Prepping a Bathroom Floor for Tile

Tile contractor Tom Boucher carries a roll of Schluter Ditra polyethylene floor underlayment to the work area. Easy to carry and store, the roll has more than enough material for the master bath floor.

Boucher cuts a piece of Ditra to length with a utility knife.

Boucher trims a piece of Ditra to fit around the base of a half-wall partition between the toilet and the main bathroom.

Step one for applying the underlayment is to thoroughly scrub the Advantech subfloor with a heavy-duty sponge. Otherwise, drywall dust and other dirt could interfere with the mortar bond between the Ditra and the subfloor.

Boucher works a slurry of latex-modified mortar into the Advantech subfloor, to ensure a good bond between the mortar and the wood structural panel.

Boucher applies a measured amount of latex-modified mortar to the prepared subfloor using a three-eighths-inch toothed trowel.

Boucher lays the Ditra decoupling membrane onto the wet mortar. The mortar will work its way into the keyed polyethylene sheet, and also lock into the fleece underside of the material for a mechanical bond. In service, the Ditra's pliability and flexibility will prevent normal expansion or contraction of the subflooring from applying excessive stress to the relatively brittle tile and grout.

To ensure a good bond, Boucher presses the Ditra membrane onto the mortar bed using an 18-inch float.

Boucher pre-creases a strip of Kerdi membrane, before mortaring the Kerdi into the corner joint where the Ditra underlayment meets the wall.

Boucher works mortar into the drywall and into the Ditra membrane at the floor-to-wall intersection.

Boucher applies a measured mortar bed to the drywall using a Kerdi trowel with one-eighth-inch notches.

Boucher gently presses the strip of Kerdi membrane into the prepared mortar bed at the wall joint.

Boucher works the membrane into the mortar using a five-inch knife. He has filed the corners off the knife so that it won't gouge or tear the membrane.

Boucher applies a mortar setting bed at the joint between two sheets of Ditra underlayment. Note: the white mortar already in place at the joint was placed the day before and has hardened overnight. This is a way to use up small quantities of extra mortar so the material won't be wasted, Boucher explains. The fresh mortar will still bond strongly to the previously placed and hardened material.

Boucher uses the notched trowel to calibrate the depth of the fresh mortar bed at a joint between two pieces of Ditra underlayment.

Boucher presses a strip of Kerdi membrane into the freshly applied mortar.

Boucher presses the Kerdi membrane into the mortar bed with his customized drywall knife.

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