I am in the process of doing a solid 3/4" over concrete slab, and I have leveled the slab, installed 6mil plastic, and just installed (screwed) 3/4" CDX plywood. One concern I have is the edge of one sheet sometimes is maybe 1/16" higher than the sheet adjacent (but spaced at least 1/2" away) from it. It is not because the floor is not flat. After 500 lbs of concrete added, that damn floor is flat! I think it's because the plywood is warped (came that way). I was a bit shocked at how bad this plywood can be, but I am at a point where I cannot return it. The issue is that at the edges the plywood still can warp up slightly even with screws installed (I shudder to think how a full floating install would have gone). I was thinking about using a belt sander to just hit the edges a bit so there are no high spots. Should I consider anything else?

Also, is this warpage normal? Some of these boards are warped in more than one direction! Some have waves! I checked moisture content, and it is around 6%, same as my solid wood. Could I just use a higher grade plywood for the other rooms I plan to do? Thanks!

I am not sure what you are referring to as warping. But if there are waves in the middle of the plywood it is not normal. As for the ends, plywood is not a precision milled product and there will be slight variations in size. Many stores will sell two sizes that are very close in the same lot Home Depot is good for doing that. There are a few things you can do to get rid of them if they are a major problem.

1. #30 felt paper will level it.
2. You can sand them.
3. Get a cheap electric planer ($19 to $30) and plain them (fastest method).

The ends are not an issue. The warping is like cupping on a grand scale, but sometimes it is in more than one direction. I just expected the not have a 4x8' sheet cup more than say 2". One of the issues is that this 3/4" is so damn strong that it's hard to screw it down and straighten it. I have been in many situations where the screw's holding rating is not enough to pull the plywood down, and the screw slips.

I did another check, and it looks like in some areas there's a 1/8" difference in height between two adjacent boards. I guess I might have to sand those. I did plan on putting down two layers of 15# roofing felt, so hopefully that will cure most of the issues. Make me wonder why I spent so much time and money on getting the concrete flat when it seems it's almost for nothing. Next time I will hand pick the plywood sheets and try another store.

I don't know what brand of plywood you got but whenever I've gotten 3/4" CDX, I have not had that "warping" problem. I have noticed small variances between thickness but it's always been small. Sounds to me like you got factory rejects. And at 6% MC, that is low. Never seen wood that low but you are in a different region.

You could have shimmed the slab with shingles or felt paper if you where putting down plywood. It saves alot (a whole lot) of time and is just as solid as feathering out leveler. leveler is more for floating and glue installations where slab defects will telegraph through the wood. Plywood usually smoothes out most of the problems with the slab and you only have to shim up the trouble spots.

Was the plywood warped on install or did it warp after it was down? If it warped after it was down then you have a moisture issue. Don’t install your floor. If it was warped before then you just have some PITA wood. Get a powder actuated nailer and shoot it every sq ft. and see if it pulls them out.

Thanks all for the help. The wood was warped before I put it down; I just didn't realize how bad it was until I started to screw it down. I think I will just pick my plywood very carefully next time. I will also take a second look at OSB, as I swear it seemed like those boards were much more consistent.

Your plywood was some how damaged before it was installed.

Personally, if it is as bad as you say, I would remove it.(but, I would have never installed it)

Plywood should remain flat at all times.

The 6 ml. plastic may not be enough to stop Concrete Moisture. . NWFA and NOFMA suggest spreading roofing mastic over the concrete surface then laying the plastic into the mastic. If the plywood was buckling before you put it down;;; you would not be have been able to screw it down.
THe description I read suggests increased moisture content after screwing the plywood down.

Ray, you must have missed this part.

Well, I have run a hand held belt sander to all the edges of the plywood. It's not prefect, but much better. Thanks for the help.

Perry,
I did not miss that part. How does one screw down warped/damaged plywood? Slight warping can be installed but can YOU screw warped plywood to concrete? It's been awhile since installing full time, but I do remember the agony of working with plywood and separating bad sheets because we could not get it to stay on the floor much less be flattened. We did have success at nailing buckled plywood to wood joists, but never to concrete.
Is there a trick I don't know about? How do you attach buckled plywood, using screws, directly to concrete - through plastic.... and get it to stay attached to concrete surfaces?
The physical description of the floor is: 1. Warped with raised edges. 2. Screwed through plastic to concrete. 3. No statement concerning concrete moisture tests.
This physical description may indicate an increase in moisture content after installation. Looking at the description of the installed plywood ONLY gives credence to looking at the possibility of moisture content increase after installation. Wise men look at all possibilities for purpose of eliminating costly failures.

There is a way, think about it.

I do 1x8 and kerf the heck out of it.

There is a way? Tell us Perry. How do you do it?
Thank you for the response Chuck.
Why would you go through so much effort? I also think you (Chuck) would check the plywood upon purchase to avoid problems on site knowing how difficult it is to install buckled plywood. I admit it......... I was a lazy installer

The screws I used have 210 lbs of holding power, and it needed most of that to get the plywood donw in somw areas. I basically had to stand on the area I was securing while putting in the screw. Before all of this, I did do a "tape plastic square" moisture test, and there was absolutely no discoloration on the original slab. FWIW, not sure if it matters, but this slab does have a moisture barrier -below- it. On the concrete patches, I did not do the same but tested using a wood moisture meter (ok, not the best tool for the job), and it started at around 18% and I waited about a week until it settled to 8-9%. I honestly don't think the sheets were warped becuase of a moist slab. Will they warp even more? Time will tell I guess. I won't be installing any wood for 3-4 days, so I will test the plywood for a change in moisture content before I start.

Also, all screw holes got a large dab of roofing mastic before inserting the screws. I verified it actually made it to the vapor barrier because I had to take 2 sheets off and trim them (to close to walls). Every hole had a liberal amount of mastic.

So, hopefully this is a case of just bad plywood. FWIW, Not every single sheet is horrible, just most I did take a look at some AC grade plywood, and wow, that stuff looked -far- more consistent & straight. For $2 more per sheet, I think I will go with that next time.