Which plywood grade do I need?
What the face grades and bond classes mean, and which plywood survives outdoors.
- Difficulty
- beginner
- Time
- 15 min
- Read
- 3 min
Short answer
The letters describe the face quality, the bond class describes water resistance. WBP or exterior-grade for anything damp, marine ply only where it genuinely gets wet. Birch ply gives the best edges and finish.
Plywood is graded twice — once for how the faces look and once for how the glue behaves in water — and people usually only check the first.
Step by step
- Read the face grades.Usually two letters, such as B/BB. The first is the front face, the second the back. A or B is smooth and near-defect-free; C and D allow knots, patches and voids.
- Check the bond class separately.Interior bond fails in damp. WBP, exterior or EN 314 class 3 glue survives outdoors. A high face grade with an interior bond is still useless in a bathroom.
- Use marine ply only where it earns it.It has a durable bond and no core voids, and costs several times more. Genuinely worth it for boats and permanently wet locations, and wasted on a shed floor.
- Choose birch ply for visible work.Consistent void-free core, clean edges that can be left exposed, and a smooth face that takes finish well. It is the choice for furniture and cabinets.
- Use softwood ply for structure and hidden work.Sheathing, hoarding, subfloors, formwork. Cheaper, rougher, and entirely adequate where it will not be seen.
- Check the thickness against the span.18mm for shelving and flooring, 12mm for cabinet backs and light panels, 6mm for facings. Thin ply sags badly over any span.
- Look at the edge before buying.The core tells you more than the face. Voids, gaps and thin veneers mean it will splinter when cut and hold screws poorly.
- Plan for edge treatment.Exposed plywood edges need lipping, edge banding or filling unless you are using birch ply deliberately for the striped look.
Tips
- Face grade and bond class are two different things. Check both.
- Look at the edge of the sheet — voids in the core tell you the real quality.
- Birch ply for anything you will see; softwood ply for anything you will not.
Common mistakes
- Using interior-bond ply outdoors — The glue lines fail and the sheet delaminates within a season, however good the face looks. Bond class is what determines outdoor durability.
- Buying marine ply for general use — It costs several times more for a durability nobody needs on a shed floor or a workbench. WBP exterior grade is the sensible choice for most damp locations.
If it doesn't work
Ply delaminating
Cause: Wrong bond class for the conditions — Fix: Replace with exterior or WBP grade. Delamination cannot be reversed.
Edges splintering when cut
Cause: Voids in the core, or wrong blade — Fix: Score the cut line first, use a fine blade, and buy void-free ply for visible work.
Screws pulling out of the edge
Cause: End-grain fixing into a voided core — Fix: Avoid screwing into ply edges. Use a batten or an insert if you must.
Questions people ask
What does WBP mean?
Weather and Boil Proof — a description of the adhesive's resistance rather than the timber. It indicates the glue survives moisture, which is what matters outdoors.
Is marine ply waterproof?
The glue is highly durable and the core is void-free, but the timber itself still needs finishing. Unfinished marine ply will still degrade if permanently exposed.