1003.5 Elevation change. Where changes in elevation of less
than 12 inches (305 mm) exist in the means of egress, sloped
surfaces shall be used. Where the slope is greater than one
unit vertical in 20 units horizontal (5-percent slope), ramps
complying with Section 1010 shall be used. Where the difference
in elevation is 6 inches (152 mm) or less, the ramp shall
be equipped with either handrails or floor finish materials
that contrast with adjacent floor finish materials.
Exception 2.
2. A stair with a single riser or with two risers and a
tread is permitted at locations not required to be
accessible by Chapter 11, provided that the risers
and treads comply with Section 1009.7, the minimum
depth of the tread is 13 inches (330 mm) and at
least one handrail complying with Section 1012 is
provided within 30 inches (762 mm) of the centerline
of the normal path of egress travel on the stair.
The old rules-of-thumb were:
2R + T = 24" to 25"
R x T = 72" to 77"
where R = riser height and T = tread width.
These rules are well over 100 years old (maybe even 150 years) and still work great today
even though we are generally taller and have larger feet..
1009.7.2 Riser height and tread depth. Stair riser heights
shall be 7 inches (178 mm) maximum and
4 inches (102
mm) minimum. The riser height shall be measured vertically
between the nosings of adjacent treads. Rectangular
tread depths shall be 11 inches (279 mm) minimum measured
horizontally between the vertical planes of the foremost
projection of adjacent treads and at a right angle to
the tread’s nosing. Winder treads shall have a minimum
tread depth of 11 inches (279 mm) between the vertical
planes of the foremost projection of adjacent treads at the
intersections with the walkline and a minimum tread depth
of 10 inches (254 mm) within the clear width of the stair.