Correcting minor discrepancies in a concrete slab for tile is part of my job. But if the surface is inches out of whack and slopes in the wrong direction, that's a whole other story. When my friend Bob asked me to tile the courtyard of his retirement home to eliminate the puddles and dress it up, I could tell at a glance that the slab was cracked and sloped toward a low spot at the entry door. Excess rainwater drained from this "birdbath" undesirably into an adjacent flowerbed. The next lowest spot was just inside the gate at the opposite side of the courtyard, and humps and valleys crisscrossed the rest of the area.

In the past, I've had builders downplay such problems, telling me to just "add a little more thinset" in the low areas. But with that approach, some parts of this slab would have required more than 3 1/2 inches of thinset to achieve the proper slope—less than ideal because the compressive and cohesive strength of thinset drops significantly when...

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