A.Patricia Davidson, TechNet professional at the Floor Covering Installation Contractors Association, responds: Before proceeding, verify that the moisture level of the substrate is not excessive. Remove any loose tiles or grout, and cap the drain. Clean the tiles to remove any grease, wax, or coatings that might interfere with adhesion. Then install a Portland-cement-based self-leveling underlayment, following the manufacturer’s recommendations. Self-leveling underlayments are available from Ardex (724/857-6400; www.ardex.com) and Mapei (888/300-4422; www.mapei.com).
Once the floor is leveled, cured, and sealed, the carpet can be installed. If you want to include a pad, you have two options: either use "double-glue cushion" — a type of pad that can be glued to both the substrate and the carpet — or install carpet with an attached cushion backing. Do not nail tack-strip into the hardened leveling compound...
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