A plywood grade stamp packs a lot into a few letters. The letters (A, B, C, D) describe the quality of the face and back veneers; the bond classification (Exterior or Exposure 1) says how water-resistant the glue line is; and the span rating and performance category say what the panel is approved to span and how thick it counts as. In the United States these are defined by the Department of Commerce standard PS 1, Structural Plywood. This page explains each part in plain language and lists the common grades and what they are good for. For real thicknesses and sheet weights see the plywood thickness and weight chart.
Veneer grades: N, A, B, C Plugged, C, D
| Veneer grade | What PS 1 allows (summary) | Typical use |
|---|---|---|
| N | Highest grade, for a natural (clear) finish. All heartwood or all sapwood, no knots, knotholes or open splits, well matched for colour and grain, at most six neat wood repairs. | Furniture and cabinet faces; rarely stocked |
| A | Smooth and paintable. No knots or open defects; neat wood or synthetic repairs allowed (up to 18). Sapwood and colour variation allowed. | Painted cabinets, shelving, visible panels |
| B | Solid surface. Sound, tight knots up to 1 in across the grain, small splits up to 1/32 in, more repairs than A. | Backs of good panels, utility cabinets, painted work where small knots are acceptable |
| C Plugged | An upgraded C: open defects limited to 1/4 x 1/2 in, tight knots up to 1-1/2 in, splits up to 1/8 in, plugs and patches allowed. Usually touch-sanded. | Underlayment faces, a smoother surface under flooring |
| C | Tight knots up to 1-1/2 in; knotholes up to 1 in (an occasional 1-1/2 in one, within limits); some splits; discoloration allowed. | Sheathing faces, the lowest grade allowed in Exterior panels |
| D | Knots and knotholes up to 2-1/2 in (occasionally 3 in, within limits) and other characteristics, as long as they do not seriously impair strength or serviceability. | Backs and inner plies of Exposure 1 sheathing; never in Exterior panels |
Panel grades: what the two letters mean
A panel grade names the face veneer first and the back second. A-C plywood has an A face and a C back; C-D has a C face and a D back. The inner plies are not in the name but are set by the standard: in Exposure 1 panels they may be D grade, while Exterior panels must use C or better throughout, because D veneer is not allowed in Exterior plywood at all. Sanded grades (A and B faces) are sanded smooth on both sides; C-D sheathing is unsanded; plugged and underlayment grades are touch-sanded, a light sanding to bring the panel to a uniform thickness.
| Grade | What it is under PS 1 | Good for |
|---|---|---|
| A-C (often sold as ACX) | A face, C back, C inner plies; Exterior | One good, paintable face; usable outdoors |
| B-C (BCX) | B face, C back, C inner; Exterior | Cheaper outdoor panel with a solid face |
| C-D (CDX) | C face, D back, D inner plies; Exposure 1 | The standard roof and wall sheathing panel |
| C-C | C face and back, C inner; Exterior | Sheathing that will stay exposed to weather |
| C-D Plugged / Underlayment | C Plugged face, D back; touch-sanded; Exposure 1 | Under resilient flooring and tile backer; smoother face |
| A-A, A-B, B-B | Both faces good; sanded on both sides | Shelving, furniture and cabinets seen from both sides |
| Structural I | A sheathing grade with extra species and veneer limits for higher strength | Shear walls and diaphragms where an engineer calls for it |
| Marine | Exterior A-A, A-B or B-B (or overlaid) from Group 1 Douglas-fir or western larch only, with B-or-better inner plies and limited core gaps | Boat building and other wet, high-stress uses |
Exterior vs Exposure 1, and what the X in CDX means
PS 1 classifies every panel as either Exterior or Exposure 1, and the difference is in the glue line. Both are tested after vacuum-pressure soaking and boiling; Exterior panels must average at least 85% wood failure in the test specimens and Exposure 1 panels at least 80% (section 5.7), and Exterior panels may not contain D grade veneer. In industry practice, Exterior plywood is the panel for permanent exposure to weather, while Exposure 1 is made to withstand moisture during construction delays and is meant to be protected once the building is closed in. The standard notes that the bond classification describes the glue line only, not resistance to decay, mold, insects or surface erosion: an Exterior panel still rots if it stays wet, unless it is preservative treated.
