In roofing terminology, sheathing plywood is the layer of material attached to the structural frame of the roof. After you frame the rafters and ridge board, you lay the sheathing on top to cover the entire frame. The tar paper and composite shingles (or other roofing material) go on top of the sheathing plywood.
Sheathing Plywood Specifications
Sheathing is just 3/8 to 1/2-inch plywood or MDF installed in 4X8-foot sheets. You may use a circular saw to cut the pieces down to size when needed. You should use 8d nails to attach the sheathing to each rafter. The pieces should never meet between rafters; rather, they should join on a rafter. A piece of sheathing plywood must also extend over at least two rafters in order to remain balanced.
After the Sheathing
After you have installed the sheathing plywood, you will add the tar paper and composite shingles. Other roofing materials, such as copper, tile, or wood, are installed onto sheathing as well. Roofing insulation fits into the underside of the sheathing in the roof crawl space between the rafters.
If you want to install vents, skylights, or any other fixtures that protrude from the roof, you will cut holes in the sheathing.
Knowing the definition of sheathing plywood will help you know if it will fit your project's needs.