IRC Stair Code Requirements
The 2021 International Residential Code rules for one- and two-family dwellings, with a checker that tests your own stair against each one.
| Rule | Requirement | IRC section |
|---|---|---|
| Riser height | 7¾" max, 4" min; max 3/8" variation in a flight | R311.7.5.1 |
| Tread depth | 10" min nosing to nosing; max 3/8" variation | R311.7.5.2 |
| Nosing | ¾"–1¼" where treads are under 11"; max 9/16" radius | R311.7.5.3 |
| Open risers | Openings must not pass a 4" sphere where the stair rises more than 30" | R311.7.5.1 |
| Width | 36" clear above handrail; 31½" at one handrail, 27" between two | R311.7.1 |
| Headroom | 6′8″ (80") min, plumb from the nosing line | R311.7.2 |
| Max rise of a flight | 151" (12′7″) between floor levels or landings | R311.7.3 |
| Landings | At top and bottom; at least as wide as the stair and 36" deep in the direction of travel | R311.7.6 |
| Handrail | Required at 4+ risers; 34"–38" above nosings; 1¼"–2" round grip; 1½" clear of wall | R311.7.8 |
| Guards | Required where drop > 30"; 34" min along stair, 36" at landings/floors | R312.1 |
| Guard openings | No 4" sphere through; 6" at the stair-tread triangle; 4⅜" on stair guards | R312.1.3 |
| Winders | 10" at the walkline (12" from narrow end), 6" min at any point | R311.7.5.2.1 |
| Spiral stairs | 26" clear width, 9½" max riser, 6¾" tread at walkline, 6′6″ headroom | R311.7.10.1 |
Many states and cities amend the IRC (some allow an 8¼" riser and 9" tread from older codes). Your building department's adopted version is the one that counts.
Check your stair
Result
Passes
- Tallest riser 7 5/8" ≤ 7¾" R311.7.5.1
- Shortest riser 7 3/8" ≥ 4" R311.7.5.1
- Riser variation 1/4" ≤ 3/8" R311.7.5.1
- Shortest tread 10" ≥ 10" R311.7.5.2
- Tread variation 1/4" ≤ 3/8" R311.7.5.2
- Nosing 1" within ¾"–1¼" R311.7.5.3
- Clear width 36" ≥ 36" R311.7.1
- Headroom 80" ≥ 6′8″ R311.7.2
- Handrail 36" between 34" and 38" R311.7.8.1
- Stair guard 36" ≥ 34" at nosings R312.1.2
- Baluster gap 3 1/2" passes the 4" sphere R312.1.3
Baluster spacing calculator
Balusters per section
13
- Actual gap
- 3 3/4"
- On-center spacing
- 5 1/4"
- First baluster center
- 4 1/2" from post
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Questions people ask
What is the IRC code for stair riser height?
IRC R311.7.5.1 sets a maximum riser height of 7¾" and a minimum of 4". The difference between the tallest and shortest riser in a flight may not exceed 3/8".
What is the minimum tread depth under the IRC?
IRC R311.7.5.2: 10" minimum, measured horizontally from nosing to nosing. Treads within a flight may not vary by more than 3/8". Winder treads need at least 10" at the walkline (12" from the narrow side) and 6" at the narrowest point.
When does a stair need a handrail?
IRC R311.7.8: a flight with four or more risers needs a handrail on at least one side, 34" to 38" above the tread nosings, graspable (1¼" to 2" round for Type I), and continuous for the full flight.
How far apart can balusters be?
IRC R312.1.3: guard openings must not let a 4" sphere pass through. The triangle under the bottom rail at the stair tread may pass a 6" sphere. In practice, space balusters so the clear gap is under 4" (commonly 3½" to 3¾").
What headroom is required on stairs?
IRC R311.7.2: 6'8" (80") minimum headroom, measured vertically from a sloped plane along the tread nosings (or from a landing floor) to the ceiling or any soffit above.
When is a guard required on a stair?
IRC R312.1.1: where the walking surface is more than 30" above the floor or grade within 36" horizontally. Guards on the open side of stairs must be at least 34" tall measured from the nosing line; guards on landings and floors must be 36".