CDX is the everyday name for C-D Exposure 1 sheathing. The X is a holdover from the days when such panels were described as having “exterior glue”; it does not make CDX an Exterior panel. Use CDX for roof, wall and subfloor sheathing that will be covered, and A-C, B-C or C-C Exterior plywood where the panel itself will face the weather.
Span ratings and performance categories
- Span rating (PS 1 section 2.57): the maximum center-to-center spacing of supports, in inches, the panel is rated for. Sheathing is marked with two numbers, roof span over floor span: 32/16 means 32 in on a roof or 16 in as subfloor. Single-floor panels carry one number, such as 24 o.c. By convention a rating of 20 means 19.2 in spacing.
- Performance category (section 2.39): the nominal thickness designation the IRC and IBC use, such as 15/32 or 3/8 Performance Category. The actual panel can be a little thinner than the fraction suggests; the plywood chart lists the tolerances. The standard notes that the IRC and IBC require at least 3/8 Performance Category for wall sheathing.
- Sized for spacing: an optional mark meaning the panel is cut slightly under 4 x 8 ft so the industry-recommended gap between sheets (commonly 1/8 in) can be left without losing the 16 or 24 in module.
- Spans, nailing and the sheathing needed in your area come from your building code and the panel maker, not from this page.
Species groups
PS 1 sorts the woods used for plywood veneers into five groups by strength and stiffness (Table 1, using ASTM D2555 values), Group 1 being the strongest. Group 1 includes coastal Douglas-fir, western larch, the southern pines (loblolly, longleaf, shortleaf, slash), sugar maple, sweet and yellow birch, American beech, tanoak, and the imported apitong and keruing, kapur, and Caribbean and ocote pine. The light, soft woods such as basswood and balsam poplar are in Group 5. A grade stamp shows the group of the face and back veneers on panels that are not span rated; span-rated panels are tested for performance instead. Profiles of most of these woods are in our wood species database, for example Douglas-fir, southern yellow pine and keruing (apitong).
Choosing a grade
- Roof and wall sheathing: C-D Exposure 1 (CDX) or OSB, at the span rating and performance category your code requires.
- Subfloor under carpet or tile: a single-floor or underlayment panel with a plugged, touch-sanded face, tongue and groove where spans need edge support.
- Outdoor projects, sheds and soffits: Exterior A-C, B-C or C-C; prime and paint all faces and edges. Use preservative-treated plywood for ground contact.
- Cabinets, shelves and furniture: sanded A or B faces, or hardwood plywood (made to a different standard, ANSI/HPVA HP-1) with birch, oak or maple veneers.
- Boats: marine plywood. US marine grade is Douglas-fir or western larch to PS 1; okoume marine plywood is usually made to the British BS 1088 standard. See okoume.
Frequently asked questions
What do plywood grades A, B, C and D mean?
They describe the veneer on the face and back. A is smooth and paintable with no knots, B is solid with small tight knots, C allows knotholes up to about 1 in, and D allows knotholes up to about 2-1/2 in. Grade N, for a natural finish, is above A (PS 1 section 5.4).
What is CDX plywood?
C-D Exposure 1 sheathing: a C face, a D back and D inner plies, with an Exposure 1 bond, intended to withstand moisture during construction but to be covered in service. The X refers to the old “exterior glue” description; CDX is not Exterior plywood.
What is the best grade of plywood?
Grade N veneer is the highest in PS 1, but A-A or A-B sanded panels are the best commonly stocked grades. For furniture, hardwood plywood with a decorative face is usually a better choice.
Can Exposure 1 plywood be used outside?
Only temporarily or where it will be covered. For permanent exposure use Exterior plywood, and keep in mind that the bond class says nothing about rot resistance.
What does 32/16 on plywood mean?
It is the span rating: rated for supports up to 32 in apart as roof sheathing or 16 in apart as subfloor (PS 1 section 2.57).
Sources
- US Department of Commerce / NIST, Voluntary Product Standard PS 1-22, Structural Plywood: sections 2.39, 2.56, 2.57, 4.2, 5.2.1 and Table 1, 5.4.1 to 5.4.6, 5.6 and Tables 2 and 3. Summarised in our own words; the standard is the authority.
- ANSI/HPVA HP-1 (hardwood and decorative plywood) and BS 1088 (marine plywood) are named for reference only; not reproduced.
Last reviewed: 24 October 2026. Related: plywood thickness and weight chart, plywood cut calculator, wood species database. Spotted an error? Please let us know.