Exterior plywood, which is coated with waterproof glue during manufacturing, can be used outdoors. The waterproof glue is resin-based and dries to a hard, crystal finish that resists moisture damage.
Plywood graded for outdoor use is categorized as exterior plywood. Plywood is graded with three letters. The first two letters indicate the quality of the plywood, while the last letter indicates an interior or exterior designation. The letter "X" is the mark for exterior use.
Exterior grade plywood can be used indoors. For example, it is often used as a subfloor because it resists damage from spills and moisture in a home's crawl space.