What Is Plywood? Beating Wood at Its Own Weakness

A piece of wood swells and shrinks substantially across the grain as it takes up and loses moisture, and barely at all along it. In a solid wide board that movement is unopposed, so the board cups, twists and eventually splits, and every traditional furniture technique for wide panels exists to accommodate it. In plywood, each layer wanting to move across its width is glued to layers that are dimensionally stable in that same direction and simply refuse, so the movement is restrained mechanically and the panel stays flat. The same arrangement distributes strength: instead of being strong one way and weak the other, the panel has useful strength in both, and it resists splitting because a crack running along the grain of one layer runs straight into a perpendicular grain in the next. That is why plywood holds a screw near an edge without splitting and a solid board frequently does not. The layer count is almost always odd so that the outer faces run the same direction and the construction stays balanced, since an unbalanced panel warps.

Manufacturing is dominated by one clever operation. A debarked log is mounted in a lathe and rotated against a full-length knife, peeling a continuous thin sheet of veneer off it like unrolling a roll of paper, which converts a round log into a flat continuous ribbon with very little waste. Logs are usually steamed or soaked first to soften them. The ribbon is clipped to width, dried to a controlled moisture content, graded, and any defects patched. Sheets are then coated with adhesive, stacked in alternating grain directions, and pressed under heat and pressure until the adhesive cures. The adhesive determines the panel's rating, with phenol formaldehyde resins giving exterior and marine grades that survive wetting and urea formaldehyde giving interior grades that do not. Finally the panels are trimmed, sanded and graded, with face grades describing appearance rather than strength, so a panel can be structurally identical on both sides while carrying different grade letters.

Plywood sits within a group of products made by breaking wood down and reassembling it, each solving a different problem:

• •Oriented strand board, made from large flakes aligned in layers, which is cheaper and structurally comparable for sheathing but swells irreversibly at cut edges if it gets wet
• •Medium density fibreboard, made from wood fibres and resin, which machines to a crisp edge and takes paint beautifully while being heavy and weak in tension
• •Particle board, from smaller chips, which is cheap, used inside flat-pack furniture and fails badly when wet
• •Hardboard, compressed fibre with one smooth face, used for backing and drawer bottoms
• •Glued laminated timber and laminated veneer lumber, which use the same principle in beam form to make large structural members from small pieces
• •Cross-laminated timber, thick perpendicular layers of boards forming structural floor and wall panels, which is what has made tall timber buildings possible

Layered wood veneer was used in ancient Egypt and revived seriously in the nineteenth century, and the rotary lathe in the 1890s is what made continuous cheap veneer possible. Aircraft use drove quality upward in both world wars, and the wooden airframes of that period, most famously a British light bomber built largely of moulded plywood, demonstrated what a strong stiff lightweight structure it could produce. Boatbuilding and mid-century furniture exploited its other property, which is that it can be moulded into compound curves under pressure and holds the shape permanently. Its failure modes are specific: delamination when an interior adhesive is exposed to moisture, voids inside cheap panels that collapse under load or leave hollow edges, and face veneers so thin that any sanding goes through them. Formaldehyde emissions from adhesives prompted regulation in several jurisdictions, and low-emission and formaldehyde-free adhesives are now widely required for interior products.

Turning the grain of each glued layer ninety degrees to its neighbours restrains the movement that makes a wide solid board cup and split, and gives useful strength in both directions. A rotary lathe peels a log into a continuous veneer ribbon with little waste, which is what made the material cheap. The adhesive sets the exterior rating. Odd layer counts keep the panel balanced, and interior-grade panels delaminate when they get wet.