Dimensional Tolerances in Construction and for Surface Accessibility

1 - Design and drawing

Current practice
It is typical practice for architects to use the values published in guidelines and standards and repeat the values on the drawings. If the value is a minimum or maximum or a range, this fact may not be communicated to the contractor, who may think that there is a tolerance ed.

In the preamble adopting the 2004 guidelines as the 2010 Standards for Accessible Design the Department of Justice clarified Sections 104.1 and 104.1.1 saying that conventional industry tolerances apply to absolute dimensions (for example, a single number) as well as to dimensions that are stated as either a minimum or maximum value. Tolerances do not apply where a requirement states a range; for example, that grab bars must be installed between 33 inches and 36 inches above the finished floor.

In addition, current practice for architectural and construction engineering drawings establish ambiguity and the potential for accumulated measurement error. For example, architects typically use chain dimensioning. If the contractor follows the chain in layout, slight errors can accumulate to the final dimension.

It is also not standard practice to assume that a fractional measurement indicates a significant figure or the implied accuracy required. An architect may dimension something as 14' - 6½" and want that to be built within a 1/16 - inch tolerance, not a ±1/4 - inch tolerance as the value 6½ inches indicates because of rounding. It is not standard practice to label the dimension as 14' - 6 4/8" to communicate this.

When a dimension is especially important, drafters may use the word "HOLD" or some similar word to indicate that a dimension is important. Less important dimensions may have a ± as a suffix to indicate that this dimension may vary slightly, although this practice is seldom used. In both cases the amount of the allowable variation is not clear.

Although significant figures could be used as a way to state expected accuracy or certainty in a measurement, the practice of using feet, inches, and fractions of an inch do not allow this as the method is currently used. However, the use of SI units can imply the accuracy intended by varying the number of digits beyond the decimal place.

1.1 Best practices for design

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.

1.1.2 When a dimension range is the regulatory requirement use the midpoint of the range as the drawing dimension.

1.1.3 A maximum overall design running slope for exterior accessible surfaces (other than ramps), such as sidewalks, of 4% (approximately 1:25) is recommended. In the ideal case, planning for a 4% running slope allows for construction inaccuracies while still not exceeding the maximum 1:20 slope for walking surfaces.

1.1.4 A maximum overall design running slope for exterior accessible ramps of 7.5% (1:13.3 or 1:13) is recommended. This allows for a potential plus tolerance of approximately 0.8% while not lengthening the ramp excessively. This also minimizes the effects of local variation while not lengthening the ramp excessively. Complying with a tolerance of +0.8% is generally possible with common methods of constructing ramps with concrete, asphalt, and pavers.

1.1.5 A maximum design cross slope for accessible exterior pedestrian paving and ramps of 1.5% (1:67 or about 3/16 in. per ft. [15 mm per m]) is recommended. This allows for a potential plus tolerance of +0.5% while still providing for drainage. ADA/ABAAG states a maximum cross slope requirement of 1:48 (1/4 in./ft. [20 mm/m] or about 2%). Pervious concrete may also be considered for surfaces that are designed to be nearly level.

1.1.6 Accessible surfaces should be as smooth as possible. This includes localized variations in slope as well as bumpiness created by small, individual units such as bricks, concrete pavers, or wood slats.

In most cases using concrete or asphalt minimizes the potential problems with bumpiness. However, smaller paving units may be used if the gap between the units and the lippage (difference in offset between units) is minimized. As a guide, research conducted at the University of Pittsburgh on concrete pavers found that limits on whole body vibration as established in ISO 2631, Evaluation of Human Exposure to Whole - Body Vibration were not exceeded if the bevel on the units was less than or equal to 6 mm (1/4 in.) and the pavers were placed in a 90 - degree herringbone pattern.

Local variation in slope can also be a problem for users of wheelchairs and other mobility aids. For example, although a ramp may meet the overall slope requirements of 1:12, a dip or hump along the ramp may create a stumbling hazard or a short incline greater than 1:12. Specifications should include limits on how many local variations will be accepted and the methods by which accessible surfaces will be measured.

For accessible surfaces constructed of small, individual units, such as concrete pavers or wood slats, the drawings (along with the specifications) should include the best method to install the material to minimize variations in smoothness and maintain a uniform surface over the life span of the material.

1.2 Best practices for drawing

1.2.1 Use the plus - or - minus symbol (±) after a dimension when it must be made clear what the expected tolerance is. This should be coordinated with the specification requirements. Although current practice dictates that tolerances only be included in the specifications, many workers reading the drawings do not read the specifications so it makes sense to show critical tolerances on the drawings as well. Also include a general note stating that no additional allowances either plus or minus will be allowed. For example, use a format such as 6' - 4" (±1/8") or 1930 (±3 mm).

1.2.2 Use datum dimensioning referenced to a primary or secondary control point on the construction site when the position of one item is particularly important. Do not dimension it based on one or more dimensions in a string.