Exterior, is the grade given to any plywood which is bonded with 100 percent waterproof adhesive and is intended for permanent outdoor exposure. Exterior plywood must be graded a C or above, meaning knots and knot holes of up to 1Ĺ inches are allowed. The species of wood can also vary and often different species are combined. Core gap size for exterior wood cannot exceed 1 inch wide. Exterior wood is good for most all wood needs and can be purchased to look almost identical to marine plywood at a much lower cost.
Plywood graded C-D or CDX is often mistaken for exterior plywood but is not approved by the APA for outdoor exposure. CDX plywood is considered Exposure 1 plywood. It can encounter high moisture content but needs to be sealed for complete outdoor use.
Made entirely of Douglas Fir or Western Larch, marine-grade is one of the highest designations which can be given to a piece of plywood and is considered to be "premium." In order to be marine grade plywood, the outer plies must be graded at least a B or better. B grading means that the wood may have some knots, but no knotholes. In order to be A-grade plywood, there cannot be any knots or knotholes present in the layer. The maximum core gap size allowed is 1/8 of an inch, and both outer panels must be sanded, Medium Density Overlay or High-Density Overlay. The durability rating must be Exterior and a fully waterproof structural adhesive must be used.