Plywood grades explained: A, B, C, D, BB, marine and birch

A plywood grade describes the quality of the face veneer, mainly how many knots, patches and open defects it allows, not the strength of the panel or the number of plies. Two different letter systems are in everyday use: the North American A to D scale, and the B/BB style used on birch and much European-made plywood. Marine grade is a separate thing again: a construction standard covering the core and the glue, not just the face. This guide covers all three, plus how to read a combined code like C-D or BB/CP, so you can pick the cheapest grade that still does the job.

The North American A to D scale

Plywood sold under this system, commonly under PS 1, the construction and industrial plywood standard, is graded on a letter scale from A (best) to D (lowest). Each face of the sheet is graded separately, and the panel is stamped with two letters, face first, back second, for example A-B or C-D.

• A grade. Smooth and sanded, with any repairs neatly made and colour matched. The best face available, ready for paint or a clear finish with minimal prep.
• B grade. A solid, sound surface. Tight knots and circular repair plugs are allowed, and some sanding marks are tolerated. Fine under paint, less fine under a clear finish.
• C grade. Unsanded. Allows tight knots up to roughly 38 mm (1.5 in) across, some knotholes and minor splits. This is the lowest grade allowed on a panel rated for full exterior exposure.
• D grade. Unsanded. Allows knots and knotholes up to roughly 64 mm (2.5 in) across. Only used on panels rated for interior use or Exposure 1 (protected, not permanent outdoor exposure), never on a panel rated Exterior.

Reading the combined code

A stamp like C-D means the face is C grade and the back is D grade. The familiar sheathing product sold as CDX is exactly this: C grade one side, D grade the other, with the X flagging that the glue bond is rated Exterior (fully waterproof) rather than Exposure 1. The glue rating and the face grade are independent of each other: a panel can have a low face grade and still use a fully waterproof glue, which is exactly the CDX case. Exposure 1 panels use a similar waterproof-type adhesive but are only intended to survive the weather during a build, not permanent outdoor use; only a panel stamped Exterior is rated for that.

BB grade and European-style grading

Outside the A to D system, plywood made in Europe (birch especially, but general-purpose panels too) is commonly graded with letters such as B, BB, S and CP, and the two systems are not a direct swap for each other. B is a single-piece face, light and even in colour, close in spirit to A grade. BB allows small, colour-matched oval patches and occasional tight pin knots rather than a flawless face. CP (cut and patched, sometimes called plugged) allows more patching and wider colour variation while staying free of open defects. As with the A to D system, a panel is usually sold under a combined code, face first: B/BB, BB/BB, or BB/CP.

Birch plywood grading

Birch plywood (often called Baltic birch) is built entirely from birch veneers, including the inner plies, rather than a cheaper softwood core under a birch face. That all-birch construction is normally specified as void-free: no gaps between the inner veneer layers, which is what makes birch ply’s cut edge look clean and layered rather than showing gaps. It’s graded with the B/BB/CP letters above. B/BB is common for one good show face with a patched back; BB/BB for a project where both faces are seen but neither needs to be flawless; BB/CP where cost matters more than the back face, a drawer box or a jig that only shows one side.

Marine plywood

Marine grade is not really a face-appearance grade at all: it’s a construction standard covering the whole panel, core included. In North America that standard is PS 1, which for marine grade requires veneers of Douglas fir or western larch throughout, every ply graded B or better (so no knotholes anywhere in the panel, not just on the visible face), a maximum core gap of about 3 mm (1/8 in), and a fully waterproof Exterior glue bond. Internationally, the equivalent is BS 1088, a standard used well beyond the country it originated in, which covers durable, naturally decay-resistant hardwood species, the same void-free core requirement, and a waterproof glue bond, independently certified in many cases (for example by Lloyd’s Register) to confirm the panel meets the standard rather than just carrying a “marine” label. Marine ply is genuinely more resistant to water at the glue line than standard exterior ply, but it is not waterproof at a raw cut edge: any edge exposed to standing water or long-term damp still needs sealing, the same as any other sheet material.

Which grade for the job

Worked example: mixing grades in one cabinet

Grade only has to be high where the panel is actually seen, and mixing grades on one project is a straightforward way to cut cost without cutting quality where it matters. Take a small wall-mounted tool cabinet: 600 mm wide, 300 mm deep, 700 mm tall, in 18 mm plywood, with a single door on the front and nothing else visible once it’s hung and closed.

The top, bottom and shelf run between the two sides, so their length is the external width minus two panel thicknesses: 600 − 2 × 18 = 564 mm. The door sits flush in the front opening: 564 mm wide, the same span as the shelves, and 664 mm tall (700 − 2 × 18, the height left once the top and bottom panels are accounted for).

Nesting the BB/CP carcass: all five hidden parts share the same 300 mm depth, so lay them out as strips 300 mm wide running the length of the sheet. Strip one takes both sides end to end plus one shelf: 700 + 3 (kerf) + 700 + 3 + 564 = 1970 mm, well inside the sheet’s 2440 mm length. Strip two takes the remaining two shelves: 564 + 3 + 564 = 1131 mm. Two 300 mm strips plus a kerf use only 603 mm of the sheet’s 1220 mm width. All five parts fit inside a small corner of one sheet, with most of it left as offcut for the next job.

The door: 564 × 664 mm easily fits on a single sheet of birch-faced ply, but it’s a different material from the carcass, so it needs its own sheet regardless of how little of it the door actually uses.

Total: 2 sheets, one inexpensive BB/CP sheet for every hidden part and one birch-faced sheet for the single part anyone will ever see. Buying a full birch-faced sheet for a 564 × 664 mm door looks wasteful next to the offcut left over, but it’s still cheaper than building the whole cabinet from the higher grade, and the leftover birch offcut is exactly the kind of piece worth keeping for the next small project. On a bigger build, more of the parts list ends up hidden, and the saving from grading down the carcass grows with it.

Standard sheet size

Grade is independent of sheet size: a full sheet in any of the grades above is normally 2440 × 1220 mm, the metric near-equivalent of an 8 × 4 ft sheet, with half sheets and birch sheets in smaller formats also widely stocked.

Plan your own cut list

Once you know which parts need the good face and which don’t, the sheet count works the same way regardless of grade: list your parts, set the thickness and sheet size, and SawReady works out the nesting and the exact number of sheets to buy for each material on your list.

Plan my cut list