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Leveling a bathroom floor to put down tile

Ask the Expert Home Improvement
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Q. I am renovating a bathroom in my house, and have a question about floor depth. I removed the tile and a concrete-like material to get to the subfloor, and discovered the thickness of the “concrete’’ and tile is 1.5 inches. I’m not sure what this concrete-like substance is: It is gray, breaks apart easily, and is clearly thicker than thin-set. My plan was to replace the floor with cement board and tile, which together will measure only ¾ of an inch in thickness. This causes problems for the toilet height and for the marble threshold in the doorway. What should I do to address the difference?

H.H.

A. It sounds as if you had a mortar thickset or “mud job’’ underlayment. A mud job tile installation consists of finish tiles set over a 1¼-inch to 2-inch thick Portland cement mortar bed. The mortar bed sits over the subfloor and provides a level and stable base for the tile installation.

Mud jobs are often used to level and adjust uneven subfloors, and it also creates an ideal surface for tile adhesion. The thickness of the mortar bed allows the floors to be sloped — to a shower drain, for example. The mortar bed can also conceal other items, such as tubes for heated floors or waterproof pans.

In order to get your floors back on the same plane, you need to redo the mud job or build it up with other materials. Assuming your floors are level and flat, you can build up your existing subfloor with a ¾-inch plywood underlayment (overlapping your seams), and then finish off with ⅜-inch tile backer board and ⅜-inch floor tile. That will equal 1.5 inches.