A.Heating contractor Bill Clinton responds: If you want a slab as thin as possible and expect to tile over it, I think regular concrete is out of the question. You’re bound to get cracking, which will telegraph through the tile. My choice would be a gypsum-cement product — either Gyp-Crete from Maxxon Industries (800/356-7887; www.maxxon.com) or Gyp-Span from Hacker Industries (800/642-3455; www.hackerindustries.com). Gypsum cement, unlike concrete, doesn’t shrink as it cures and usually won’t crack. When it comes to preventing cracks, the most important factor is the rigidity of the floor framing and subfloor.

Gypsum cement can, with care, be poured as thin as 1 1/4 inches. You can thin-set the tile to a gypsum-cement slab. But before tiling, you need to allow the gypsum underlayment to dry completely, and then seal it. In bathrooms, you will need a waterproof membrane, like NobleSeal (800/678-6625; www.noblecompany.com).

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