Plywood Grading & Span Rating Terms Glossary

July 20, 2026

The stamp printed on a plywood sheet packs in more information than it looks like at a glance. Here’s what the terms mean, per APA — The Engineered Wood Association, the industry group that developed the standard trademark system most North American structural panels use. Once a grade and size are chosen, use the plywood calculator to figure out how many sheets to buy.

Veneer Grade (A, B, C, D)

Plywood grade stamps often show two letters (like A-B or B-C), representing the veneer grade used on the face and back of the panel. A is the smoothest, most defect-free appearance grade; D allows the most knots and repairs. A panel stamped “B-C” has a moderate-quality face (B) and a rougher, utility-grade back (C) — common for sheathing where only one side will be visible or matters cosmetically.

CDX

A common shorthand for a C-D panel with an Exposure 1 (X) bond classification — not an official APA grade name itself, but widely used in lumberyards to describe a utility-grade sheathing panel with lower-grade veneers on both faces, suitable for structural use but not appearance-critical applications.

Span Rating

Two numbers separated by a slash, printed on structural sheathing panels — the left number is the maximum recommended support spacing (in inches) for roof use, the right number for floor/subfloor use. A panel marked “32/16,” for example, is rated for supports up to 32 inches apart on a roof or 16 inches apart on a floor. Always match a panel’s span rating to the actual framing spacing on the job — see plywood subfloor installation: fastening schedule & panel spacing for how this plays out in practice.

Exposure Rating (Exterior vs. Exposure 1)

Describes how much moisture exposure a panel’s adhesive bond is designed to withstand. “Exterior” panels use a fully waterproof bond suitable for permanent outdoor exposure. “Exposure 1” panels use a similarly durable bond but are intended for construction that may see temporary moisture exposure during building (before final weatherproofing) rather than permanent outdoor use — not the same thing as “exterior grade,” despite sounding similar.

Product Standard (PS 1 / PS 2)

The specific manufacturing or performance standard a panel is certified to. PS 1 covers structural plywood specifically; PS 2 establishes broader performance criteria that can apply to plywood or OSB alike, based on how the panel actually performs rather than exactly how it’s constructed.

APA RATED SHEATHING / STURD-I-FLOOR / SIDING / UNDERLAYMENT

Product-specific trademark names indicating a panel’s intended end use, printed instead of (or alongside) a veneer-grade letter pair. RATED SHEATHING covers general wall/roof/floor structural use; STURD-I-FLOOR is engineered specifically as combined subfloor-and-underlayment in one panel; UNDERLAYMENT is a thinner panel meant to go over an existing subfloor, not replace it.

Species Group Number

A classification (Group 1 through 5) reflecting the strength and stiffness of the wood species used, with Group 1 being the strongest. Typically referenced only when a panel doesn’t carry a span rating.

Using This Glossary

Match these terms against a plywood sheet’s actual grade stamp before buying, then use the plywood calculator to figure out how many sheets of the correct size and grade to purchase. See plywood grades explained: choosing the right grade for your project for how to translate these terms into an actual buying decision.