It’s not a Flat Earth: Why your curb ramp is too steep.

April 7, 2009

It happens all the time on inspections.  I’m standing in the August heat trying to explain to a contractor or architect why the 6 foot long curb ramp is too steep.

They argue that since the curb is 6 inches high, the ramp need only be 6 foot long to achieve the 1:12 (8.3%) slope permitted by the ADA.  The fact that my electronic level reads a slope of 10.4% is usually ignored in a fit of cognitive dissonance.

So how can it be that a 6 foot long ramp is insufficient for a 6″ curb? 1″ in 12″, right? Nope, because you aren’t trying to get to the top of the curb.  Take a look at this diagram:


Most sidewalks running along buildings (strip shopping centers are a good example) have a slope of 1/4″ per foot to allow for drainage away from the building.  This slope is usually continuous to the curb.  The curb ramp is fighting this 1/4″ per foot rise as it cuts into the sidewalk; for every foot of ramp length, the height increases by 1/4″.  In the example above a 6′ ramp will have a running slope of 10.4% (7-1/2″ rise with a 72″ run).

To comply with the ADAAG, the curb ramp must be 8 foot long to achieve a maximum running slope of 8.3%.  And, no, handrails are not required on curb ramps regardless of the length.

If 8′-0″ is too long, try using a parallel curb ramp that slopes parallel to the curb and doesn’t have to fight the incline.