Initiative on Dimensional Tolerances in Construction

Mark Macy, P.E.
Director of Engineering
Nashville and Davidson County
Nashville, Tennessee
Member, APWA UPROW Committee

The U.S. Access Board, the federal agency that develops accessibility guidelines under the ADA and other laws, is developing an initiative on dimensional tolerances in the design and construction of curb ramps, sidewalks and walkways. Additionally, the Board wants to look into slopes, flatness, smoothness and planarity with respect to the various materials used for interior and exterior walking and rolling surfaces. On March 16, 2007, the Board invited a group of key design and construction principals to their offices to assist in this initiative. APWA, as a recognized leader in the management of the public right-of-way, was asked to designate and send a representative for their 29,000 members.

The participants included representatives from the following organizations:

  • Associated Builders and Contractors, Inc. (ABC)
  • American Concrete Institute (ACI)
  • The American Institute of Architects (AIA)
  • American Public Works Association (APWA)
  • American Society of Concrete Contractors (ASCC)
  • American Society of Civil Engineers (ASCE)
  • American Society of Landscape Architects (ASLA)
  • Building Owners and Managers Association International (BOMA)
  • The Construction Specifications Institute (CSI)
  • National Institute of Building Sciences (NIBS)
  • National Institute of Standards and Technology (NIST)
  • US Army Corps of Engineers (USACOE)
  • Hitt Contracting, Inc.
  • US Access Board
  • Architectural Research Consulting, Inc.

At the March 16 meeting, Lois Thibault, Coordinator of Research for the U.S. Access Board, reviewed the following objectives for the project:

  1. Gather information related to applicable design standards and opinions from industry professionals.

  2. Use the experience of industry leaders to develop standard industry tolerances and measurement protocols for a range of materials used in interior and exterior walking and rolling surfaces.

  3. Provide consulting services to support, coordinate, and advise industry trade organizations on accessibility issues during the development process, and

  4. Develop technical assistance for design and construction professionals, contractors, code officials, and others on where to find appropriate construction tolerances and measurement protocols for new construction, including recommendations for best practices in dimensions construction documents, selecting appropriate units of measure and applying tolerances information.

The meeting began with presentations from three experts in the design of accessible facilities:

  • David Kent Ballast, AIA, CSI, of Architectural Research Consulting, speaking on "Tolerances in Design"

  • Coleby Cyrtmus, Senior Project Superintendent, Hitt Contracting, Inc., speaking on "Tolerances in Construction"

  • Rory Cooper, Ph.D., Director and VA Senior Research Career Scientist of the Center for Wheelchairs and Associated Rehabilitation Engineering, speaking on "Tolerances and Usability"

Their papers can be found on the Access Board website at

Following a short presentation by each of the above speakers, the participants discussed tolerances for various walking surface characteristics for which there are currently no applicable industry tolerance measures: walkway slope, flatness/planarity and smoothness. For example, everyone agrees a cross-slope of 2% allows positive drainage and is tolerable for a person in a chair to traverse, but you can't build it perfectly at every place every time. If the tolerance was +/-1%, it would allow a slope between 1 and 3%, but according to previous studies, a cross-slope of 3% is 50% harder to traverse by a person in a chair.

Additional points summarizing the discussion include:

  1. Available methods to determine smoothness appear to be based on roadway standards, which typically involve bigger, wider, and more sophisticated equipment than used to build sidewalks and curb ramps.

  2. Some industries use the "F-number" system to measure surface flatness, which may be difficult in exterior sidewalk and ramp construction.

  3. Different materials—concrete, asphalt, brick, tile—may create a need for different tolerances.

  4. The effect of accumulating tolerances, e.g., road, gutter, curb, sidewalk, first floor, needs to be considered and addressed.

  5. The maintenance of surfaces, as well as repairs or alterations, are also issues.

  6. Workers will need to be trained to inspect using new tolerance guidelines.

  7. Inspection procedures need to be able to be readily understood and easy to follow for the construction/inspection population.

  8. Contractors and inspectors responsible for compliance want a more clearly defined set of requirements.

The primary intent of the Access Board is to encourage the industry and trade organizations to update their standards, specifications, guidelines, etc., to include dimensioning and measurement protocols appropriate for the design and construction of accessible routes. Plans for the next phase of the project include the following:

  1. Work with trade associations to encourage and support them in the development of materials-specific tolerances.

  2. Encourage organizations such as the AIA, CSI, and master specification providers to develop and communicate "best practices" for design, construction documentation, and specification of tolerance and measurement information for walking and rolling surfaces covered by accessibility standards.

  3. Continue to coordinate with ACI and ASCC on their efforts, already underway, to address tolerance standards and tolerance compatibility.

  4. Pursue the development of measurement protocols for ramps and walkways in a range of surface materials.

  5. Using information gained from this phase, expand our efforts to include the development of industry tolerances for other materials and assemblies, where they currently do not exist.

Workshop participants will be invited to recommend strategies, expertise, and targets for involvement in the initiative. It is anticipated that many, like APWA, will want to stay involved in support of the ongoing effort, which will be overseen by David Kent Ballast, AIA, CSI, of Architectural Research Consulting, Inc., the author of the newly updated Handbook of Construction Tolerances, Second Edition, published this year by Wiley. The Board will track progress in a planned series of presentations to industry meetings. For more information or to make recommendations on the initiative, contact Lois Thibault (

Mark Macy is a member of APWA's Utilities and Public Right-of-Way (UPROW) Committee and SAFETEA-LU Task Force, and represents the public works field on the Common Ground Alliance (CGA) Board of Directors. He can be reached at (615) 862-8764 or