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Installing Ceramic Tile Backerboard on Subfloors

How to Install Ceramic Tile Backerboard on Subfloors

By Mark J. Donovan

Question: I’m going to ceramic tile a second floor bedroom. My understanding is that I need to put down a concrete backerboard first. Can I just screw the ceramic tile backerboard down directly to the subfloor? If so, what sort of spacing should I do?

Answer: Yes, you should install 1/4" or 1/2" ceramic tile backerboard over the subfloor first IF there is a reasonable amount of foot traffic expected and the subfloor is questionable in terms of its rigidity. The subfloor should be at a minimum constructed out of 3/4" tongue and groove plywood installed over 16" on center (maximum) floor joists.

If the subfloor is warped, or is observed to have some flexing when you walk on it, then you should install another 1/2" of exterior grade plywood over it. You should secure this layer of exterior plywood with screws every 6 inches on-center. Make sure you use screws that are not longer than the total thickness of the two subfloor layers.

Once you are sure your subfloor is structurally adequate you should then install 1/4" or 1/2" thick cementitious ceramic tile backerboard over it. The backerboard should be installed to the subfloor using a non-modified thinset mortar and 1.25" screws or nails spaced on 6" centers. Use a 1/4" x 1/4" trowel for applying the thinset mortar.

Note that the thinset mortar is not used as a backerboard fastening agent. 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 set up, you are ready to install the ceramic tile.

If you still have concerns on whether or not you should use a backerboard, check directly with the ceramic floor tile manufacturer, and not the local tile reseller. The ceramic floor tile manufacturer will know best about the appropriate subfloor preparation, and will more than likely give you the most accurate answer for your specific ceramic floor tile.

For information on ceramic tile and installing a shower pan membrane liner, see the Shower Pan Membrane Liner Installation Ebook from HomeAdditionPlus.com. The Shower Pan Membrane Liner EBook will quickly teach you the step-by-step process for installing the shower pan membrane liner correctly. It includes instructions on framing the shower stall, pouring the pre-slope and shower base mortar, and installing the shower pan membrane liner.

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