Field Tolerances are permitted during construction, but not allowed for design. It is important that designs do not include ± on dimensions, as this would provide a moving target rather than complying dimensions. The field “tolerance” or adjustment” is allowed in construction due to minor differences in finish materials and their installation.
An exception to the allowance for field tolerances is when the ADA specifies a range. (see ADA Standards Figure 604.2) Water Closets (toilets) are shown in the figure as having a minimum and maximum dimension. The tolerance is “built-in” to this range so that no field condition would warrant acceptable deviations. The designer may wish to show the range with a note to the installer that no field tolerance outside this range is allowed. Another common use of ranges is in ADA 2010 Standards Figure 505.4 Handrail Height. The dimensions are now expressed as 34”-38” to top of the handrail. Many designers are still using the old “center of handrail” dimension, which could lead to unacceptable field compliance.
Whenever the ADA indicates both endpoints of a range with a number, then designers and installers should be aware that the field tolerance is already a part of the dimension and lies between the two points.
NOTE: The answers provided are the opinion of the author, and should not be relied upon as ADA interpretations may vary.