Plywood or MDF warping when stored?

I had some half inch mdf that was stored on end, it definitely warped, but it flattened out from it’s own weight afterblaying flat.

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Yes. You can minimize the warping though by keeping the sheet as flat and vertical as possible (e.g. put it against the wall, and then push some stuff up against it).

-- Laziness minus Apathy equals Efficiency

I’ve had mixed results. The Chinese crap definitely will warp, curl, and separate. but if you set domestic ply and MDF on 2×4 pieces to keep them off the concrete floor and I clamp mine to a large shelf that I store 5 gallon water jugs and liquid nutrition cases on for my disabled son. This works very well for me. I have sheets of 19mm ply that have been in this rack for over a year now and no problems.

You can see my system in my shop photos. Look just to the right of the TS.

I have definitely had plywood warp when standing it on edge in the garage. It was a major pain to deal with afterwards.

I don’t have the room to set up ideal storage, so I have to lean sheet goods (and lumber) against the walls. The more vertical I keep it, the less it warps, and flipping it periodically seems to straighten it out again.

-- Adversity doesn't build character...................it reveals it.

If you have some 2x and can make some sort of holder for it that would be the best. Keep it as vertical as possible and I clamp a few pieces together to give it a little more stability. Make sure you give it good stability so it cant sag, support it well, and you should get good results.

-- Bill - "Freedom flies in your heart like an Eagle" Audie Murphy

Yup! That is why commercial shops use flat storage systems.

-- Folly ever comes cloaked in opportunity!

Often it will warp. It is not 100% certain it will but do plan for it. I saw a
design somewhere for a system that clamped sheet goods stored on edge flat. It’s usually warping on the 4’ side that causes problems so clamping the long
dimension is not as crucial to avoid warpage.