How the stair check works
Measure the total rise from finished floor to finished floor (or to the landing) and decide on a riser count. Everything else follows from division:
- Riser height R = total rise ÷ number of risers.
- Fewest risers that keep R at or under the maximum = total rise ÷ 7.75, rounded up to a whole number.
- On a straight flight that lands on the upper floor, treads = risers − 1, and total run = treads × tread depth.
- Each limit is a simple comparison: the riser and the flight's rise must not exceed their maximums; tread, headroom and width must reach their minimums.
These are arithmetic steps, not code text. The limit values and section numbers are cited so you can find them in your own code book; read the full sections there, including their exceptions.
Which sections, in which edition
The 2024 IRC reorganized Chapter 3. Stairways moved from Section R311.7 to Section R318.7, so the rule that a flight of four or more risers needs a handrail moved from R311.7.8 to R318.7.8. The handrail details (height, clearance, continuity and grip size) now have their own section, R320. The base dimensional limits used here did not change. Tab the right numbers for the edition you will bring to the exam.
- Clear width (above and at handrail height): R311.7.1 in the 2021 IRC, R318.7.1 in the 2024 IRC.
- Headroom: R311.7.2 and R318.7.2.
- Vertical rise of a flight: R311.7.3 and R318.7.3.
- Riser height and riser variation in a flight: R311.7.5.1 and R318.7.5.1.
- Tread depth and tread variation in a flight: R311.7.5.2 and R318.7.5.2.
- Handrail trigger: R311.7.8 in the 2021 IRC; R318.7.8 in the 2024 IRC.
Worked example: 8 ft 9 in floor to floor
A basement stair has a total rise of 105 in (8 ft 9 in) and 10 1/2 in treads.
- Fewest risers: 105 ÷ 7.75 = 13.55, rounded up to 14 risers.
- Riser height: 105 ÷ 14 = 7.5 in (7 1/2 in), 0.25 in under the 7.75 in maximum.
- Treads: 14 − 1 = 13. Total run: 13 × 10.5 = 136.5 in (11 ft 4 1/2 in).
- Tread depth: 10.5 in is 0.5 in over the 10 in minimum. The flight's rise, 105 in, is under the 151 in maximum.
- With 14 risers, a handrail is required on at least one side.
Try 13 risers instead: 105 ÷ 13 = 8.08 in, which is over the maximum, so 13 risers fails in both editions. These are original examples, not exam questions.
Worked example: a tall flight that needs a landing
A total rise of 155 in on 20 risers gives 155 ÷ 20 = 7.75 in per riser, exactly on the maximum, so each riser passes. The flight still fails: 155 in is 4 in over the 151 in maximum vertical rise between floor levels or landings. Split it with an intermediate landing so that each flight rises no more than 151 in.
What this tool does not check
It assumes one straight flight with equal risers and treads. It does not check winder or spiral stairs, landing size, nosing projection, open-riser openings, guards, handrail height or graspability, or stair illumination. The 3/8 in variation limits apply to the built stair, so measure the tallest and shortest risers and the deepest and shallowest treads on site. Check those items in your own code book and with the AHJ.
Stairs on the B1 exam
Stairs fall under the public-safety content area of the ICC Residential Building Inspector (B1) outline. Exam questions usually test whether you can find the right section fast in an open-book setting, so practice locating each limit in your edition rather than memorizing it. The B1 study guide sets out a study plan from the outline, and the B1 practice page has the free 10-question check.