The 2010 ADA Standards and ICC A117.1 – Accessible and Usable Buildings and Facilities contain similar requirements regarding changes in level within an accessible route. When thresholds are provided in a door opening, the maximum allowable threshold height is 1/2 inch, with the exception of existing or altered thresholds which are permitted to be 3/4-inch maximum in height (refer to the standards for additional criteria).* These limitations on threshold height apply to manually-operated doors and automatic doors.
In existing buildings where space is an issue, the accessibility standards allow a steeper slope for ramps. If the rise is 3 inches or less, the slope shall not be steeper than 1:8. When the rise is 6 inches or less, the slope may be a maximum of 1:10. A ramp with a slope greater than 1:8 is not allowed.
Although the accessibility standards require walking surfaces to be slip-resistant, the standards do not include prescriptive requirements to establish whether thresholds and ramps used in doorways are required to be treated for slip-resistance. Most thresholds and ramps are grooved to provide some level of slip-resistance, and many manufacturers have optional finishes which create even more friction. These treatments may be helpful for particularly wide thresholds or ramps, or those that would otherwise be smooth and could pose a hazard to building occupants.
The accessibility standards include additional requirements for ramps and sloped walkways, beyond what is addressed here with regard to changes of level within a doorway. The US Access Board publishes an online guide which explains the requirements of the 2010 ADA standards. Chapter 3 is titled Floor and Ground Surfaces, and covers surface characteristics, flooring, openings, and changes in level. Chapter 4 addresses Accessible Routes, including a section on Entrances, Doors, and Gates which contains the requirements for thresholds. The online guide can be accessed here, and the complete requirements can be found in the ADA standards (downloadable here).
* In order to be considered an existing threshold, the installation would have to pre-date the adoption of a code or standard which limited the threshold to 1/2 inch.