I just had 4 sheets of MDF delivered to my house. The box store picked out the 4 sheets themselves.

As I moved the last sheet to my basement, I noticed a very bad cup in the sheet (about 1 inch over 8 feet.) I called the box store, and they handled it very well by having the driver bring me a new sheet.

However, in between the time I called and the new sheet came, I cut another sheet to 4 X 6. When I put a straight edge on it, it had a 3/8'' cup. I can't use this for the top of my torsion box, which will function as an assembly table.

The replacement sheet was dead flat. I checked one of the other sheets, and it seems dead flat, too.

My question is how to make sure other sheets I buy are flat. If I go to the box store alone, I can't pull a sheet off the big pile and test it. Even if I could pull the sheet off the pile, you can really only test it by placing it on a relatively flat surface. (Maybe I could put it on the floor of the store and test it?)

I choose this box store because it delivers for very good price. I don't have a pick up truck. I could rent a truck and buy from a real lumber yard, but the problem then is unloading it. My wife is not always available.