IRC Stair Code: Residential Stair Requirements (2021)
Under the 2021 International Residential Code (IRC), a residential stair riser can be at most 7 3/4 in (196 mm) and a tread must be at least 10 in (254 mm) deep, with no more than 3/8 in (9.5 mm) of variation within a flight (R311.7.5). The stair needs 36 in (914 mm) of clear width, 6 ft 8 in (2032 mm) of headroom and, once it has four or more risers, a handrail 34 to 38 in (864 to 965 mm) high on at least one side (R311.7.1, R311.7.2, R311.7.8). Open sides more than 30 in (762 mm) above the floor below need a guard at least 34 in (864 mm) high along the stair, with openings that stop a 4 3/8 in (111 mm) sphere (R312.1).
Summary table
| Rule | Imperial | Metric | 2021 IRC |
|---|---|---|---|
| Riser height, maximum | 7 3/4 in | 196 mm | R311.7.5.1 |
| Tread depth, minimum | 10 in | 254 mm | R311.7.5.2 |
| Variation of risers or treads in a flight, maximum | 3/8 in | 9.5 mm | R311.7.5.1, R311.7.5.2 |
| Nosing projection (treads under 11 in / 279 mm) | 3/4–1 1/4 in | 19–32 mm | R311.7.5.3 |
| Clear width above handrail height, minimum | 36 in | 914 mm | R311.7.1 |
| Width at handrail height: one / two handrails | 31 1/2 in / 27 in | 787 mm / 698 mm | R311.7.1 |
| Headroom, minimum | 6 ft 8 in | 2032 mm | R311.7.2 |
| Rise of one flight, maximum | 12 ft 7 in | 3835 mm | R311.7.3 |
| Landing depth on a straight run, minimum | 36 in | 914 mm | R311.7.6 |
| Handrail height | 34–38 in | 864–965 mm | R311.7.8.1 |
| Guard required where the drop exceeds | 30 in | 762 mm | R312.1.1 |
| Guard height: open side of stair / landings and floors | 34 in / 36 in | 864 mm / 914 mm | R312.1.2 |
| Guard openings: stair / elsewhere | 4 3/8 in / 4 in sphere | 111 mm / 102 mm | R312.1.3 |
| Open risers more than 30 in (762 mm) up | 4 in sphere | 102 mm | R311.7.5.1 |
R311.7 applies to every stairway that is required or provided, including stairs serving a porch or deck. It excludes stairways not within or serving a building, porch or deck, and stairways leading to a nonhabitable attic or a crawl space.
Riser height (R311.7.5.1)
The riser is measured vertically between the leading edges of adjacent treads, without carpets, rugs or runners (R311.7.5). The tallest riser in a flight may not exceed the shortest by more than 3/8 in (9.5 mm). Risers must be vertical or slope back no more than 30 degrees.
Common mistake: dividing the framing height instead of the finished floor-to-floor height. If the treads and upper floor get 3/4 in hardwood but the bottom slab stays bare, the bottom riser grows by 3/4 in, twice the tolerance.
Tread depth (R311.7.5.2)
Tread depth is measured horizontally from nosing to nosing, at a right angle to the leading edge, with the same 3/8 in (9.5 mm) tolerance. A 7 3/4 in rise on a 10 in tread is the steepest stair the IRC allows, about 38 degrees.
Nosings (R311.7.5.3)
Treads under 11 in (279 mm) deep need a nosing projecting 3/4 to 1 1/4 in (19 to 32 mm), and the largest projection may not exceed the smallest by more than 3/8 in (9.5 mm). The edge radius is limited to 9/16 in (14 mm), or a bevel to 1/2 in (12.7 mm). Check the top landing too: the 2024 edition states that the landing at the top of the flight is part of the uniformity rule.
Open risers
Open risers are allowed, but where the opening is more than 30 in (762 mm) above the floor or grade below, a 4 in (102 mm) sphere must not pass (R311.7.5.1). With normal riser heights the gap is bigger than that, so those gaps need a riser bar or partial riser. Spiral stairs are exempt.
Width (R311.7.1)
The stair must be at least 36 in (914 mm) clear above handrail height. At and below the handrail, the clear width may drop to 31 1/2 in (787 mm) with one handrail or 27 in (698 mm) with two, since a handrail may project up to 4 1/2 in (114 mm) (R311.7.8.2). Common mistake: framing exactly 36 in, then losing width to drywall and skirt boards.
