2010 ADA Standards for Accessible Design Pocket Guide
502.4 Floor or Ground Surfaces.
EXCEPTION: Slopes not steeper than 1:48 shall be permitted.
Advisory 502.4 Floor or Ground Surfaces. Access aisles are required to be nearly level in all directions to provide a surface for wheelchair transfer to and from vehicles. The exception allows sufficient slope for drainage. Built-up curb ramps are not permitted to project into access aisles and parking spaces because they would create slopes greater than 1:48.