Accessibility guidelines and standards, including those issued under the ADA, generally recognize construction tolerances, but they do not identify acceptable tolerances for specific dimensions. Tolerances must be based on the construction methods and materials used. Most guidance on tolerances available from industry and other sources does not specifically address requirements in accessibility requirements such as the ADA standards.
The Access Board undertook an initiative to encourage stakeholders in the construction industry to develop information on specific tolerances, as well as dimensioning and measurement conventions. This effort focused on specifications for the surface of exterior and interior routes, including ramps. The project, which was conducted by David Ballast, FAIA, CSI of Architectural Research Consulting, offered a forum for collaboration among representatives from industry, trade groups, professional organizations such as the American Institute of Architects and the Construction Specifications Institute, design professionals, contractors, code officials, and others.
A report on this effort, "Dimensional Tolerances for Surface Accessibility," that summarizes the information collected is now available on the Board’s website. The report offers suggested tolerances for the running slopes and cross slopes of routes and ramps and for surface uniformity or flatness. It also provides design recommendations to accommodate tolerances and construction variations. For example, the report notes that designing ramps with a more gradual slope (7.5% instead of the 8.33% maximum) will accommodate tolerances for common methods of constructing ramps with concrete, asphalt, and pavers. The report also provides guidance on protocols and available tools for measuring running and cross slopes, flatness, gaps, and surface variations.
Several trade groups involved in the project have issued guidance on tolerances for certain materials and methods. This includes information on ceramic tile from the Tile Council of North America and on concrete pavers from the Interlocking Concrete Pavement Institute. The Brick Industry Association is also developing guidance on tolerances.
For further information on this Board project, contact Marsha Mazz at mazz@access-board.gov, (202) 272-0020 (v), or (202) 272-0076 (TTY).