If you’re considering building a boat, pier, dock, or any other wooden structure you want to withstand constant assaults from the sea, you should know the facts about marine-grade plywood. This type of plywood is the only grade of plywood made to withstand constant immersion in water.
Plywood, by definition, is composed of an odd number of thin layers of wood glued together under pressure, usually with their grains at right angles to one another. Among the various grades of plywood, exterior-grade plywood, pressure-treated plywood, and marine-grade plywood all include various levels of safeguarding against rotting and warping. Although similar in quality, these plywood types are distinct.
Marine-grade plywood is composed of lumber derived from Douglas fir or Western Larch trees. Each of the plies is grade A or B. A-grade veneer lacks any knots or knotholes, and B-grade may have some knots, but no holes. Each panel is sanded on both faces, and the maximum core-gap size is 1/8”. Only wood with an “Exterior” durability rating is used, and fully waterproof structural adhesive is used to combine the veneers.
Instead of being chemically treated in order to increase decay resistance, marine-grade plywood should be pressure-treated. However, even though marine-grade plywood may be pressure-treated, “pressure-treated plywood” is technically distinct from “marine-grade plywood.” Pressure-treated plywood (also called “Wolmanized” or P.T. plywood) has usually been subjected to pressure treatment that includes chemical treatment, in order to prevent decay or rotting. The chemicals typically used are copper and arsenic compounds, which not only fail to make plywood waterproof, but also risk being leached from the treated wood, if continuously exposed to water. Another important distinction between pressure-treated and marine-grade plywood is that the former is made from regular, interior-grade plywood, often comprised of softer woods with common gaps and voids.
In addition to distinguishing between marine-grade plywood and pressure-treated plywood, it’s important to distinguish marine-grade plywood from other exterior-grade plywood, as well: Not all plywood panels with an “exterior” rating are marine-grade. While all exterior-grade plywood uses water-resistant glue, only the exterior shell is completely free of voids and gaps; when cutting exterior-grade plywood, you may expose such weaknesses. Those rated as A-B, A-A, or C-C Exterior may contain species of wood other than those approved for marine-grade plywood, in addition to C-grade veneer. The C-grade veneer can include knots and knotholes up to 1-1/2 inches in diameter, and inner plies may include core gaps up to an inch in width.
As you might imagine, marine-grade plywood is more costly than pressure-treated plywood or other exterior-grade plywood, so it may seem like a better bargain to use an inferior grade in order to save money. However, the higher price of the kind of plywood that’s been prepared especially to withstand constant exposure to water is worthwhile. This grade of plywood won’t bubble, buckle, or warp, and its lamination will not wear. If you’re not sure whether your project requires marine-grade plywood, you can ask the professionals at J. Gibson McIlvain about the specific project you have in mind.