How to Prep a Plywood Subfloor for Tile
For residential use, one form of underlayment the Tile Council of North America’s handbook allows is plywood over joists that are 16 in. O.C. The go-to product is 1/2-in. void-free underlayment grade panels, which both stiffen the existing floor and provide uniform support to prevent cracks in the tile above. Installation is a little counterintuitive though. Carpenters are used to installing all sorts of panels, from subfloor to wall sheathing to drywall so that the panel edges fall on joists, rafters, or studs. That’s exactly what you don’t want to do when installing underlayment. The panel edges should fall between the floor joists, and avoid fastening into them. The idea is to isolate the underlayment from movement in the framing below.