How Much Plywood Needs to Be Under a Ceramic Floor?

Standard Ceramic

• For the typical ceramic tile that is up to around 16 inches in facial dimensions you can use standard 5/8-inch thick plywood. You need to install at least two sheets to create a stable subfloor, and the sheets need to be securely fastened to each other as well as the joists. The floor joists should be no more than 16 inches in center from each other.

Large-Bodied or Natural Stone Tiles

• Large-bodied tiles over 16 inches in facial dimensions require thicker plywood for the base. This is also true of natural stone tiles as they are heavier than regular ceramics. In these cases you need to install two sheets of 3/4-inch thick plywood. The overall installation methods are the same as long as you securely fashion the two sheets to the joists and each other.

Mud Installations

• While these are normally reserved for high-end work, mud installations are where the homeowner opts to use a cement-based concrete mixture to float an underlayment in place for the tile installation. You need to use the same underlayment as for natural stone, but the joists need to be a minimum of 12 inches on center from each other, so the floor under the tile installation area needs to be reinforced accordingly or built to spec beforehand.

Interior or Exterior

• The type of plywood you use depends on the type of installation you are working on. For dry areas, such as living room floors or bedrooms, you can use interior-grade plywood. However, for all wet areas, including entryways, washrooms, bathrooms and laundry rooms, you need to use an exterior-grade plywood. The resins and glues in exterior grade plywood are meant to hold up to more weight and water exposure. You should also use exterior plywood for mud installations.