Keeping Plywood Flat

Pros discuss causes, cures, and prevention of warping in plywood. July 9, 2005

I am wondering, what is the best way to stack 4x8 sheets of plywood? Also, are there any tricks I can use to flatten some cabinet shelves that have bowed/warped?

Forum Responses
(Cabinetmaking Forum)
From contributor R:
I do not think that it is possible to flatten bowed or warped plywood. I once had a couple of sheets of 3/4" red oak plywood that were badly bowed. I lightly moistened them on both sides and then lay them on the bottom of a stack of 40 sheets of 3/4 melamine.

Although I always knew they were there, I left them on the bottom of the pile for nearly 10 years (of course the stack of melamine would sometimes dwindle down to a couple of sheets but it would get replenished promptly). Then one day I needed to pull those sheets of oak plywood when I ran short on a job. Those sheets quickly sprang back to their original bowed state in a matter of minutes. I think that a plywood straightener can only be purchased from the same outfit that sells board stretchers.

From contributor J:
The best way to stack 4x8 sheets of plywood is horizontally flat. I would recommend turning the shelves over and letting the crowned plywood work in your favor to defy gravity.

From contributor R:
I would suggest laying them on the grass in the sun during the summer with the bow up. The moisture from the ground will wet the short side of the plywood, and working in combination with the sun drying the longer side of the sheet, it will be flat as a pancake in half a day.

From contributor S:
Contributor R is the closest to a correct answer, if one exists. Putting the sheet on the grass will do two things. One, it will introduce moisture into the dry side, and two, it will remove moisture from the wet side. If this is a moisture problem, it will correct it.

When you store your panels flat, you need to make sure that you have a cover sheet on top - this well help to maintain equal moisture on both sides of the panel. Otherwise it will cup/bow. It will shrink to the side that has less moisture (dry side). Think of it as shriveling up.

Introducing moisture to this side or removing moisture from the other side will help to correct this. Putting it on the grass in the sun will both introduce moisture and remove it. This is not always the cause of warping, though. Sometimes the panels are not glued well, and there is not anything that you can do to change that.

From contributor K:
Moisture is rarely the problem. When it is, both sides exposed to air will flatten the sheet. If they're flat, they'll stay flat unless temporarily unbalanced on one side by moisture. Some nice looking knotty alder was giving me grief with its bow shape. After looking closely at the edge of the sheets I realized they weren't symmetrical - that is, some on one side were different thicknesses than those on the other side.

There really isnít any solution except maybe moistening one side and working fast. This isnít any help if it's to be used as a refrigerator panel though. I have a piece of DF 3/4" ply about 22" x 66" that is over 40 years old. It's been used as a ramp, table-top on saw horses, and generally abused in every way short of using a flame-thrower on it Ė and it's perfectly flat.

From contributor H:
To the original questioner: The best way to store plywood is in pallet racking, with a cover sheet on the bottom, and one on top - ten feet racking works well if you are using standard 4x8 plywood. Itís flat, easily accessible, takes up little space, limits humidity problems, and isnít too expensive.