Model codes are not automatically law
The International Residential Code (IRC) and International Building Code (IBC) are model codes written by the International Code Council. They become enforceable only when a state, county or city adopts them, usually a specific edition and often with local amendments. Your jurisdiction may enforce the 2021 IRC, the 2024 IRC, an older edition, or its own code. The building department that issues your permit is the authority on which rules apply.
Which rulebook covers which stair
| Situation | Rulebook usually involved | Note |
|---|---|---|
| Detached houses, duplexes, townhouses | IRC, as adopted locally | Includes stairs serving decks and porches. It exempts some stairs not serving a building, and stairs to non-habitable attics and crawl spaces. |
| Commercial, multi-family and public buildings | IBC, as adopted locally | Stricter geometry than the IRC. |
| Stairs that are part of a means of egress in facilities covered by the ADA | 2010 ADA Standards §504, alongside the building code | Federal civil-rights standard. A typical private single-family house is not an ADA-covered facility. |
| Workplace stairs (general industry) | OSHA 29 CFR 1910.25 | Protects workers. Not a residential design rule. |
| Stairways on construction sites | OSHA 29 CFR 1926.1052 | Temporary and permanent stairs used during construction work. |
| Outside the US | National or regional regulations | This site's references are US documents. Use your own country's rules. |
Residential stairs: the IRC
In the 2024 IRC the stair section was renumbered: stairways are now R318.7, handrails R320 and guards R321. In the 2021 IRC the same provisions are R311.7 and R312. The values below are the same in both editions. Many articles still quote only the older numbers, so check which edition your plans reviewer uses.
| Provision | 2024 IRC | 2021 IRC | Value |
|---|---|---|---|
| Maximum riser height | R318.7.5.1 | R311.7.5.1 | 7 3/4 in (196 mm), measured vertically between the leading edges of adjacent treads |
| Riser uniformity | R318.7.5.1 | R311.7.5.1 | Tallest riser at most 3/8 in (9.5 mm) more than the shortest in a flight |
| Minimum tread depth | R318.7.5.2 | R311.7.5.2 | 10 in (254 mm), measured horizontally between the foremost projections of adjacent treads |
| Tread uniformity | R318.7.5.2 | R311.7.5.2 | Deepest tread at most 3/8 in more than the shallowest in a flight |
| Nosing projection | R318.7.5.3 | R311.7.5.3 | 3/4 in to 1 1/4 in; not required where the tread depth is at least 11 in; larger allowed with open risers |
| Open risers | R318.7.5.1 | R311.7.5.1 | Where the stair is more than 30 in above the floor or grade below, openings must not pass a 4 in sphere |
| Maximum rise of one flight | R318.7.3 | R311.7.3 | 12 ft 7 in (151 in) between floors or landings |
| Width | R318.7.1 | R311.7.1 | 36 in clear above handrail height; 31 1/2 in at the handrail with one handrail, 27 in with two |
| Headroom | R318.7.2 | R311.7.2 | 6 ft 8 in, measured vertically from the sloped line of the nosings, with a limited exception at the floor opening |
| Landings | R318.7.6 | R311.7.6 | At the top and bottom of each flight, at least as wide as the stair and 36 in deep on a straight run; exceptions apply |
| Handrail required | R318.7.8 | R311.7.8 | On at least one side of each flight with four or more risers |
| Handrail height | R320.2 | R311.7.8.1 | 34 in to 38 in above the sloped line of the nosings |
| Guards | R321.1.1 | R312.1.1 | Required on open sides more than 30 in above the floor or grade within 36 in horizontally |
Two measurement details matter for calculators. Riser height is measured between the leading edges of adjacent treads, which on a uniform stair equals total rise ÷ risers. Tread depth is measured between the foremost projections of adjacent treads, which is the going, not the depth of the tread board. Dimensions are taken without carpet, rugs or runners (2024 R318.7.5).
The steepest stair these limits allow, 7 3/4 in risers on 10 in treads, is about 37.8°. That angle is derived from the two limits; the IRC does not state an angle.
Commercial stairs: the IBC
The 2024 IBC (section 1011.5.2) requires risers between 4 in and 7 in and treads of at least 11 in, with the same 3/8 in uniformity tolerance (1011.5.4). Nosing projection is limited to 1 1/4 in (1011.5.5.1), headroom is 80 in (1011.3), a single flight may rise at most 12 ft (1011.8), and the minimum width is generally 44 in, or 36 in where the occupant load is under 50 (1011.2). Occupancy-specific exceptions exist.
Accessible stairs: the ADA Standards
Where the 2010 ADA Standards apply, stairs that are part of a means of egress need uniform risers of 4 to 7 in and treads at least 11 in deep (504.2). Open risers are not permitted (504.3). The radius of the leading edge may be at most 1/2 in and nosings may project at most 1 1/2 in (504.5), and handrails are required on both sides (505.2).
Workplace and construction stairs: OSHA
OSHA's general-industry rule (1910.25) sets standard stairs between 30° and 50°, with risers of at most 9 1/2 in and treads of at least 9 1/2 in for stairs installed on or after January 17, 2017. The construction rule (1926.1052) requires stairs between 30° and 50°, limits variation in riser height and tread depth to 1/4 in, and requires handrails on stairways with four or more risers or more than 30 in of rise. These numbers protect workers. A 9 1/2 in riser is not acceptable in a house built to the IRC.
Common misreadings
- "7 3/4 in is the target." It is a maximum. Most comfortable house stairs use lower risers.
- "The tread board must be 10 in." The 10 in minimum is the going between nosings; the board is deeper by the nosing.
- "No nosing needed." Only true where the tread depth is at least 11 in; below that the IRC calls for a 3/4 to 1 1/4 in nosing on solid-riser stairs.
- "Every stair needs a handrail." In the IRC the trigger is four or more risers, but guards on open sides have their own height trigger.
- "ADA applies to my house." Generally not for a private single-family home, though local accessibility rules may apply to some dwellings.
- "Uniform on paper means uniform built." Floor finishes added later change the top and bottom risers. See why finishes matter.
How to find the rules that apply to you
- Find your building department (city or county) and ask which residential code edition is adopted and whether it is amended.
- Ask whether your project needs a permit. Stair work often does, especially new stairs, decks and structural changes.
- Read the adopted sections themselves. The ICC publishes free read-only versions of its codes at codes.iccsafe.org.
- Bring a dimensioned plan, for example a printout from the stair calculator, to the plan review.
What this site's checks do
The calculators compare your geometry with a chosen reference (IRC, IBC, ADA or OSHA values) and label each item "within reference", "outside reference" or "check". They never say a stair passes or complies, because that depends on adoption, amendments, exceptions, the building's use and the inspector. Headroom, width, guards and handrail details are flagged as things to check, not calculated.
Review history
- 2026-10-24: first published. IRC 2024 (current ICC version V2.1) and 2021 values and section numbers checked on ICC's public code viewer; IBC 2024, 2010 ADA Standards and OSHA 1910.25 / 1926.1052 checked on their official sites.