Dimensional Tolerances in Construction and for Surface Accessibility

Introduction

Purpose of the project

Accessibility standards reference industry guidance in the measure of field and construction tolerances. However, few trade and materials specifications include tolerances for features that are relevant to usability by people who have disabilities. For instance, when this project began, tolerances for the measurement of the slope, flatness, and smoothness of exterior walking and rolling surfaces in concrete, asphalt, unit masonry, and other materials had not yet been developed by industry groups.

The current project was undertaken to encourage and support construction industry stakeholders to fill in the gaps in existing tolerances and procedures and to set specific tolerances and procedures for measurement for particular materials and assemblies. The purpose was not to suggest or revise the dimensional limits set in the ADA/ABA Accessibility Guidelines nor to set specific tolerances within the body of the Guidelines, but rather to assist industry in developing dimensioning and measurement conventions appropriate to the different materials and methods used in exterior and interior walking/rolling surfaces so that practitioners and regulatory agencies could refer to them for guidance.

The Access Board contracted with David Ballast, FAIA, CSI with Architectural Research Consulting to manage the project and provide assistance and advice to industry associations as required to meet the goals of the project.

Background

While the current ADA/ABA Accessibility Guidelines (ADA/ABA Guidelines) make reference to standard industry construction tolerances as they relate to the dimensions given in ADA/ABA Guidelines, architects, contractors, code officials, courts, and others still have questions about what is required with respect to accuracy in dimensioning and constructing accessible elements. The position of the U. S. Access Board - as formalized in the ADA/ABA Guidelines in Section 104.1.1 - is that industry standard tolerances should be relied on when questions regarding this issue arise. However, few trades have codified specific tolerances for many of the elements mentioned in ADA/ABA Guidelines, nor have they developed accepted industry protocols for measuring when questions of accessibility arise. This is especially the case with issues related to surfaces. Further, industry standards, specifications, and measurement procedures do not even address some measures important to facility usability in new construction, such as how to measure the slope and flatness of a ramped surface designed for accessibility.

The absence of specific direction from industry regarding these issues creates significant problems for users trying to comply with surface accessibility requirements. These problems include increased construction costs, unnecessary demolition and rebuilding, litigation, uncertainty, and - too often - animosity among members of the building team, regulatory agencies, and the disabled community.

U.S. Access Board staff report that a large number of calls to their technical assistance hotline from architects, contractors, and others are seeking clarification of tolerances for specific materials or assemblies. Anecdotal evidence of the need for greater specificity also exists as comments from architects, contractors, building owners and managers, the disabled community and regulatory agencies. In the absence of clear direction, tolerances have often been determined by the courts. Evidence of these problems was significant enough that the U.S. Access Board funded this project to deal with them.

Many trade groups have developed standard tolerances for certain aspects of construction. In particular, valuable work has been done on the measurement of flatness, smoothness, and vibration for some applications. The accompanying list of publications in Part 3 of this report includes references to many of these resources. In addition, a number of organizations and individuals have attempted to develop general guidance for tolerances based upon ADA/ABA Guidelines provisions. These plus - or - minus ances have generally not been based upon materials and fabrication criteria that underlie standard industry tolerances, nor have any testing protocols been provided for their measure.

Construction tolerances, measurement protocols, and related issues affect all areas of construction where there is a concern for accessibility. Some of the questions and problems related to construction tolerances may also raise issues of metrology - selecting appropriate units of measure and significant digits and establishing appropriate measurement protocols. Local practice adds another dimension to the issue.

This project focused on surface measures, including slope, flatness/waviness, smoothness, and planarity with respect to the various materials used for exterior walking and rolling surfaces.

In order to eliminate uncertainty in compliance with accessibility standards, the Access Board believes it is best for industry stakeholders to fill in the gaps in existing tolerances and procedures and to set specific tolerances and procedures for measurement. As part of this project tolerances have been established by the Tile Council of North American, the Interlocking Concrete Pavement Institute, and work is underway by the Brick Industry Association. Part 1 of this report summarizes these efforts. Unfortunately, other industries and trade groups were reluctant to establish additional tolerances for their materials and assemblies. Part 2 of this report includes early work by the concrete industry to develop tolerances and measurement protocols.

Part 1 of this report summarizes the tolerances developed by those industries that participated in this project. It also includes other work that is ongoing by other groups and agencies.

Part 2 of this report provides suggested best practices for designers and the construction industry as a way to avoid construction tolerance problems. It also includes suggested tolerances and measurement protocols that the concrete industry may want to consider in developing an industry standard. These tolerances and measurement protocols could also be used by other industries that use their materials for both exterior as well as interior accessible surfaces.

Part 3 of this report includes some of the background information that was generated during the course of this project.