Tom Silva
Q: I'd like to install flooring over the uneven, out-of-level concrete slab in our basement. Is there any way to level the slab without reducing the already limited headroom?

—  Milivoj Antic, Williston Park, New York



A: Tom Silva replies: The easiest way to level a floor is to use a self-leveling compound, also called liquid floor underlayment or floor resurfacer. You mix it with water and pour it out on a floor; the material flows out like thick syrup, then hardens into a smooth, perfectly level surface, sometimes in less than an hour. The only places you'll lose any appreciable headroom will be in the slab's low spots.

In a basement, you should use a compound that contains portland cement rather than gypsum because it won't be affected by moisture. Before you pour, give the existing slab a thorough cleaning with a degreaser to remove any contaminants that might interfere with getting a good bond. And before you lay your flooring, check the installation instructions to be sure the compound is dry enough.
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