Marine-grade plywood, as its name implies, is plywood specifically designed to be able to withstand constant immersion in water. Marine-grade plywood is, in fact, the only grade of plywood approved for constant immersion in water, and as such, it is perfect for use in applications such as piers, docks, boat building, and any other project which must withstand spray and moisture related to either saltwater or freshwater.
Marine-grade plywood bears many similarities to regular plywood. Plywood is manufactured from various thin layers of wood glued together and then placed under immense amounts of pressure. In order to ensure optimum strength, these layers of wood are typically glued with their grains at right angles to one another. Plywood boards are widely used due to their versatility and affordability, and marine-grade plywood is just one type of plywood. Other types of plywood include pressure-treated, exterior-grade, and decorative (overlaid) plywood.
Marine-grade plywood is commonly constructed using Douglas Fir or Western Larch tree lumber, as these types of wood generally lend themselves very well to the durability required within a marine type environment. The layers of veneer used to construct the plywood boards are glued together using a waterproof structural adhesive, and the boards are then graded with either an A or a B grade. These grades depend on the board’s appearance, with A grade veneer lacking both knots and knotholes, and B grade veneer containing some knots but no knotholes. Regardless of grade, each board is then sanded on both faces before being treated for waterproofing.
Marine-grade plywood, because of the special treatments it must receive in order to meet industry standards for the label “marine-grade,” costs more than standard plywood. This is a necessary price adjustment, however, as marine-grade plywood is often very rot and warp resistant compared to other plywood types. Rather than being chemically treated for decay resistance, marine-grade plywood is typically pressure treated. The chemicals used to treat wood for decay resistance are usually copper and arsenic compounds, which not only fail to completely waterproof the wood but can also leech when submerged, thus poisoning the water.
When researching plywood for your project, keep in mind that although marine-grade plywood is pressure treated, so-called “pressure-treated plywood” is a separate grade of plywood. This type of plywood is made from interior-grade plywood and is usually treated with the aforementioned chemicals to inhibit decay. Marine-grade plywood, on the other hand, generally does not receive the same chemical treatment and also often contains fewer gaps than pressure-treated plywood.
Although marine-grade plywood is only constructed from exterior-grade plywood (which is more durable than interior-grade plywood, as it contains fewer gaps and voids and is made from harder woods), a distinction must be made between exterior-grade plywood and marine-grade plywood. Not all exterior-grade plywood can be called marine-grade plywood. Although all exterior-grade plywood is required to use water-resistant glue, the composition of the wood itself varies between exterior-grade and marine-grade. Exterior-grade, for example, is only free of voids and gaps on the exterior shell; the interior layers may very well contain both voids and gaps, which is not the case in marine-grade plywood. Exterior-grade plywood often also uses species of wood that are not approved for use in marine-grade plywood.
If you are considering starting a project that requires the use of marine-grade plywood, contact the professionals at J. Gibson McIlvain lumber company. They have the knowledge to answer each of your questions, whether those questions are related to marine-grade plywood or other types of wood, and their professional staff has the expertise to help you make your next lumber project a success. For more information, visit their website at www.mcilvain.com.
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