🪵 Plywood Types & Grades Explained — CDX, ACX, BCX, Marine & More

CDX is construction sheathing with a C-grade face, a D-grade back and X-rated exterior glue. The X does not mean the panel is waterproof or outdoor-safe — it means the adhesive line survives exposure during construction delays. Left exposed to weather for a season, CDX delaminates at the edges and the C/D faces roughen; used under siding, roofing or flooring, it is the cheapest panel that does the structural job. There is no furniture use for CDX: the D-face knotholes can reach 2½ inches and internal core voids will telegraph through any thin finish.

The face veneer only. ACX has an A-grade face — smooth, sanded, paint- or clear-coat ready — over a C back. BCX has a B face, which tolerates tight knots and football-shaped plugs but is not glass-smooth. Both carry exterior glue, so both survive incidental moisture. The practical decision is price versus finish: ACX for anything that will be painted to a furniture standard or clear-coated, BCX where a textured paint job or a hidden-but-dry face is fine.

No panel is waterproof by itself — wood absorbs water, and even the glue line only holds the plies together. 'Marine grade' means two things: the adhesive is fully exterior (boiling-water-proof, tested to soak/dry cycling), and the core is void-free so water cannot migrate through open internal gaps. US marine ply is typically Douglas fir or meranti to standards such as BS 1088. Water resistance in a finished project still comes from edge sealing, epoxy or paint — marine ply gives the coating a void-free substrate instead of fighting one.

For structural stiffness, thicker is stronger — span ratings and thickness drive it far more than face letters. Among common panels, 18 mm Baltic birch is exceptionally stiff for its thickness because it carries 13 plies versus 7 in standard 3/4-inch construction ply, and film-faced formwork ply uses dense hardwood cores for the same reason. For screw-holding and impact, hardwood-faced cabinet ply beats softwood sheathing. 'Strongest for the application' is a span-and-thickness question — the span calculator on this site gives the deflection numbers by panel and support spacing.

Either — the panel type and the face species are separate decisions. Softwood plywood (Douglas fir, southern yellow pine, SPF) dominates construction sheathing; hardwood plywood (birch, maple, oak, poplar cores) serves cabinets and furniture. The botanical terms also do not equal hardness: balsa is a hardwood, and southern yellow pine is a softwood that out-screws some 'hardwood' poplar cores. Judge panels by face species, core construction and glue type, not the soft/hard label.

Plywood is an odd number of thin wood veneers glued with alternating grain directions — face, core, back, with each layer cross-laminated 90° to its neighbors. Mills steam or boil logs, peel them into continuous veneer ribbons on a lathe, clip and dry the sheets, lay up a stack with adhesive, then press the stack hot. Cross-lamination is the entire point: it equalizes wood's natural shrinkage in both directions, which is why a plywood panel stays flat where a solid board of the same width would cup.

A two-number stamp like 32/16 gives maximum support spacing in inches: the first number is rafters or roof supports, the second is floor joists. A 32/16 panel can span 32 inches on a roof or 16 inches on a floor at standard loads; a 48/24 panel (Sturd-I-Floor) reaches 48-inch rafters or 24-inch joists. The stamp also carries the actual thickness — 23/32 where the label says '3/4-inch' — so it is worth reading the edge stamp before setting saw blade height or ordering screws.

Pressure-treated ply is impregnated with a copper-based preservative under vacuum, rated for above-ground or ground-contact exposure depending on the retention level. Use ground-contact rated (UC4A and up) anywhere the panel touches soil, stays wet, or vents poorly — shed floors, planter boxes, permanent forms. The copper corrodes plain steel: fasteners and hangers in contact must be hot-dip galvanized or stainless, and the panel cannot be glued with standard PVA adhesives. It is also not rated for direct food contact or aquarium use.

Luan (lauan) is a cheap 1/8″–1/4″ meranti-family ply from Southeast Asia. Its legitimate jobs: 1/4″ underlayment under resilient flooring (where the flooring manufacturer permits it), dado-panel backs, bending laminations, and temporary protection. It warps with any humidity swing, holds screws poorly, and delaminates outdoors in one season — so it fails at every job sanded or marine ply is specified for. Treat it as a disposable interior material and it is excellent value.

Poplar-core ACX or A2 birch ply, 3/4″, painted. The A face takes primer level; the C back is hidden inside the carcass. If the interiors will be visible, UV-prefinished birch skips the topcoat entirely and Baltic birch offers the crispest painted edges. Spend one grade up on anything a visitor sees and one grade down on everything screwed behind — the classic cabinetmaker's split is A on doors and fronts, C on everything else.