Plywood

Frequently asked questions

Does the X in CDX mean exterior?

It means exterior GLUE, which is not the same as an exterior board, and the difference has soaked a lot of roofs. The letters read in order: C is the face veneer grade, D is the back, and X indicates the bond will survive moisture. But the veneers themselves are low grades full of knots and voids, and the board as a whole is classified Exposure 1 — engineered to tolerate the weather during construction, not to live in it. So CDX left uncovered for a season delaminates at the edges, swells and never recovers its thickness. The genuinely exterior product is a different classification again, and for permanent wetting the answer is neither: it is a board specified for that service class. The comparable British trap is asking for WBP and receiving whatever the merchant calls WBP, since the grade was withdrawn and the term now means roughly weatherproof rather than anything testable.

Is marine plywood just plywood with better glue?

No, and that misconception is why it disappoints. The glue in a good exterior plywood is already fully weatherproof — the same phenolic chemistry. What marine grade adds is VENEER quality and construction: veneers of a durable species, graded to permit very few defects, and above all no core VOIDS, because a void inside a board is a pocket that fills with water, cannot dry, and rots from the inside where nothing shows on either face. That is the property being bought, and it is about what is inside the sheet rather than what holds it together. Two consequences follow: marine grade is not automatically rot-proof, because the timber itself may not be durable unless the species says so and the edges are sealed; and for many wet applications a cheaper exterior board with sealed edges and a proper detail outperforms marine ply bought as a talisman.

What does a span rating tell you that a thickness does not?

How far it can go between supports, in the direction that matters — which thickness alone cannot say because plywood is not equally strong both ways. The veneers alternate, but the FACE grain runs one way, and a sheet is substantially stiffer along that direction than across it. An American span rating is stamped as two numbers, the first for use as roof sheathing and the second as subfloor, and both assume the face grain runs ACROSS the supports, which is how the sheet is meant to be laid. Turn it ninety degrees to make the cuts work out and the rating no longer applies, even though the thickness is unchanged — a real and common site error that shows as a springy floor or a visibly sagging roof line between rafters. European practice publishes the bending properties for both directions instead of a single stamped rating, which is more information and rather less likely to be read at all.