Plywood is used almost universally for cabinetry, floors, roofs and walls. Almost any type can be used outdoors when properly sealed, but some specific types are more appropriate than others.
Interior or Exterior
Plywood is graded for application. Structural plywood is made with water-resistance adhesive. Interior plywood is made with water-soluble adhesives. Plywood is also graded for appearance.
What The Stamps Mean
Plywood is stamped or marked with a standard coding system that uses the alphabet to describe it. The letter "X" stands for exterior-grade, which means it's made with exterior adhesive.
The letters A,B,C and D, are stamps indicating the amount of allowable defects on each side. The letter "A" represents prime quality, with no defects. As grades descend, the wood contains more defects.
The Most Common Exterior Grade Stamp
The appearance of structural plywood is typically not an issue, and grades below A and B are to be expected. For example, the most common exterior plywood is typically stamped CDX. The C, or face side, has knots, streaks, patches and visible splices. The D side, or back, has more of the same, with unfilled defects.
CDX plywood is the most common for sub-roofing, sheathing and exterior applications. It's also common for interior sub- flooring, primarily because of it's water-resistance.
Plywood siding, also known a T1-11, is manufactured with exterior glue. T1-11 can be attached directly to CDX plywood sheathing. T1-11 is textured to look like wood, without the patches or defects found on common CDX. It can be stained for a natural appearance, or primed and painted just like any other exterior siding.
Marine Grade Plywood
Marine-grade is the premium exterior grade plywood -- and also the most expensive. Marine-grade can withstand complete immersion in water. Use it for applications, such as docks, piers, boat building, exterior benches, cabinets or furniture that require superior moisture resistance. Marine-grade is also manufactured with aesthetics in mind. A and B grades -- unlike the structural grades of D and C, are the most common.
Pressure-treated plywood is a different type of plywood, separate from structural and marine grade. This type of plywood is made from interior-grade plywood and is injected with chemicals to improve weather resistance.
When pressure-treated plywood is used as exterior sheathing, it has one side facing the interior, so chemicals in the plywood become a safety issue. While formulations differ -- with mixed results regarding safety -- plywood treated with borate is one of the safest of the pressure-treated options.