How to Install Ceramic Tile Backerboard on Subfloor

By Mark J. Donovan

When installing ceramic floor tiling it is best to install a cementitious ceramic tile backerboard over the subfloor first.

A ceramic tile backerboard should be installed on a subfloor that is expected to have high foot traffic. A backerboard of 1/4" or 1/2" thickness should be installed over the subfloor. The subfloor should be constructed out of 3/4" tongue and groove plywood installed over 16" on center floor joists. If the subfloor is warped, or flexes when you walk on it, then another 1/2" layer of exterior grade plywood should be installed over it. This second layer of subfloor should be secured to the base layer with screws or nails spaced every 6 inches on-center. The length of the screws or nails used should not be longer than the thickness of the two combined sublayers.

Once the subfloor is structurally sound you should next install 1/4" or 1/2" thick cementitious ceramic tile backerboard over it. The ceramic tile backerboard should be secured to the subfloor using a non-modified thinset mortar and 1-1/4" screws or nails spaced on 6" centers. Use a 1/4" x 1/4" trowel for applying the thinset mortar.

It is important to note that the thinset mortar is not used to secure the backerboard to the subfloor. Instead, it is used to fill voids between the backboard and the subfloor to eliminate any flexing between the two layers.

When installing the backerboard, space the boards 1/8" apart from one another. Fill the gaps with thinset mortar, as this will help to bridge any ridges between panels. Also, apply fiberglass tape into the mortar over the seams. Trowel a skim coat of mortar over the tape and smooth it out with a flat trowel.

Once the thinset mortar has dried, the ceramic tile can be installed.