Under 2021 IRC Section R311.7.5, a stair riser may be no taller than 7-3/4 inches and a tread no shallower than 10 inches, and within one flight the tallest riser and the deepest tread may each exceed the smallest by no more than 3/8 inch. Headroom, width, nosings and landings have their own limits, all quoted below from the code text.
These are prescriptive minimums for the dwelling stairs the IRC covers. The code edition your jurisdiction has adopted, and any local amendments, govern. The numbers below are copied from the 2021 IRC as reproduced in the 2021 Seattle Residential Code. The sections covered here carry no Washington amendment markings, but check your local text before you rely on a figure.
Which stairs does R311.7 cover?
Section R311.7 applies wherever the code requires a stairway or one is provided. Three exceptions are listed: stairways not within or serving a building, porch or deck; stairways leading to nonhabitable attics; and stairways leading to crawl spaces. Spiral stairways and bulkhead enclosure stairways follow R311.7 with their own adjustments in Sections R311.7.10.1 and R311.7.10.2, which this article does not cover.
For deck stairs, R311.7 applies because the stair serves a porch or deck. See the deck framing plan guide for where the stair sits relative to the deck framing.
Stair riser height code: the 7-3/4 inch maximum
Section R311.7.5.1 sets the rules for risers:
- The riser height is not more than 7-3/4 inches (196 mm).
- It is measured vertically between the leading edges of the adjacent treads.
- The greatest riser height within any flight may not exceed the smallest by more than 3/8 inch (9.5 mm).
- Risers are vertical, or sloped from the underside of the nosing of the tread above at an angle not more than 30 degrees from the vertical.
The code gives a maximum only. It does not set a minimum riser height, and it does not say how many risers a flight must have. The riser count falls out of your total rise, and the 3/8 inch limit is why every riser in the flight has to be cut from the same divided dimension, with the first and last adjusted for the floor finishes.
Section R311.7.5 also says dimensions and dimensioned surfaces are exclusive of carpets, rugs or runners. Measure to the hard surface, not the carpet.
Stair tread depth code: the 10 inch minimum
Section R311.7.5.2 sets the tread depth at not less than 10 inches (254 mm). It is measured horizontally between the vertical planes of the foremost projection of adjacent treads, at a right angle to the tread's leading edge. As with risers, the greatest tread depth in a flight may not exceed the smallest by more than 3/8 inch.
That measurement is nosing to nosing, not the flat top of the board. A tread with a nosing projection still has to give 10 inches between the foremost edges.
Summary of 2021 IRC Sections R311.7.5.1 and R311.7.5.2, straight flights.
| Item | Limit | Section |
|---|---|---|
| Riser height | Not more than 7-3/4 in. | R311.7.5.1 |
| Riser variation in a flight | Not more than 3/8 in. between greatest and smallest | R311.7.5.1 |
| Tread depth | Not less than 10 in. | R311.7.5.2 |
| Tread variation in a flight | Not more than 3/8 in. between greatest and smallest | R311.7.5.2 |
| Riser slope (if not vertical) | Not more than 30 degrees from vertical | R311.7.5.1 |
Stair rise and run code: how the numbers fit together
The code never states a "rise and run" formula. It gives a riser maximum and a tread minimum and leaves you to fit them to your floor-to-floor height. The method is:
- Measure the total vertical rise from finished floor to finished floor, using your own dimension.
- Pick a whole number of risers that divides that rise into riser heights not over 7-3/4 inches.
- The run of the flight is the number of treads multiplied by the tread depth you choose, which must be at least 10 inches. A flight has one fewer tread than risers when the upper floor acts as the top step.
- Check the vertical rise limit in Section R311.7.3: a flight may not have a vertical rise greater than 12 feet 7 inches between floor levels or landings.
Taller risers mean fewer steps and a shorter run, so the stair takes less floor area. Shallower risers mean more steps and a longer run. The code limits fix the steepest stair you may build, and the run you choose fixes how much floor it consumes. That run is what sets the stair length on your plan, and it is why the stair is often the first thing placed on a layout.
Nosings and open risers
Section R311.7.5.3 controls the nosing:
- The radius of curvature at the nosing is not greater than 9/16 inch (14 mm), or the bevel is not greater than 1/2 inch (12.7 mm).
- A nosing projection of not less than 3/4 inch (19 mm) and not more than 1-1/4 inches (32 mm) is provided.
- The greatest nosing projection may not exceed the smallest by more than 3/8 inch within a stairway.
- Exception: no nosing projection is required where the tread depth is not less than 11 inches (279 mm).
