Q.: We've been searching for published standards for the tolerances allowed in finishing residential flatwork. Are there any you know of? Specifically, are there any standards for basement floor slabs that are intended for possible future living space? We have a 3-inch drop within distances of 6 to 10 feet in a basement floor. If we apply a topping, the floor level will be above the bottom of a door.

A.: Actually, there are two main considerations: flatness and levelness. Flatness is concerned mostly with the small, shallow waves of about a foot in length. Levelness is concerned mostly with the major tilting, upward humping or downward dishing of the floor. Recently developed studies and methods of measurement are making it easier to sort these out and to express the flatness mathematically by a single number known as the F number. ACI Committee 117, Tolerances, is currently preparing a document that is expected to assign reasonable F numbers for flatness and levelness for various kinds of use. This committee document is recent and has not yet been standardized, so its values are still not officially recognized. Nevertheless, the values you cite are very large. Based on common practice and experience they would undoubtedly be considered unacceptable by most authorities.