Severely out-of-level floors are a fact of life in the New England homes that I work in. For a tile setter, these floors can be a nightmare—especially when the project calls for large-format tile that requires the subfloor to be within 1/8 inch of flat and level in 10 feet (“Working With Large-Format Floor Tile,” Nov/17).

Recently, clients asked me to install large-format tile on the floor of an L-shaped bathroom that they were remodeling and expanding with a new walk-in shower. When I arrived at the project, the carpenters had already installed the plywood underlayment, but I could tell at a glance that the outside corner of the L was very high. A long level confirmed that the floor dipped a whopping 1 inch in about 4 feet in the direction of the toilet and shower, and more than 3/4 inch toward the entry door.

New options. In the past, my options would have been limited. If the deviations were 1/4 inch or less, I could have built up low spots with layers of thinset. In an extreme case such as this project, I would have had to do a full mortar bed, which is messy and time consuming. Instead, I opted to use one of the self-leveling underlayment products now on the market.

Self-leveling concrete products have been around since the late 1970s. The product I used, Ardex Self-Leveling Backerboard, is a polymer-modified, Portland-cement-based product that mixes with water and is then poured onto the low areas of the floor. It has high compressive strength, making it ideal for installation under tile floors, and it can be feathered into the high areas, maintaining strength and adhesion as it approaches zero thickness.

Before I could mix and pour the self-leveling compound, the subfloor had to be primed with Ardex P51, a milky liquid that I applied to the subfloor with a coarse broom. Ardex says that the primer should be allowed to dry a minimum of three hours and a maximum of 24 hours before you mix and apply the self-leveling compound.

Self-leveling underlayment products are not cheap. We used four 50-pound bags for this particular floor at just under $50 per bag. In addition, the primer cost about $70 per gallon jug. This price may seem a bit steep to some, but the time it saved me was invaluable. And in one pretty simple and quick operation (shown in the photos), I was able to form a strong, solid substrate for the tile floor that was level, flat, and perfectly smooth.

Photos by Roe Osborn