Types of plywood vary widely, and can all be classified according to appearance, manufacture, source, and dimensions. Standard manufactured size is an 8' x 4' sheet, which can then be cut and sold according to the individual lumber yard's needs. Almost all plywood begins life as pine or fir, though other wood-types are available for indoor use and furniture manufacture. Description is made along the following qualities:
Species: the type of wood used in the manufacture. Pine is most common, also the least expensive (usually running around $10 for a 4' x 8').
Face Quality: measure of the appearance of a single external side of the board, possible to give a separate letter-rating for each side.
- A: Smooth, with a uniform grain and no knots along the entire surface, useful for furniture or other situations where appearance is important.
- B: Smooth, with a relatively uniform grain, but some closed knots.
- C: Rough, grain is unregulated, freely allowing closed knots.
- D: Very rough and unfinished, with many knots, often open.
rating is the most common, with a single finished side for plating and a rough side which won't be visible. Outside work usually requires CD-X, (C on the outer surface, D on the inner, X for exterior use). Plywood is also graded according to its treatment and manufacture, different for different applications:
- Exterior: the wood is treated with a waterproof glue and laminated under high heat.
- Interior: laminated under lower temperatures with a water soluble glue.
- Marine: similar to the exterior, but further treated with waterproofing agents and sealants, and lacking any inner voids in the core. Neither marine nor exterior grade plywood is absolutely rot-resistent.
- Structural: laminated as the exterior, but with thicker fibres specifically designed to increase strength and thickness.
- Foundation: wood treated with sealants and special preservatives to prevent long-term rot.
Thickness: this is pretty straight-forward, and simply describes the thickness of the board. Common thicknesses available are 1/4", 1/2", 3/4", and 3/8". The 1/2" or 3/8" are usually enough for almost any job. Simple shelves or cabinet backing can make do with just the 1/4", while only true structural or foundation jobs need the 3/4" or more.
Exposure: aside from the classification based on manufacture above under Face Quality, plywood is often rated separately for the treatment of its surface by various agents. An exposure rating of 1 means the board has been specially prepared, and is thus suitable for very wet conditions, including constant outdoor exposure. A rating of 2 means the wood is pretty much untreated, and thus useful only for extremely dry, indoor conditions.
Other "specialty" types of plywood exist; one of the most common is Melamine
, usually a 1/4" board with a finished, grade A veneer over a particle board core, laminated under high heat and pressure with a special glue called melamine, particularly suitable for cabinets and table-tops.