Hello All-

I have a 5x9 frame and I just attached a 4x8 piece of plywood (1/2" thick). For the rest, I am going to cut down another 4x8 into a 1x5 piece and a 1x8 piece. I am attaching with drywall screws per the manufacturer's suggestion. My question is when I am lining the plywood up, should I leave a 1/8" space for the wood to expand like construction sites recommend, or can I line it up flush? For what it's worth, I am building in a basement that runs between 25 and 50%(ish) humidity. I will be covering the wood with 2" insulation foam, which obviously will be flush.

Thanks!

Original Post

I am sure you will probably get varying opinions here, but I used 1/2" plywood on my 6' x 16' layout. Plywood was cut into 6 pieces, 2' wide by 8' long all butted as tightly together at the joints as I could get them. That was 5 years ago and no problems so far.

The layout is in an unfinished basement, but I have a couple of vents in the main ductwork to condition the space. The vents remain open year round. Humidity averages about 35% to 55% year round. It's 53% right now and our outdoor humidity was very high today, it's raining.

 I would be more concerned about the wood you are attaching it to - is it dimensional lumber ie 2 x 4 or 6 since that type of lumber is more affected by temperature and humidity.  If high humidity is a concern, use exterior grade plywood.  I have a fairly leaky basement and I store my modules down there and I don't have issues.

Brendan

Plywoood's construction of alternating layers, each layer has its grain pattern rotated 90degrees from the prior layer. This effectively eliminates expansion/contraction, thus you can put each piece of plywood as tight as you want against another piece of plywood (talking edge against edge).

One thing to consider is where the seam created by butting two pieces of plywood together is created. Is the seam supported sufficiently for your application? For an extreme example, if your 1x8 foot piece butted against your 4x8 piece creates an unsupported seam and you try to stand in the seam area, you foot would probably go through the seam.

Hi everyone, thanks for the advice! I have to confess that I did not consider the seam where the plywood sheets come together. I solved that by attaching a shim underneath with wood glue, I'll be putting my 2" foam insulation board down and over the seam as well so that way the "holes" don't' all line up with each other.

-Ken

This is 23 x 13, 1/2" multi-ply and topped with 1/2" Homasote, everything was butted as close as possible.  It's all fastened down with around 5,000 staples in the ply, and around 700 drywall screws in the Homasote.  I seriously doubt it's going to have any expansion issues.

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GVDobler posted:

I would put the plywood in the basement and let it sit for 3-4 days to settle/swell/dry-out or whatever before nailing it down.

You frame has already done that.

Depending on where you get your wood, it's probably more likely to shrink than expand. Much of the wood you get at the big boxes has moisture in it, and shrinks as it dries. (Found this out the hard way years ago building a deck.) It's good advice to bring wood inside and let it acclimate for awhile before starting to build.

Mine sat in the basement for a few weeks before I started, and it was humidity controlled the whole time.  That's as stable as it's going to get.

gunrunnerjohn posted:

This is 23 x 13, 1/2" multi-ply and topped with 1/2" Homasote, everything was butted as close as possible.  It's all fastened down with around 5,000 staples in the ply, and around 700 drywall screws in the Homasote.  I seriously doubt it's going to have any expansion issues.

If it does, you're going to end up with a bigger room !

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