Stair Building Code Requirements

The 2021 IRC R311.7 sets a maximum riser height of 7 3/4 inches and a minimum tread depth of 10 inches. These are the non-negotiable stair code requirements that apply to every residential stair in a one- or two-family dwelling. Divide your total rise (floor-to-floor height) by a target riser between 4 and 7 3/4 inches to get the number of steps, then check that every riser in the flight is within 3/8 inch of every other. That uniformity rule is the single most common failure point. Measure the finished floor height, not the subfloor, or every riser will be wrong.

The 2021 IRC R311.7.5 requires riser height between 4 inches and 7 3/4 inches. Tread depth must be at least 10 inches and is capped at 13 3/4 inches. The tolerance for both is 3/8 inch between the largest and smallest in a single flight. A stair with a 7 3/4-inch riser and a 10-inch tread is legal but feels steep. Apply Blondel's Formula, 2R + T, to check comfort. The target range is 24 to 25 inches. At the code limits, 2 × 7.75 + 10 = 25.5, which sits outside the comfort zone. If you can, aim for a 7-inch riser and an 11-inch tread for a 2R + T of 25.

Riser slope matters too. Solid treads cannot have a riser that slopes more than 30 degrees from vertical. Open risers are permitted but the gap between treads must not allow a 4-inch sphere to pass through. Nosing projection is a separate regulated dimension, covered below.

The 2021 IRC R311.7 sets headroom at a minimum of 6 feet 8 inches measured vertically from the nosing of any tread to the nearest overhead obstruction. This applies to the full width of the stair and the full length of the flight. A sloped ceiling or a floor joist that drops below that line is a headroom violation. Measure from the finished nosing, not the subfloor or the stringer. Headroom failure is common in basement stairs with a low ceiling or in stairs built under a sloped roof. If the headroom clearance is tight, your only fix is to reduce the total run (which steepens the stair) or re-plan the floor opening.

The 2021 IRC does not set a minimum stair width for a straight residential flight beyond what is needed for a 36-inch landing. The landing at top and bottom must be at least as wide as the stair and 36 inches deep in the direction of travel. The maximum vertical rise between landings is 12 feet 3 inches per IRC R311.7.6. For a deck or basement stair that serves a single dwelling, a 36-inch-wide stair with a 36-inch-deep landing at both ends is the practical minimum. A winder cannot replace a required landing. If your total rise exceeds 12 feet 3 inches, you must insert a landing.

A rail is required on at least one side of any stair with four or more risers per 2021 IRC R311.7.7. The rail height must be between 34 inches and 38 inches, measured vertically from the nosing of the tread to the top of the rail. The rail must be graspable: 1 1/4 to 2 inches in diameter or an equivalent shape. Clearance from the wall must be at least 1 1/2 inches. The rail must run continuously the full length of the flight and extend horizontally 12 inches at the top and bottom. A guard is not a rail.

For a deck stair with an open side, the guard and rail are separate elements. The guard sits at 36 inches minimum height per R312.1. The rail is mounted at 34 to 38 inches. Do not combine them into one rail unless the top of the guard falls within the rail height range and is graspable.

Any open side of a stair or landing with a drop of more than 30 inches requires a guard per 2021 IRC R312. The guard must be at least 36 inches high measured from the walking surface. Infill openings, balusters, mesh, or glass, must not allow a 4-inch sphere to pass through. Baluster spacing is 4 inches on center maximum. Measure the drop from the finished walking surface, not from the edge of the tread. A deck stair that is 24 inches above grade does not need a guard. A deck stair that is 36 inches above grade needs a 36-inch guard.

The 2021 IRC is a model code. Adoption varies by state. Use the ICC code adoption map to see which edition your state has adopted. Some states, like California and Florida, have state-specific codes (California Residential Code, Florida Building Code) that amend or replace IRC sections. Local cities and counties can add their own amendments, such as a stricter riser limit of 6 inches or a different rail height. A stair built to the 2021 IRC limits may fail inspection in a jurisdiction with a local amendment. Call your building department before you cut stringers. The cost of a failed inspection and a rebuild far exceeds the time it takes to verify the local code.