|Marine Grade Plywood|
Marine-grade plywood is made entirely of Douglas-fir or Western Larch. 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 or Medium Density Overlay (MDO) or High Density Overlay (HDO). 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. Sheets up to 5x12-feet are also available. Available grades are A-A, A-B, B-B (face-back), MDO and HDO.
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.
Sample Specification For Marine Grade
APA 3/8" B-B Marine Grade 4x8 10 pieces
Other Exterior Plywood Grades
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.
Â©1996 APA - The Engineered Wood Association
The above is a Technical data sheet from the American Plywood Association. Plywood grading in the US is not regulated by the Federal Government, but is a voluntary set of standards administered by APA.