Open risers (no board between treads) are allowed, with a limit written into R311.7.5.1: openings located more than 30 inches, measured vertically, to the floor or grade below may not permit the passage of a 4-inch-diameter (102 mm) sphere. The sphere limit does not apply to spiral stairways.
Winders and the walkline
A winder is a wedge-shaped tread used to turn a stair without a landing. Section R311.7.4 defines the walkline: it is concentric to the turn and parallel to the direction of travel entering and exiting the turn, located 12 inches (305 mm) from the inside of the turn, measured from the widest point of the clear stair width at the walking surface.
Section R311.7.5.2.1 then sets winder treads:
- Not less than 10 inches (254 mm) deep at the walkline, measured between the vertical planes of the foremost projection of adjacent treads.
- Not less than 6 inches (152 mm) deep at any point within the clear width of the stair.
- Within a flight, the largest winder depth at the walkline may not exceed the smallest by more than 3/8 inch.
- Consistently shaped winders at the walkline may share a flight with rectangular treads, and do not need to be within 3/8 inch of the rectangular tread depth.
Stair width, headroom, and the floor opening
Stair width, headroom and vertical rise, 2021 IRC Sections R311.7.1 to R311.7.3.
| Item | Requirement | Section |
|---|---|---|
| Clear width above handrail height and below headroom | Not less than 36 in. | R311.7.1 |
| Clear width at and below handrail height, one handrail | Not less than 31-1/2 in. | R311.7.1 |
| Clear width at and below handrail height, two handrails | Not less than 27 in. | R311.7.1 |
| Headroom | Not less than 6 ft 8 in. | R311.7.2 |
| Vertical rise of one flight | Not more than 12 ft 7 in. | R311.7.3 |
Headroom is measured vertically from the sloped line adjoining the tread nosings, or from the floor surface of a landing or platform. This is the dimension that sizes the floor opening. Exception 1 to Section R311.7.2 lets the nosings of treads at the side of a flight extend under the edge of a floor opening through which the stair passes, provided the floor opening does not project horizontally into the required headroom by more than 4-3/4 inches (121 mm).
In practice, the header at the top of the opening has to sit far enough along the stair that the nosing line has dropped to give the headroom under the floor framing, ceiling finish included. A deeper floor system pushes the opening longer. Take the depth of your own floor assembly and your own nosing line, and work it out for the stair you have drawn. Framing a stair opening covers the headers, trimmers and tail joists once the size is known.
Landings at the top and bottom of a stair
Section R311.7.6 requires a floor or landing at the top and bottom of each stairway. The width perpendicular to the direction of travel is not less than the width of the flight served. Where the stairway has a straight run, the depth in the direction of travel is not less than 36 inches (914 mm). For landings of other shapes, the depth at the walk line and the total area may not be less than a quarter circle with a radius equal to the required landing width.
There is one exception: a floor or landing is not required at the top of an interior flight, including stairs in an enclosed garage, provided that a door does not swing over the stairs.
Landing and walking surfaces may not slope steeper than 1 unit vertical in 48 horizontal (2 percent), per Section R311.7.7. Where a landing must drain surface water, the limit is 1 in 20 (5 percent) in the direction of travel.
Handrails
Section R311.7.8 requires a handrail on at least one side of each flight with four or more risers. Height, projection, clearance, continuity and grip size are covered in the handrail height code guide.
How the stair shows up on a framing plan
On a floor framing plan the stair appears as an opening in the joist field, a direction arrow, and an annotation. Three code results drive what you draw:
- Opening length follows from the run you chose and the headroom rule.
- Opening width follows from the stair width plus wall finishes, with R311.7.1 as the floor for the clear width.
- Landing zones at the top and bottom need floor framing under them, and an intermediate landing on a turning stair needs its own support.
The same stair appears on both levels of a two-story house, so the opening on the upper floor and the footprint on the lower floor have to agree. The second floor framing plan and floor framing plan for a two-story home show how the two levels are coordinated. If you are new to reading these drawings, start with how to read a floor framing plan.
Using Framing Plan for the opening
Framing Plan draws joist layouts, beams and openings and exports PNG, PDF and DXF. It does not check stair geometry against the IRC: its sample profile uses demo span figures and metric sizes, and the editor works in millimeters. Work out the rise, run, headroom and landing sizes yourself from the code text above and your adopted code, then open the framing studio to draw the opening into a preliminary layout for your designer or plan reviewer. Prefabricated or engineered stair components follow the manufacturer's instructions, and the building department has the final say.