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: 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.
Formula and source
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.
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