ADA Stair Treads and Risers (2010 ADA Standards 504)
A stair that satisfies the building code can fail the ADA, and the usual builder's flight does.
Example
You enter
- Riser height (in) 7.75
- Tread depth (in) 10
- Total rise of the flight (in) 108.5
- Open risers? no
- Nosing projection over the tread below (in) 1
- Radius at the leading edge (in) 0.25
You get
- Riser (4 to 7 in) OVER by 0.75 in
- Tread (11 in min) short by 1.00 in
- Risers as built 14
- Risers required 16
- Compliant riser (in) 6.78125
- Run added (in) 35
Details, formula, and sources
504.2 wants risers 4 to 7 in and treads 11 in MINIMUM, where the IRC commonly allows 7-3/4 in on a 10 in tread -- missing both numbers at once. The fix is not a trim adjustment. Holding the same total rise, dropping to a 7 in riser adds risers, and every added tread adds 11 in of run: a 108-1/2 in rise goes from 130 in of run to 165, nearly 3 ft longer, which is why an accessible stair rarely fits the opening a code stair was framed for. Open risers are not permitted at all.
risers 4 in minimum to 7 in maximum; treads 11 in deep minimum; open risers not permitted; leading-edge radius 1/2 in maximum; nosing projection 1-1/2 in maximum over the tread below; riser slope under the tread 30 degrees maximum from vertical. Compliance cost at a fixed total rise: risers required = ceil(total rise / 7), and the run required = (risers - 1) x 11 in.
2010 ADA Standards for Accessible Design, 504.2, 504.3, 504.5, and 504.6. A US federal standard in the public domain, quoted directly.
The 2010 ADA Standards are published in full at no cost at ada.gov and by the US Access Board.
Estimate. AHJ and licensed professional govern.
Field names used by the API: riser_height_in, tread_depth_in, total_rise_in, open_risers, nosing_projection_in, leading_edge_radius_in, riser_deficit_in, tread_deficit_in, risers_as_built, risers_required, compliant_riser_in, run_added_in
- Risers 4 in high minimum, 7 in high maximum2010 ADA Standards 504.2
- Treads 11 in deep minimum2010 ADA Standards 504.2
- Open risers not permitted2010 ADA Standards 504.3
- Leading edge radius of curvature 1/2 in maximum2010 ADA Standards 504.5
- Nosing projection 1-1/2 in maximum over the tread below; riser slope 30 degrees maximum from vertical2010 ADA Standards 504.5
- Compliance cost total rise held constant; risers = ceil(rise / 7) and run = (risers - 1) x 11 inderived from 504.2