What is a stair nosing?
A stair nosing is the part of a tread that overhangs the face of the riser below it. It gives the foot more landing on a short tread and is the line every stair dimension is measured from: tread depth runs nosing to nosing, and headroom and handrail height are measured from the sloped line through the nosings[1].
Stair nosing code requirements (IRC)
The IRC sets the nosing size in R311.7.5.3 (R318.7.5.3 in the 2024 edition)[1][2]:
| Rule | Requirement |
|---|---|
| Projection | at least 3/4 in and at most 1-1/4 in |
| Uniformity | largest and smallest projection in a flight within 3/8 in (2024: includes the landing at the top) |
| Edge shape | radius at most 9/16 in, or a bevel at most 1/2 in |
| When not required | treads 11 in deep or more |
| Open risers (2024) | projection may exceed 1-1/4 in where risers are open |
Before 2018 the rule applied only to stairs with solid risers; the 2018 and 2021 editions apply it to all stairways[3][1].
Stair nosing outside the US
- England: the going is measured nosing to nosing; on open-riser stairs the treads must overlap by at least 16 mm and a 100 mm sphere must not pass between them[4].
- Canada: the NBC has no projection rule like the IRC's; the top edge of a tread is rounded or bevelled 6 to 14 mm (3 mm minimum with a resilient covering)[5].
Sizing the tread board with the nosing
A tread board must be the unit run plus the nosing deep. With 10 in runs and a 1 in nosing, cut 11 in treads; with an 11 in run no nosing is required and an 11-1/4 in board leaves a 1/4 in overhang. The stair tread calculator sizes treads and riser boards together, and the stair calculator checks the nosing along with the risers and treads.
Common nosing mistakes
- Mixed projections. A thicker finish tread on the top step changes its nosing; keep all within 3/8 in.
- Square, sharp edges. A radius or bevel up to the limit is allowed and safer on the foot.
- Forgetting the nosing in the run. The run is measured nosing to nosing, not riser to riser plus nosing; see the stair code summary.
Frequently asked questions
What is the code for stair nosing?
Under the IRC, a projection of 3/4 to 1-1/4 in, edge radius at most 9/16 in or bevel at most 1/2 in, all nosings within 3/8 in of each other. Not required on treads 11 in deep or more.
Is stair nosing required?
Only when the tread is shallower than 11 in under the IRC. With 11 in treads you may leave it off.
How far should stair nosing overhang?
Between 3/4 and 1-1/4 in past the riser below, and the same on every step within 3/8 in.
Is tread depth measured with the nosing?
Yes. The IRC measures tread depth horizontally from the leading edge of one nosing to the next.
Sources
- 2021 International Residential Code, Chapter 3: Building Planning (R301.5, R311.7 stairways, R312 guards)
- 2024 International Residential Code, Chapter 3 (R318.7 stairways, R320 handrails, R321 guards)
- 2015 International Residential Code, Chapter 3 (147 in flight rise)
- Approved Document K: Protection from falling, collision and impact (2013 edition, for use in England)
- National Building Code of Canada 2020, Division B, Part 9, Section 9.8 Stairs, ramps, handrails and guards