Headroom (R311.7.2)
Headroom is at least 6 ft 8 in (2032 mm), measured vertically from the sloped line of the nosings or from the landing floor. Where the nosings run under the edge of the floor opening, that edge may project up to 4 3/4 in (121 mm) into the headroom. The usual low point is the header of the floor opening.
Landings (R311.7.6)
Every stairway needs a floor or landing at the top and bottom, at least as wide as the flight served and, on a straight run, at least 36 in (914 mm) deep. A landing that is not rectangular must at least equal a quarter circle whose radius is the required width. No top landing is needed on an interior stair if no door swings over the stairs. One flight may rise no more than 12 ft 7 in (3835 mm) between floors or landings (R311.7.3).
Handrails: height and continuity (R311.7.8)
Each flight with four or more risers needs a handrail on at least one side, its top 34 to 38 in (864 to 965 mm) above the sloped plane of the nosings (R311.7.8.1). It runs from directly above the top riser to directly above the lowest riser, and its ends return to a wall, guard or walking surface, or stop at a post (R311.7.8.4). A newel post may interrupt it at a landing, at winders or over the lowest tread. Keep 1 1/2 in (38 mm) from the wall (R311.7.8.3). Round rails are 1 1/4 to 2 in (32 to 51 mm) in diameter (R311.7.8.5); a flat 2x4 fails the grip-size rules.
Guards: height and openings (R312.1)
Guards are required on open-sided stairs, landings and floors more than 30 in (762 mm) above the floor or grade below, measured within 36 in (914 mm) of the edge. Minimum height is 36 in (914 mm) at landings and floors and 34 in (864 mm) above the nosing line along a stair; where the guard top is the handrail, 34 to 38 in. A 4 in (102 mm) sphere must not pass, except 4 3/8 in (111 mm) on the open side of a stair and 6 in (153 mm) in the triangle under the bottom rail. Common mistake: keeping the stair baluster spacing on the landing, where 4 in applies.
Winders and spiral stairs
Winder treads need at least 10 in (254 mm) at the walkline, 12 in (305 mm) from the inside of the turn (R311.7.4), and at least 6 in (152 mm) everywhere within the clear width (R311.7.5.2.1). Spiral stairs have their own limits: 26 in (660 mm) clear width, treads at least 6 3/4 in (171 mm) at the walkline, risers up to 9 1/2 in (241 mm) and 6 ft 6 in (1982 mm) of headroom (R311.7.10.1).
Which edition applies: 2021, 2024 and local amendments
The IRC is a model code: it becomes law when a state or local government adopts it, and some jurisdictions amend it. The ICC reports that it is in use or adopted in 49 states, the District of Columbia, Guam, Puerto Rico and the U.S. Virgin Islands. The 2024 IRC keeps the numbers above but renumbers them: stairways become R318.7, handrails R320 and guards R321. It also allows a nosing over 1 1/4 in where risers are open, and runs the handrail from above the top landing nosing to above the lowest nosing. Ask your building department which edition and amendments apply.
FAQ
What is the maximum stair riser height under the IRC?
7 3/4 in (196 mm), with no more than 3/8 in (9.5 mm) between the tallest and shortest riser in a flight (R311.7.5.1).
What is the minimum tread depth for residential stairs?
10 in (254 mm), measured nosing to nosing (R311.7.5.2). Treads under 11 in (279 mm) also need a 3/4 to 1 1/4 in nosing.
How wide does a residential stair have to be?
36 in (914 mm) clear above the handrail, and at least 31 1/2 in (787 mm) at the handrail with one rail or 27 in (698 mm) with two (R311.7.1).
When is a handrail required on stairs?
On at least one side of any flight with four or more risers, 34 to 38 in (864 to 965 mm) above the nosings (R311.7.8).
Does the IRC stair code apply to deck stairs?
Yes. R311.7 covers stairways serving a building, porch or deck, so deck stairs follow the same riser, tread, handrail and guard rules. Check local amendments as well.
Stair3D gives you a design and a code check to guide your work. Check the measurements on site and with your local building authority before cutting.