Dimensional Tolerances

The dimensions on a construction drawing may show a code compliant design. However, when the project is constructed, the dimensions may not comply with accessibility codes because the construction may not be exactly as drawn. There are many possible reasons for this, including inaccurate measurement of the existing building or site, mistakes in construction, field conditions that necessitated constructing the project in a slightly different manner or changes in the design made during the construction. Regardless of the cause of the differences and who is to blame, something must be done about the fact that things are not as they should be. In fact, in many cases, no one is to blame. For example, walls do not get erected perfectly square, floors are not perfectly level and ground settlement occurs after construction. These conditions cause irregularities in dimensions that could not be predicted or completely controlled. Another issue is the use of nominal dimensions instead of absolute dimensions. For example, a 1 1/2" metal tubular section has an inside dimension of 1 1/2" while a metal pipe has an outside dimension of 1 1/2". A bathtub is nominally 5'-0" and the width of the space it is installed in is dimensioned the same. But, when the tile is installed over the flange of the tub, the inside dimension of the bathtub area is 4'-11".

In many cases, the difference between what is built and what the code requires is not significant. It is not necessary to demolish and rebuild parts of buildings if this is the case. In other situations, however, it is necessary in order to insure that the building meets the intent of the law. What determines when variations in dimensions are significant or not?

Most existing accessibility codes and model standards, including UFAS, have a paragraph entitled "Dimensional Tolerances." In the UFAS the paragraph reads as follows:

"All dimensions are subject to conventional industry tolerances for field conditions."

Unfortunately, this does not provide any guidance to NYCHA staff because it does not define exactly what a conventional industry tolerance is. Moreover, the tolerances for each industry that makes construction products varies. The tolerances for plumbing are different than for carpentry.

In many situations, designers can avoid problems by including "margins of error" in their dimensions. Then, some variation in constructed dimensions will not result in a violation of accessibility requirements. Some examples are:

• Design a ramp with a 1:14 or 1:16 slope so that variations in the slope over the course of the ramp will not exceed the maximum slope of 1:12

• Specify base cabinets heights of 33 1/2 in. to the top of the counter so that variation in floor slabs will not result in the cabinets being over 34" at any one point

• Use a 36 in. wide door so that the doorway will exceed 32" clear even if the stop dimension or door width is slightly different than expected

• Design a bathroom to be 91" long so that it is sure to be no less than 90" long wherever it is measured.

By planning ahead, much trouble can be avoided after construction. Moreover, when a field condition does result in a technically non-compliant situation, code authorities often respond by saying that the designer should have considered the potential for that to happen by including a margin of error. However, there are many dimensions in the accessibility codes that are absolute dimensions rather than minimum or maximum thresholds. Moreover, as everyone knows, "things happen." In these cases, it is impossible to use such a strategy. Moreover, in some cases, extending the dimension of one room will mean that another room on the other side of the wall will not comply with minimum standards or something just will not fit at all, e.g. a ramp. In other cases, the dimensions of equipment will govern the design and those dimensions may vary from product to product.

In general, tolerances should only be allowed under these conditions:

1. The construction drawings met code and were properly labeled, or

2. There is a significant conflict between UFAS and another regulation, e.g. fire codes, minimum floor area guidelines, etc.

And….

3. Field measurement during construction identified serious constraints to meeting the UFAS requirements, or

4. The project is already constructed and the contractor is not at fault for not following the construction documents.

The following is a list of acceptable dimensional tolerances based on research, precedent in codes and practical knowledge of disability: