Stair Building Code Requirements (IRC R318.7)

Quick reference — IRC vs IBC

Worked example: checking a stair

Say the finished floor-to-floor height is 105 inches. Divide by the 7¾-inch maximum and round up to get the fewest risers that pass:

A straight flight has one fewer tread than risers, so 13 treads. At the 10-inch minimum that is a total run of 130 inches (10'10"). Our stair calculator does this, plus stringer length, for any height.

Riser height — IRC R318.7.5.1

Maximum riser height is 7¾ inches, measured vertically between the leading edges of adjacent treads. The IRC sets no minimum. Risers must also be uniform within a flight:

You cannot make the bottom or top step a different height to absorb rounding. Divide the total rise evenly across all risers so every step is identical. A stair with 13 risers at 7¼ inches and one at 7¾ inches fails, because the difference is ½ inch.

Open risers (no riser board) are allowed, as long as the gap between treads will not let a 4-inch sphere pass through. The limit applies only to openings more than 30 inches above the floor or grade below — gaps on the lowest few steps are not restricted — and spiral stairs are exempt.

Tread depth — IRC R318.7.5.2

Minimum tread depth is 10 inches, measured horizontally between the vertical planes of the foremost projection of adjacent treads, at a right angle to the tread's leading edge. The same ⅜-inch uniformity rule applies to treads.

Winder treads — IRC R318.7.5.2.1

Winders are the wedge-shaped treads used to turn a stair without a landing. Because they are narrow at one end, the IRC measures them along a walkline located 12 inches from the inside of the turn (R318.7.4):

• At least 10 inches deep at the walkline
• At least 6 inches deep at any point within the clear width
• Winder depths at the walkline within ⅜ inch of each other; they do not have to be within ⅜ inch of the rectangular treads in the same flight

Nosing — IRC R318.7.5.3

Stairways need a nosing projection of ¾ inch to 1¼ inches. The largest and smallest projection may differ by no more than ⅜ inch within a flight, including the landing at the top of the flight. Treads, landings and floors need a nosing radius of no more than 9⁄16 inch or a bevel of no more than ½ inch. A nosing projection is not required when treads are at least 11 inches deep, and with open risers the projection may exceed 1¼ inches.

Stair width — IRC R318.7.1

Minimum clear width is 36 inches at all points above the permitted handrail height and below the required headroom height. Handrails may project up to 4½ inches (R320.3), so at and below handrail height the minimum is 31½ inches with a handrail on one side, or 27 inches with handrails on both sides.

Headroom — IRC R318.7.2

Minimum headroom is 6 feet 8 inches (80 inches), measured vertically from the sloped line adjoining the tread nosings, or from the landing surface. Where the side of a flight passes under the edge of a floor opening, the floor opening may project into the required headroom by up to 4¾ inches.

Handrails — IRC R318.7.8 and R320

A handrail is required on at least one side of each flight with 4 or more risers. Key requirements:

• Height: 34–38 inches, measured vertically from the sloped plane adjoining the tread nosings (R320.2)
• Continuity: runs the full flight, from directly above the top landing nosing to directly above the lowest nosing. Ends return continuous to themselves or toward a wall, guard or walking surface, with no more than a ¼-inch gap at a wall. A newel post may interrupt the rail at a winder turn, at a landing or over the lowest tread (R320.5)
• Graspable profile: round rails 1¼–2 inches in diameter; other shapes need a 4–6¼ inch perimeter and a cross-section no wider than 2¼ inches. Larger profiles need a finger recess on both sides (Type II, R320.6)
• Wall clearance: at least 1½ inches between the rail and the wall (R320.4)
• Strength: must resist a 200 lb concentrated load applied in any direction at any point along the top (Table R301.5)

Guards on open sides — IRC R321

A handrail is for gripping; a guard stops people falling off the side. Guards are required on open-sided stairs and landings more than 30 inches above the floor or grade below, measured at any point within 36 inches horizontally of the open edge.

• At least 34 inches high on the stair, measured from the tread nosings (36 inches on landings and floors)
• Balusters spaced so a 4⅜-inch sphere cannot pass (4 inches on landings and floors)
• The triangle formed by the riser, tread and bottom rail must stop a 6-inch sphere

Landings — IRC R318.7.6

A landing or floor is required at the top and bottom of each flight. Key rules:

• At least as wide as the flight it serves
• On a straight run, at least 36 inches deep in the direction of travel
• Landings that are not square or rectangular need a depth at the walkline and a total area at least equal to a quarter circle with a radius equal to the required landing width
• No flight may rise more than 12'7" (151 inches) between floors or landings (R318.7.3)
• On an interior stair, the top landing may be on the other side of a door at the top, as long as the door does not swing over the stairs

Lighting — IRC R325.6 and R325.7

Interior stairs (R325.6): a light must be able to illuminate the treads and landings to at least 1 foot-candle, measured at the center of treads and landings. If the stair has 6 or more risers, there must be a wall switch at each floor level — unless remote, central or automatic control of the lighting is provided.

Exterior stairs (R325.7): a light is required at the top landing — and at the bottom landing for outdoor stairs down to a basement.

Spiral stairs — IRC R318.7.11.1

Spiral stairs get their own, looser rules because of their compact geometry:

Deck and exterior stairs

There is no separate, looser code for deck stairs — stairways serving a building, porch or deck follow the same R318.7 rules for risers, treads, width and handrails. Outdoors, three more items apply:

• Guards: any open side more than 30 inches above grade needs a guard (see R321 above)
• Lighting: a light source at the top landing (R325.7)
• Bottom riser: risers are measured to the landing or floor the stair lands on, so measure total rise to the finished surface — patio, pad or grade — not to the ground before it is levelled

The IRC's deck construction rules are in Section R507. The American Wood Council's DCA 6 prescriptive deck guide is based on the 2015 IRC — ask your building department whether it is accepted where you build.

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