Dimensional Tolerances in Construction and for Surface Accessibility

1.1.1 When a maximum or minimum dimension is a regulatory requirement use a drawing dimension that is less than a maximum limit or more than a minimum limit. The dimension should be determined by the expected tolerance of the construction element.

The simplest way for design professionals to avoid problems with construction tolerances related to surface accessibility and other accessible elements is to design for slopes and dimensions that are slightly less than maximums and slightly more than minimums. For example, the 1:12 slope stated in ADAAG and ADA/ABAAG is a maximum slope for ramps, not a design requirement. ADAAG and accessibility experts recommend that ramps be built with the least slope possible but in no case should a ramp exceed a 1:12 slope (except for curb ramp flares, and other approved exceptions). Although ramps with a slope slightly less than 1:12 take up more floor space, the negligible loss in usable space will more than compensate for potential problems caused by rebuilding or litigation due to ramps exceeding the 1:12 slope.