Bookmark and Share RSS

Recent Posts



    Essence of Life

    Civil engineering, fly fishing, surfing and other things that make life worth living.

    ADA Accessible Routes - What the Law Means by Percent Cross Slopes

    Clark Stoner - Saturday, September 06, 2014


    It is not uncommon for property owners to have Code Compliance Inspectors and citizens assert that as-constructed and intended accessory paths of travel, or parking spaces, do not comply with ADA Standards.  Purchase a $25 "smart level" from the local hardware store and everybody is an expert.  The common claim is that the constructed improvements, intended to comply with ADA standards, are excessively steep, or exceed two percent cross slope, or 2%.  The common dimension noted on engineering and architectural plans for accessible paths of travel is "2% max."  That statement, "2% max", becomes ingrained in everybody's mind.


    These inspectors sometimes assert non-compliance with a smart level reading of 2.1 percent.  I've seen several instances where a smart level reading of 2.1% led to demolition and re-construction (prior to my involvement), costing property owners thousands of dollars, all because of a misinterpretation of the ADA Standards and the California Building Code.


    What do the ADA Standards and the California Building Code actually say?


    Section 403.3 of the ADA Standards and 2013 CBC Section 11B-403 both state that "(t)he cross slope of walking surfaces shall not be steeper than 1:48."


    The ratio of 1/48 equates to 2.08 percent. 


    Section 104.1.1 of the ADA Standards, and 2013 CBC Section 11B-104.1.1 both state that "(a)ll dimensions are subject to conventional industry tolerances except where the requirement is stated as a range with specific minimum and maximum end points."  The 2010 ADA standards fell under substantial criticism for the above statement, because in reality, as shown below, construction and manufacturing tolerances apply to absolute dimensions as well as to dimensions expressed as a maximum or minimum.  In California, the Division of the State Architect recognized this criticism of the ADA Standards and in 2013 issued Advisory 11B-104.1.1 stating that the "(a)pplication of conventional industry tolerances must be on a case-by-case, project-by-project basis."


    Tolerances are always at play, because there are no absolutes in measurement statistics.  To illustrate, take for example the 2.08% maximum allowed for ADA walking surfaces.  What is 0.08%?  In terms of walking surface grades, 0.08% amounts to just under one inch elevation change over a length of one hundred feet.  That amounts to about 0.25 millimeter rise over a distance of one foot.  Breaking this down further, 0.01% grade amounts to about 0.03 millimeter elevation change in one foot.  We are talking about particle sizes smaller than the minimum granular particles included in concrete mixes.


    In terms of pavement and concrete construction precision, taking a percent grade out to the one hundredth decimal place amounts to insanity.  Contractors cannot build to that level of precision/accuracy, and surveyors and smart level readers cannot measure to that level of accuracy.  The Federal Highway Administration (FHWA) acknowledges that the construction tolerance for Portland Cement Concrete is based on 1/4-inch per 10 feet, or 0.2%(+/-) ( See FHWA:  Accessible Sidewalks and Street Crossings - An Informational Guide).  And smart level manufacturers typically state their accuracy to be near 0.1%.  This means that a reading of 2.1% cross slope could result in an actual value of 2.0% or 2.2%.  Furthermore, a smart level reading of 2.2% cross slope could mean that the actual value is 2.1% or 2.3%, borderline compliant or non-compliant.  So, for this reason a smart level reading of less than 2.3% may be considered compliant based on the stated accuracy of a typical smart level.  Now, for example, also taking into account the 0.2% tolerance for Portland Cement Concrete construction as noted above, it is plain to see that we are not dealing with absolutes.


    For this reason, analysis and commentary to the 2010 ADA standards concerning Section 104.2, Calculation of Percentages, provides:  "where the required number of elements or facilities to be provided is determined by calculations of ratios or percentages and remainders or fractions result, the next greater whole number of such elements or facilities shall be provided." and furthermore, Section 104.2 "permits rounding down for values less than one-half where the determination of the required size or dimension of an element or facility involves ratios or percentages. Such practice is standard with the industry, and is in keeping with model building codes." 

    So, what is that smart level reading telling you?  It is telling you to stop and think.


    Capitola Office | 1840 41st Ave., Suite 102-264 | Capitola, CA 95010 | T: 831-477-9215
    Sonoma Office | 617 Broadway, #1962 | Sonoma CA 95476 | T: 707-996-8449
    Copyright © CFS Engineering. All Rights Reserved. Photography Copyright © Todd Kaplan Photographics
    Website Powered by Bullsprig a Reno Web Design Company.