For boat-building applications, a special grade of plywood, APA Marine Exterior, is available. Marine-grade plywood is a premium panel using the same fully waterproof glues as regular Exterior plywood, but manufactured with only Douglas-fir or Western Larch veneers, and with additional restrictions on veneer grade, core gaps, patches and other characteristics. The grade of all plies of veneer is B or better. B-grade veneer may have knots but no knotholes. A-grade veneer has no knots or knotholes. Both A and B grade may contain wood or synthetic patches. Panels are sanded on both faces. The maximum core-gap size permitted is 1/8 inch. Its exposure durability rating is EXTERIOR and the glue used is a fully waterproof structural adhesive. It is considered a "premium" panel grade for use in situations where these characteristics are required. It is available in 4x8-foot sheets of 1/4, 3/8, 1/2, 5/8 and 3/4-inch thickness.
Marine-grade plywood is not treated with any chemicals to enhance its resistance to decay. If decay is a concern, it should be pressure-preservative treated to an appropriate standard. The detailed description of veneer grades and Marine-grade plywood is contained in Voluntary Product Standard PS 1-95 Construction and Industrial Plywood.
Marine grade should be used where bending strength is important, such as boat hulls. Basic Exterior-type plywood may be used for most other applications in boat construction. Plywood panels rated as Exterior but not Marine, such as A-B, A-A or C-C Exterior, may contain any other permissible species and contain C-grade veneer. Unless specially improved, C-grade veneer is permitted to contain knots and knotholes up to approximately 1-1/2 inches across, and the inner plies may have core gaps up to 1-inch wide.
• Slow growth inland Douglas Fir and Western Larch yield fine-grained, smooth faces
• All Group 1, Struc 1 species Faces
• "A" grade faces shall be limited to a total of nine repairs in a 4'x 8' panel. Backs, Centers & Cores All backs, centers and inner plies shall be "B" grade or better and shall be full length and full width. Neither edge of a panel shall have any cross band gap or edge splits in excess of 1/8" wide or 4 in number. No inner ply laps are allowed.