CDX Plywood - What Does the "X" Really Mean?

CDX plywood is likely the most popular type of plywood used for rough construction. You'll find it in roof sheathing (just under the shingles and roofing felt, on walls (just behind the siding and insulation, and sub floors (just under the carpet pad or a tiled floor's durock. It's easy to find at your local home center, and is a relatively cheap material for constructing large-scale projects (like a house!)

The three letters (CDX) do in fact mean something in regard to the type of plywood we're talking about. The letters "C" and "D" represent the quality of the surfaces - front and back. If we consider that the letter "A" is the best level of plywood veneer you can buy - and that means plywood without any noticeable knots, holes, or repaired defects - CDX is certainly a lower grade of quality, and the price will reflect this. CDX plywood is fairly easy to spot on the shelf - you'll see plenty of football-shaped patches and shallow holes and indentations on both sides. Considering what CDX plywood is normally used for (see below) it's not going to matter really what the panel looks like - as long as it is in one piece.

The letter "X" in CDX indicates for what purpose this plywood should be used. Some mistakenly think the "X" stands for "exterior" - which is not true, exactly. The letter stands for "exposure" - which means the plywood is tough enough to withstand a little moisture - but for only a short time.

For example, let's say you're in the middle of building a garage - and you've just finished putting up CDX plywood as the sheathing for the walls, which will eventually be covered with siding. But now you've discovered that your siding won't be delivered till the end of the week. Can you leave your CDX plywood exposed for a couple days? According to grading standards for CDX plywood - you certainly can leave it exposed- temporarily. In fact this is one reason why CDX plywood is used so extensively on the job site. The glues and adhesives are made to be water resistant, but not made to withstand direct exposure to the outdoors over time