DSA IR 11B-8 - USE OF PREDETERMINED CONSTRUCTION TOLERANCE GUIDELINES FOR ACCESSIBILITY (with revisions issued through Jan. 2011)

3. MINIMUMS, MAXIMUMS AND SPECIFYING AT THE LIMIT: Where an element is to be installed at the minimum or maximum permitted dimension, such as "48 inches maximum", it is not good practice to specify 48 inches, although it is allowed. Rather, it would be good practice to specify a dimension less than the required maximum (or more than the required minimum) by the amount of the expected field or manufacturing tolerance and not to state any tolerance in conjunction with the specified dimension.

In other words, dimensions noted in accessibility provisions as "maximum" or "minimum" should not be considered dimensions for design, because they represent the absolute limits of a requirement. To be sure that field tolerances result in usable construction, notes and dimensions in construction documents should identify and incorporate expected tolerances so that a required dimensional range is not exceeded by the addition of a finish material or a variation in construction practice, to the maximum extent feasible.

Specifying dimensions for design in the manner described above will better ensure that facilities and elements accomplish the level of accessibility intended by the requirements. It will also more often produce an end result of strict and literal compliance with the stated requirements and eliminate enforcement difficulties and issues that might otherwise arise.

On the other hand, by voluntarily choosing to specify the exact maximum limit or the exact minimum limit of a requirement for design, field construction based on such specification may unnecessarily fail to achieve the compliance that is required. In such cases the failure is not necessarily a consequence of field tolerance, but rather it is a result of the decision to design at the very edge of the prescribed limit.