I just bought some 1/2" plywood from HD for a project, and the best piece I could find was still pretty warped. Is there a way to fix this? Currently, it's sitting flat on my garage floor with a sheet of 3/4" plywood on top of it. Other than adding more weight (maybe some half-empty paint cans?) is there anything else I can do to help it flatten out?
Most corespondents are right in that the warp is caused by an imbalance of moisture. The problem has little or nothing to do with the type of store from which it was bought because even if it was absolutely flat in the store if you store it incorrectly when you get it home it will warp. This is especially true if you stand it upright anywhere near a radiator or other heat source.
What happens is the side nearest the heat source drys out too quickly causing the wood to shrink on that side resulting in a 'bow' or 'warp'
Now the thing is to rebalance the moisture content. This is accomplished by wetting the CONCAVE side (hot water works best) then laying the board CONCAVE side down and applying weights to flatten the board. The 'wetted' side will expand as moisture is absorbed on that side and the board will flatten. Some people like to apply heat to the CONVEX side. This will certainly speed up the process but you have to be careful not to cause the warp to reverse. Check the process from time to time. As soon as the flatness you want is achieved store the board flat. It is usual to 'condition' newly purchased material for at least 48-hours in the environment in which it is going to be used, before cutting and fixing. Hope this makes sense. Albatross
I ended up leaving the warped 1/2" plywood under the 3/4" plywood and some paint cans for about a week. The plywood was visibly less warped afterwards, and almost lies flat on the ground with just a slight arch in the center. Not sure if it was significant, but it rained a few of the days, so humidity levels might have played a factor.
(adding this because I came across it while reading about installing plywood T&G subfloor)
A warp means the convex face is larger than the concave side.
Set it out on a sunny day, with the convex face towards the sun. As it dries, it will shrink.
To help things along, add moisture to the concave side. Either a light misting of water (spray bottle or hose) or lay it on the ground, exposed to the earth's moisture.
Watch closely, because if you leave it too long it will warp the other way!
I think that these answers may be right in a small range of specific circumstances, but not right in mine. My plywood pieces had been sitting indoors in a constant environment, with both sides exposed to the same air, and they warped and they stayed warped.
If these answers were correct, plywood cabinets and furniture would be writhing around all over the place all the time.