What is Marine Grade Plywood?
Marine grade plywood is essentially the highest-quality plywood you can get. It’s called “Marine grade” in part because it has to be fully capable of standing up to rain, snow, and moisture on a regular, repeated basis in order to qualify. This isn’t the only qualification a piece of plywood needs to meet to be labeled as marine grade, however.
Marine grade plywood can only be made with Western Larch or Douglas Fir wood. It also must have 5 or more layers, with far fewer air pockets and small voids than regular plywood. It can have some knots in the exterior plies, but no knotholes are allowed. It must be bonded with waterproof glue.
How is Marine Grade Plywood Made?
Marine grade plywood is made by arranging 5 or more plies of wood veneer in a perpendicular manner so that the grains are alternating from vertical to horizontal. This perpendicular arrangement of the individual plies is what gives the plywood increased strength.
The plies are bonded together under heat and pressure, using a special type of glue that is waterproof. A water-resistant finish is then added to the exterior. The waterproof glue as well as the lack of voids, knots, and air pockets are the defining features that separate Marine grade plywood from various other types of plywood.
Also, the fact that Marine grade plywood is made with more, thinner layers of high-quality wood as opposed to fewer, thicker layers of lower-quality wood is what sets it apart from other types of plywood.
Marine grade plywood is not to be confused with similar types of plywood like pressure-treated plywood. While Marine grade plywood is technically “pressure-treated”, it is not soaked in or made with any special chemicals that increase its resistance to rot, mold, and moisture. The term “pressure-treated” is usually used to refer to plywood that has, in fact, been chemically soaked to improve its durability.
Grades of Marine Plywood
- A-A: This grade of Marine plywood comes in ½ inch thick pieces and is usually found in either 4×8 or 5x12ft pieces.
- A-B: This grade is found in the same length and width dimensions but is ¾ inch thick instead of ½ inch. A-B grade Marine plywood is stronger and heavier than A-A.
- B-B: This grade of marine plywood is also known as face-back Marine plywood.
- MDO: MDO stands for medium-density overlay. This is exterior-grade plywood that has been made with a resin-impregnated, high-quality thermosetting fiber overlay under heat and pressure. The veneer beneath this fiber surface is smooth and takes paint very well.
This grade is both durable and strong while possessing dimensional stability as well. MDO is usually found in 4x8ft pieces. It can be had in a variety of thicknesses including ¾ inch, ½ inch, ⅜ inch, 1 inch, and ⅝ inch.
This grade of marine plywood is resistant to chemicals, weather damage, scratching and denting. For this reason, it’s often used for siding.
- HDO: HDO stands for high-density overlay.
Marine Grade Applications
You might expect to see Marine grade plywood used in the installation and construction or birdhouses, furniture, boats, siding for buildings, signs, pergolas, gazebos, and more. Because Marine grade plywood is able to withstand 24/7 immersion in water, you’ll find it in many different docks, piers, boats, lighthouses, and other buildings and features that are constantly exposed to a high level of moisture.
Bathrooms, kitchens, patios, decks, and porches are also commonly built with at least some Marine grade plywood during construction. These are household areas that see a lot of water and moisture, thus Marine grade plywood makes an excellent choice for their construction.
