Plywood is a manufactured timber board that consists of multiple thin layers of wood veneer glued together. For extra strength and stability, the layers are placed so that the grain of each veneer is at right angles to the adjacent layers. There are a myriad of plywood products available, but to be suitable for exterior use it must be constructed with an adhesive able to withstand moisture and outdoor weather. In the United States, plywood suitable for outdoor use is generally identified by the code External Use (EXT).
Softwood vs. Hardwood
Plywood can be made from either softwood or hardwood timber species. Hardwood plywood is generally used for decorative applications such as the construction of furniture or musical instruments, with a strong emphasis on high-quality face veneers. On the other hand, softwood species generally are more suited to structural applications. Both are available in varieties suitable for outdoor use, as long as external-grade glues are used in the manufacturing process.
Marine plywood is a superior quality plywood that is specially treated to resist rotting in high-moisture environments. It is used primarily in the marine industry, for projects such as the construction of docks and boats. Fully waterproof structural-grade adhesives and higher-grade timbers are used in its construction process to ensure added durability and strength. It is suitable for heavy-duty outdoor use, and is therefore much more expensive than general exterior-grade plywood, up to three times more per sheet.
Tropical plywood, commonly known as "Luan" plywood, is made from wood species originating in the Southeast Asian region. It is a soft, light plywood that is commonly used for crafts and furniture construction. Due to it's lightness it can also be used to build small boats such as kayaks. However, it requires intensive surface treatment to prevent the ingress of water and eventual warping and rotting. It is available in varieties suitable for both indoor and outdoor uses.
Structural plywood is manufactured to a minimum stress grade suitable for high-end construction, such as concrete form work and other intensive uses where the plywood will be used in a load-bearing capacity. It's added strength is achieved through the use of high-quality bonding and more durable timber species such as birch and maple. Almost all structural plywood is made with high-grade bonding that is suitable for outdoor use.
Any plywood identified as internal-grade plywood by the manufacturer is exactly that, designed for internal use only. In the United States, internal-grade plywood is identified by the code "INT." Some of these products are usable outdoors when combined with surface treatments such as paint or sealer. However, it is always recommended to select the plywood product designed for its intended purpose.