Wade Shaddy – The application of plywood depends on the manufacturing process. One size doesn’t fit all, and there are distinctive differences between interior and exterior plywood — even if they are not visible. Exterior plywood can be used indoors or outdoors, but interior plywood should never be used outdoors under any circumstances.
Glue is the differentiating factor in exterior plywood. When the plywood is manufactured for exterior use, waterproof glue is used. It is a resin-based glue similar to the glue used in wooden ships. It dries to a hard, crystal substance that will never yield to moisture of any kind. This glue is applied between every layer of the plywood. Even though the plywood will hold up to almost any weather conditions, it’s recommended that some type of paint or finish be used on exterior plywood if it is exposed directly to sun or water.
When purchasing exterior plywood, read the marks on the plywood to identify it. Common plywood markings will have three letters indicating the plywood’s application. The most typical marking for exterior plywood is “CDX.” Plywood quality is ranked by letters of the alphabet, with “A” being the best ranking. Using this formula, CDX means that the plywood has one “C” side and one “D” side. The “X” means that it is designated for exterior use.
Exterior plywood is not restricted to outdoor use. It’s almost always used as a subfloor in most homes so that its waterproof qualities will resist moisture from below or water that may be spilled from above. Exterior plywood can also be used on countertops around sinks or even inside cabinets. Exterior plywood works fine for just about any interior application, except for the fact that it has numerous defects or patches in the surface.
The majority of all exterior plywood is made from fir. This lumber is widely produced and harvested in the Northwest, making it the favorite for affordability and availability. Exterior plywood may be slightly more expensive, but typically it is not because it doesn’t need to be finished. It is full of defects that do not detract from its usefulness but do restrict it to floors, exterior sheathing or other applications that cover the plywood.
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About the Author
Specializing in hardwood furniture, trim carpentry, cabinets, home improvement and architectural millwork, Wade Shaddy has worked in homebuilding since 1972. Shaddy has also worked as a newspaper reporter and writer, and as a contributing writer for Bicycling Magazine. Shaddy began publishing in various magazines in 1992, and published a novel, “Dark Canyon,” in 2008.