Plywood

How it is built and graded

Softwood plywood, used mainly for structural work, is generally made from rotary-cut veneer: the log is mounted in a lathe and turned against a long blade so the veneer peels off in one continuous sheet, rather like unrolling a length of paper from a roll. Hardwood plywood, intended for furniture, cabinet work and panelling, is made from a mix of rotary-cut and sliced veneer, since a sliced sheet can show a straighter, more even figure on the face. Porter (1999) explains that a plywood's grade mostly describes the condition and quality of the veneers used in it, and that the type of glue used to bond the layers, together with the natural durability of the wood itself, decides which exposure class, meaning how much weather and damp a sheet is fit to withstand, it is sold under. Coatings and films can be applied afterwards to adapt a sheet to particular jobs, and Sato (1987) notes that numerous textured and coloured prefinished plywood panels have come into wide use as interior finishing material.

Strength, movement and bending

Because the grain direction alternates from layer to layer, a sheet of plywood is far more even in strength along its length and across its width than a solid plank, where strength differs sharply with the direction of the grain; the USDA (2021) records that the gap between plywood's along-panel and across-panel bending strength and stiffness is much smaller than the equivalent gap in solid wood. Hoadley (1980) warns of the opposite weakness, though: plywood splits easily between its layers, in the plane of the panel itself, which matters when screwing hinges or other fastenings into a panel's edge rather than its face. Joyce (1987) adds that for ordinary furniture work the seasonal movement of plywood can usually be ignored, though on an untreated panel it can still show as a slight, gradual creep along the edges of the long-grain veneers.

Flat plywood can also be bent after it has been glued up, by methods much like those used to bend solid timber, and the USDA (2021) lists what governs how far it can be curved: the moisture content of the wood, the direction of the grain, the thickness and number of plies, the species and quality of the veneer, and the technique used to bend it.

Joining and fixing plywood

Because it is a sheet material rather than solid timber, plywood is often joined with corner joints designed specifically for it. Rogowski (2002) describes a tongue-and-groove corner joint suited to plywood cases, cut with a groove sized to the thickness of the plywood stock itself, while Frid (1979) describes a lock mitre joint used for either solid wood or plywood, and a more elaborate double-tongued version reserved for plywood alone. For nailing, Sato (1987) suggests a nail roughly two to two and a half times the thickness of the panel it is fixing.

Marine plywood and other specialist uses

Marine plywood is built to resist both structural stress and damp; Benson (1988) rates it highly for strength and moisture resistance but notes that its cost is prohibitively high next to ordinary plywood. Plywood is also used as the lining, called lagging, inside the timber formwork that shapes poured concrete; Brett (2005) explains that plywood or close timber lagging is generally used for brickwork-faced concrete and open timber lagging for stone-faced concrete, and that the grain pattern of the plywood itself transfers to the finished concrete surface, so that Douglas fir plywood leaves a visible decorative grain while birch plywood leaves a smoother finish.