Exterior Plywood
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 1½ 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.
Marine Plywood
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.
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