Marine plywood is a form of plywood that has been constructed using waterproof adhesives to make it suitable for use in wet environments, including a home-built boat, the sub-floors of a home, the sheathing of a utility shed's walls and of a pier, boat house and other structures. It may serve as the underlayment of your boat's vinyl deck cover, the backing of the voids under wall and compartment overhead panels on your boat, and may even be part of your boat's hull.
Marine plywood is generally made of Douglas fir or Western Larch. Non-U.S. sources of marine plywood may use cedar or other woods for the fascia plies. All plies of wood are at least B grade, having knots, but without knotholes. The panels are glued together with a waterproof structural adhesive. The core may have a gap no larger than 1/8-inch throughout.
Marine plywood comes in sheets that are 4 x 8 feet, or 5 x 12 feet, with thicknesses of 1/4-inch, 3/8-inch, 1/2-inch, 5/8-inch and 3/4-inch. These dimensions are suitable for most uses on a boat or ship, or in a residential setting requiring a waterproof engineered wood, such as sub-flooring. Marine plywood is available with veneers in grades A-A, A-B, B-B, Medium Density Overlay (MDO) and High Density Overlay (HDO).
No decay-resistant chemicals are used in the manufacturing of marine plywood, although pressure treated marine plywood is available and is both rot- and insect-resistant. Some of the non-U.S. made plywood, such as cedar, are rot- and insect-resistant by the nature of the wood used in their manufacture.
Marine plywood played a part in the 1944 D-Day Invasion during World War II. Many soldiers arrived on the beaches of Normandy in "Higgins Boats," the brainchild of Frank Higgins, who convinced his father, Andrew Jackson Higgins, president of the Higgins Boat Company, that marine plywood was the best material available from which to build the Landing Craft Personnel.
Marine plywood is not waterproof; only the adhesive that binds the plies together is waterproof. If you plan to use marine plywood in a wet environment, it should be painted and any joints caulked just as you would with any other material.