Q: We're designing the concrete floor for an auto dealership's service department. Trench drains are to be installed in the floor, but we don't know what slope is required to make sure that liquids flow into the drains. Also, what is a reasonable tolerance on the slope we decide upon?

A: We could find no reference to recommended slopes when trench drains are used in a floor. The recommended slope for flat roofs is usually 1% (1/8 inch per foot) or 2% (1/4 inch per foot) according to The Professional Handbook of Building Construction, published by John Wiley & Sons, New York.

A 2% slope will probably ensure better drainage.We could also find no slope tolerance for use when a floor slope is specified. ACI 117-90, Standard Specifications for Tolerances for Concrete Construction and Materials, gives a level alignment tolerance of 3/4 inch for top-of-slab elevation of slabs on grade. If the slab elevation at the trench drain is within 3/4 inch of the plan elevation, and the slab top elevation farthest from the drain is also within 3/4 inch of plan elevation, the slab would meet ACI 117 tolerances. This tolerance is useless, however, as a means for ensuring floor drainage.