sand them further, except where one sheet meets the next. I would use a small hand orbital sander and 180-220 grit to smooth out the edges, the big sanders will eat through it so fast that you will likely ruin it. I would also probably want to put down at least a few mechanical fasteners (aka, nails) as well as glue. Id probably use a nail gun with 16 gauge nails, and mainly go around the edges. Be sure to putty the holes. Then stain it all and poly it, just like you would a piece of furniture. Because that is basically what you are using, cabinet grade plywood.
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User
15 years ago
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.)
ashug987
Original Author
15 years ago
Thanks Handymac for your reply
marys1000
15 years ago
I wonder if priming and painting the finish ply with a good marine paint, both sides would work as well? Just asking for reference.
User
15 years ago
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.
sombreuil_mongrel
15 years ago
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?
Casey

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