Basics of Exterior Plywood
Exterior grade plywood is specifically designed for outdoor applications. It has been put together with different types of resins and glues to hold the sheets together and is manufactured to withstand the bluntness of Mother Nature and the various seasons. It is not a finish material and will always need to be covered with something else, as it is a functional rough material used for roughing in and building up other materials.
Exterior plywood can be cut down to use as strips for installations that need to be put down on top of concrete slabs, such as when you want to install a wood or carpet floor and need to provide extra spacing for the installation to breathe on top of the concrete. Another layer of plywood is generally installed on top of these strips to provide a base for the finish material.
Subfloor for General Materials
While most wood decking can be installed directly on top of the joists for the deck or porch, you can also opt to install a layer of exterior plywood first. In addition, some types of composite decking will require a base layer of plywood on top of the joists so they can be mounted across the length and width of the deck. A single layer is generally sufficient for wood or outdoor carpet installations and other lightweight general finish materials.
Subfloor for Tile
Tile floors require a heavy-duty layer of doubled-up exterior grade plywood in order to support the weight of the tile and provide stability. Two layers of 5/8-inch-thick plywood are required for normal tile installations, while 3/4-inch plywood doubled up is required for natural stone installations. A layer of concrete or fiber board underlayment needs to be installed on top of the exterior plywood before the tile can be set.