An uneven floor or wall surface can lead to several problems in a tile job. Tiles may crack, pop loose or “lip” — a condition that occurs when the corners or edges of some tiles stick up higher than those around them. Many older homes have uneven surfaces, but several techniques can be used to help lay the tiles in a way that will not lead to damage.
Floor Leveling Compound
If the substrate the floor tiles are being installed on is uneven but structurally sound, a self-leveling floor compound can be used to even it out. Self-leveling floor compounds are liquid bases that seek out the low areas on a floor and fill them in. They are typically poured over the entire floor; not just uneven areas, and they require no smoothing or additional leveling once dry. They take 24 to 48 hours to cure and the tiles can be installed right on top. Keep in mind, however, that a self-leveling floor compound will increase the thickness of the finished floor, so transitions to adjacent rooms may need thresholds.
Back buttering is a trick used for tiles of uneven thickness, like ungauged slate or handmade tiles, extremely large tiles or for tiles being installed on an uneven surface. The mortar is spread over the substrate and keyed, or grooved, as usual. A second application of mortar is placed directly onto the backs of the tiles. Extra mortar is placed on those areas that need it, such as thin spots on tiles or low spots on the substrate. The extra mortar in the low areas helps even out the appearance of the final installation and creates a smooth surface.
Large-format tiles are the most difficult to install over an uneven surface because they have no way to bend or flex over a hump or valley. Mosaic tiles, however, can be installed over extremely uneven surfaces with little problem. Because each individual tile measures 2 inches or less in size, the tiles are able to move with an uneven surface, going up or down as the surface demands. The more uneven or curved a surface is, the smaller the tile should be to accommodate it easily. Mosaic tiles are available in sizes as small as 3/8-inch, which can accommodate any surface.
If the surface is only mildly uneven and is only producing mild lipping at the corners of the tiles, beating the tiles into the mortar bed can reduce the issue. Use a trowel with a 1/4- to 1/2-inch groove to produce an extra thick mortar bed for the tiles. Lay a flat board over the tiles, paying attention to the uneven spots, and beat the board with a rubber mallet. This will drive the tiles deeper into the mortar; if more pressure is placed on low spots than on high ones, it can even out the finished surface of the tile installation.
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