Dealing with Wet and Warped Plywood

One of the many advantages of working with plywood comes from its inherent dimensional stability. Since the composition of plywood consists of layers of alternating grain direction, any negative effects of wet plywood tend to cancel each other, and the board remains, for the most part, straight and level. However, extreme saturation cases or prolonged exposure to disparate humidity levels often result in sheets of plywood that aren’t perfectly level. How the typical do-it-yourself (DIY) type handles this sort of setback depends mainly on a few things: the location of the plywood (has it already been purchased or is it at the home center), the type of defect (cupped, bowed, or warped) and the type of work you want to do with it. Below is a list of tips to help you handle these cases.

Purchasing and Storing

If you haven’t purchased the plywood yet and you notice that the stock at the home center is warped, cupped, or bowed, it’s usually a good idea to pass on those sheets—you’re paying good money for them, and the dimensional stability they’re famous for. However, and we’ve all done this sort of thing before, you will sometimes store plywood you’ve already purchased in a way that causes it to warp—sometimes you store it with one side subjected to much more humid conditions than its opposite side.

Concrete floors are a good example. An extreme example would be storing your plywood sheet, even briefly, on the damp ground while the sun bakes the other side dry. To guard against warping your plywood, if you choose to store it flat, ensure you do so well above the floor, and if you have multiple pieces of plywood, place stickers between these sheets to allow airflow between the sheets. Since not every shop has the space to store large sheets flat, another option would be to store your plywood sheets vertically; again, make sure it’s elevated to minimize moisture absorption along the edges. Normally, if plywood is stored in this manner, slight deformations such as cupping or bowing will dissipate as the moisture levels equilibrate and the sheet becomes dimensionally stable again.

Concave and Convex Sides

Let’s now assume you have plywood that has cupped— it now has a concave and a convex side. The convex side has had the extra moisture, swelled, and expanded. The side that dried out and shrunk a bit is the concave side. See the graphic on the right (exaggerated for clarity). The concave side needs moisture, and the convex side needs to dry out. Spray a light mist on the concave side and expose the convex side to sunlight or the drier conditions of your shop, usually facing up. The sheet should be flat when the two sides reach moisture equilibrium (equal dryness on both sides). This can take a little time, and if you don’t have that time, then using cupped plywood is ok as long as you know its limitations.

Check the price on Amazon

Flattening

If the plywood you wish to use is a full sheet with supports for attaching the sheet itself, such as on a roof or flooring where the joists can help flatten sheets when connected, then using slightly cupped sheets of plywood is not problematic. The attachment of the sheet to these supports flattens the sheet, and any subtle changes that occur as it dries out are minor. However, there are times when more minor, cupped/warped pieces of plywood are required for a project. These pieces may not have the advantage of an attachment point like a joist to help flatten it. If these smaller pieces of plywood require an intricate fit, it would be better to ensure it’s flat before cutting it for use in your project, as the piece will eventually equilibrate of its own accord. If a piece is cut while wet or warped, it runs the risk of not fitting as well once it dries out. The take-home lesson is to make sure your plywood is dry and flat before you use it for smaller pieces without support to help flatten it.

Using slightly cupped or bowed plywood generally isn’t a problem for larger and less delicate applications where the plywood will have time to equilibrate, or slight imperfections aren’t detrimental to the overall project. For smaller projects that require the use of smaller pieces of plywood that have cupped, warped, or bowed… the best advice would be to restore them to their naturally flat state or consider using pieces that are already flat and straight.

Check the price on Amazon