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Probably 90% of the tile installation problems I’ve encountered are a result of the tilesetter failing to allow for substrate movement. Typical problem spots are vertical wall planes that intersect with horizontal surfaces, concrete substrates with control joints or random cracks, and the junctures of dissimilar materials. In these situations, a "movement joint" (my term for a broad class of expansion or control joints) should be used. Inside corners should be left free of grout, then caulked after the grout has cured. Use a high-quality silicone caulk, and install a foam backer rod in the open joint before caulking. It’s also important that all "hard" material be cleaned out of an open joint before caulking. Small bits of hardened adhesive or grout can apply point pressures to adjacent wall surfaces, causing cracks as the substrate expands. When installing tile over a concrete slab, creating movement joints can be a little trickier. One approach is to position an open tile joint directly above any slab joints, install backer rod, and caulk. The trouble with this approach is that the location of this open joint may disrupt a patterned tile layout, or you may be forced to follow a slab joint that is crooked or out of square. A more adaptable approach is to use a crack isolation membrane like the one manufactured by the Noble Company (614 Monroe St., Grand Haven, MI 49417-0350; 800/878-5788). Although its use will not completely eliminate cracks, the 1/32-inch membrane material isolates the tile from the substrate movement and allows tile movement joints to be located near the slab joint instead of directly over it. The width of the membrane should be three times the tile size, and it should be bonded to the slab using either Noble’s proprietary latex-based adhesive or an acrylic or polymeric modified thinset mortar (see Figure 1).
Figure 1. A tilesetter uses a floor roller to adhere a crack isolation membrane over a slab control joint. The membrane should be three times the width of the tile.