Stair dimensions and building code
Stairs are the part of a DIY project inspectors look at hardest — for good reason. These are the key residential dimensions and why each one matters.
On this page
Key IRC stair dimensions
Most US jurisdictions base their residential rules on the International Residential Code (IRC), sometimes with local amendments. Its main stair requirements (Section R311.7):
| Item | Requirement |
|---|---|
| Riser height | 7¾ in maximum |
| Tread depth | 10 in minimum, measured nosing to nosing |
| Variation | No more than ⅜ in between the largest and smallest riser (or tread) in a flight |
| Nosing | ¾ to 1¼ in projection where treads are less than 11 in deep |
| Width | 36 in minimum clear width |
| Headroom | 6 ft 8 in minimum, measured from the line of the nosings |
Handrails and guards
- Handrail: required on at least one side of any flight with four or more risers, 34 to 38 in above the nosing line, continuous for the full flight.
- Grip: round handrails 1¼ to 2 in in diameter, with at least 1½ in clearance from the wall.
- Guards: open sides of stairs and platforms more than 30 in above the grade below need guards, generally at least 36 in high (34 in along the stair itself where the guard doubles as a handrail), with openings small enough that a 4-inch sphere can't pass through (4⅜ in on the open sides of stairs).
Landings
A landing is required at the top and bottom of a stairway, at least as wide as the stair and at least 36 inches deep in the direction of travel. Very tall flights also need intermediate landings — the IRC limits a single flight's rise. Check your local code for exceptions, such as doors that swing over the top step.
Comfort: the 2R + T rule
Code sets limits; comfort sets the target. A stair feels natural when two risers plus one tread add up to about 24 to 25 inches — for example a 7 in riser with an 11 in tread. Steep stairs at the code maximum (7¾ in rise, 10 in tread) are legal but feel steep, especially for children and older adults.
Laying out a stair
- Measure the total rise from finished floor to finished floor.
- Divide by your target riser height and round up to get the number of risers; then divide the rise by that number for the exact, equal riser height.
- Treads = risers − 1. Multiply by the tread depth for the total run, and check it fits the space.
- Check headroom along the whole flight, especially under beams and floor openings.
The stair calculator does steps 1 to 3 and flags anything outside the IRC limits.
Frequently asked questions
Do deck stairs have to meet the same code?
Yes, exterior stairs serving a deck follow the same riser, tread, handrail and guard rules in most jurisdictions.
Can risers be different heights?
Only within ⅜ in across the flight. Uneven risers are a major trip hazard — often at the top or bottom where flooring was added later.
Do I need a permit to build stairs?
Usually yes for new stairs and deck stairs, and they're inspected. Ask your building department before you start.
Sources
- International Residential Code, Sections R311.7 (stairways) and R312 (guards).