As a general rule, any basic shingles require a minimum of 1/2-inch-thick plywood to be used. However, this only holds true for rafters that are no greater than 16 inches on center. This means the rafters are 16 inches apart from the center of one rafter to the center of the other rafter. The sheets need to be installed so that any termination points rest halfway across a rafter so that the adjacent piece can also attach to the rafter, rather than float in between rafters.
The wider the spacing in the rafters, the thicker the material needs to be to stand up to the weight of the finish shingle material and to avoid deflection, also known as flexing. Plywood that is 5/8 inch thick is preferred for rafters that have 24-inch spacing. However, if you know you are working with a heavier type of roofing material, you may want to use a 3/4-inch material for safety’s sake.
Slate Tile Shingles
Given the fact that slate tile shingles are one of the heaviest types you can install, it is recommended that you use at least a 3/4-inch-thick plywood sheet material for the base. Some professionals will actually recommend that you use double layers of 3/4-inch material, similar to what is used in slate tile installations on floors. However, slate shingles don’t have the same deflection requirements as tile floors, so a single sheet is generally sufficient.
If you are working with a typical shingle type, such as asphalt shingles, 1/2- or 5/8-inch material is general sufficient. However, you should always check with the shingle manufacturer to determine its recommendations. Always follow manufacturer recommendations to preserve any warranties attached to the products. If you install shingles with plywood that is too thin, you void any warranties associated with your roofing products and may have issues insuring your home.