How is Marine Plywood Different?

There are many unique types of plywood on the market today. In addition to standard plywood, there is a special type called marine plywood. As the name implies, this grade of plywood is used to handle applications that may be exposed to moisture or submerged in water. Discovered in the 1930s, marine plywood was first invented at about the same time as the formulation of a new waterproof adhesive. In its earliest version, this grade of plywood was used to build ships during World War II. After that era, marine plywood has provided a case for outdoor furniture, marine components and coastal homes. So, what exactly makes a plywood marine grade?

Waterproof Pine 15MM 12MM CDX Plywood

Standard and Marine Plywood

Standard plywood is made from softwood or hardwood veneers or laminates. These planks are then glued together to form thicker boards. The most common plywood is made of cedar, pine and spruce. The grain of each layer is rotated 90 degrees to form a strong product. For marine plywood, the main species used in the creation are Douglas fir or western larch. These materials are favored for their fine grain structure, durability and moderate density. Marine plywood also typically has thinner layers than standard plywood. Having more, thinner layers is better for structural integrity than fewer, thicker layers. These thinner layers also promote cleaner sanding, sawing and machining.

Wood species is not the only unique feature of marine plywood. The adhesives used to make marine grade plywood are water and heat resistant. Typically, marine plywood uses melamine or phenolic binders. These types of compounds make the entire board very resilient to water, as each layer does not crack or peel. The adhesives are also heat resistant, so even boiling water cannot penetrate the bonded layers.

Overview

Marine plywood uses melamine or phenolic binders that make it very resilient to water.

Grading system

There are several grades for the different plywoods. Standard plywood is sold in grades A, B, C and D, with A being the best quality. Marine plywood follows the principles of these typical grading levels and has a double grading. Double grading describes double-sided plywood. For example, AB grade plywood has a clean, almost perfect side and a rougher, looser side. Many times, projects involving plywood will only see one side in critical condition, or only one side, so it makes economic sense to invest in higher quality veneer for the critical side.

Marine plywood is available in grades AA, AB, BB, and two other special grades, medium density cladding and high density cladding. The last two grades have a resin-impregnated fiber layer that acts as a weatherproofing layer. The high density overlay has a higher percentage of resin, resulting in a brighter, harder surface.

Marine Plywood Performance

Most marine plywood is not completely waterproof. The wood used to make it is not chemically treated, so it does not resist decay or weathering. If left untreated, the outer layer may degrade, but with proper fabrication, it will not get into the first adhesive layer. Pressure treated lumber or chemically treated lumber is better at preserving rot than marine grade lumber. In fact, pressure-treated marine plywood is available at your local hardware store and is even suitable for saltwater applications. That said, you can always use a protective sealer or polyethylene coating to provide the ultimate weatherproofing protection for marine plywood.

There are also differences between marine plywood and exterior plywood. These use high-end phenolic binders that are weather and boil resistant, but for marine grades rather than exterior grades, the quality of the cord fabric layer is more closely regulated. This is because exterior plywood is likely to be masked, so appearance is not as critical as marine plywood visibility on boats or outdoor furniture.

Marine plywood is an extraordinary plywood product. Few options are available to protect your project from water and heat as well as marine grade plywood. Your next project will last for generations when you use this weather-resistant grade of plywood.