Plywood comes in two families with separate standards. In the US, construction and industrial plywood is covered by PS 1 and PS 2, and hardwood and decorative plywood by ANSI/HPVA HP-1.
How plywood is built
Plywood is a panel of thin wood veneers, called plies, bonded with an odd number of layers. The grain of each layer runs at right angles to the next, which equalizes strains, reduces splitting and limits warping and changes in size.
For screws near the edge of a board, see how to keep wood from splitting.
Sheathing and CDX
Sheathing is construction plywood for covering roofs, subfloors and walls, and its stamp carries a span rating. PS 2 sets performance rules for sheathing whether the panel is plywood or OSB, and the IBC and IRC building codes use the term wood structural panel for panels made to PS 1 or PS 2. Panels commonly called CDX are Exposure 1 panels.
The thickness on a sheathing stamp is a Performance Category, such as 23/32 for a sheet called 3/4. The plywood thickness chart lists every category, and plywood grades explained covers the letters and bond classes.
Sanded plywood
Sanded grades such as A-C and B-C have an A or B face and are sanded on both sides at the mill. Roseburg makes its Exterior Core panels from western wood veneers with C grade or better core and back, offers AC and BC with a fully sanded face, and its touch-sanded CCPTS panel has a tighter thickness tolerance.
Hardwood and cabinet plywood
Hardwood plywood goes into furniture, cabinets and decorative wall panels, where appearance matters more than strength. HP-1 grades it by face veneer species and quality, bond durability and the makeup of its center layers, and its 2004 edition recognizes two exposure classes, Interior and Exterior. PS 1's thickness ranges don't cover it.
To see where a measured hardwood panel falls against the PS 1 sizes, use the plywood thickness converter.
Marine plywood
Marine is a PS 1 Exterior grade with Douglas-fir or western larch veneers throughout and tighter rules for its inner plies.
The full rules, and what the British BS 1088 covers, are in marine plywood.
MDO and HDO
Medium Density Overlay (MDO) and High Density Overlay (HDO) are Exterior panels with a sheet of resin-impregnated cellulose fiber bonded to the working face. They're made for uses that need a durable, uniform surface. HDO Industrial is typically used for highway signs or industrial applications, and both overlays come in concrete form grades.