Washington State Stair Code: Risers, Handrails, and Landings

The Washington state stair code for residential construction is the International Residential Code as adopted and amended in Chapter 51-51 of the Washington Administrative Code, overseen by the Washington State Building Code Council.¹ Any residential stair project that needs a building permit must meet these standards, and local jurisdictions verify compliance through inspections. The rules cover step size, width, clearance, handrails, guards, lighting, and load capacity.

Riser Height and Tread Depth

Under WAC 51-51-0311, the maximum riser height is 7¾ inches and the minimum tread depth is 10 inches.² Riser height is measured vertically between the leading edges of consecutive treads. Tread depth is measured horizontally between the front projections of adjacent treads, at a right angle to the leading edge.

Uniformity matters as much as the individual dimensions. Within a single flight, the tallest riser cannot differ from the shortest by more than ⅜ inch, and the same ⅜-inch tolerance applies to tread depth variation.² Inspectors measure this closely, and a flight that fails often has to be rebuilt.

Nosing Profile

Where a tread projects past the face of the riser below, the overhang must be between ¾ inch and 1¼ inches. The leading edge must be rounded to a radius no greater than 9/16 inch. If the treads are solid with no overhang, such as a closed-riser stair with a tread depth of 11 inches or more, the nosing projection requirement does not apply.

Stairway Width and Headroom

The minimum clear width of a residential stairway is 36 inches, measured at all points above the handrail and below the required headroom. Handrails may project into that space by up to 4½ inches per side, but the clear width at and below handrail height cannot drop below 31½ inches with one handrail installed, or 27 inches with handrails on both sides.

Headroom must be at least 6 feet 8 inches along the entire length of the stairway. It is measured vertically from a sloped line connecting the nosings of consecutive treads up to the ceiling or any obstruction above. Ductwork, soffits, and headers that dip below this line will fail inspection. Basement stairs framed into existing floor joists are the usual trouble spot.

Flight Rise and Landings

A single flight cannot rise more than 12 feet 7 inches between floor levels or landings. Stairs serving non-habitable attics, crawl spaces, or exterior areas not attached to a building are exempt from this limit. Homes with standard 8- or 9-foot ceilings sit comfortably under the cap; tall great rooms and some split-level designs need an intermediate landing.

Every stairway needs a floor or landing at both the top and the bottom. The landing must be at least as wide as the stairway it serves. For a straight run, the landing depth in the direction of travel must be at least 36 inches. A landing is not required at the top of an interior flight, including stairs in an enclosed garage, as long as no door swings over the stairs.³

Where a door opens onto a landing, it cannot reduce the required landing depth to less than 29 inches when fully open, and its swing cannot block more than half the landing width at any point during its travel.

Winder Stairs

Winder treads are permitted with extra rules. Tread depth at the walkline must be at least 10 inches, measured 12 inches from the narrow side of the winder. At the narrowest point, no winder tread can be less than 6 inches deep. Within a flight, the largest winder tread at the walkline cannot exceed the smallest by more than ⅜ inch.

Handrail Requirements

Handrails are required on at least one side of every flight with four or more risers. Handrail height, measured vertically above the stair nosings, must be between 34 and 38 inches.⁴ The rail must run continuously for the full length of the flight, from directly above the top riser to directly above the bottom riser.⁵ Continuity can be interrupted by a newel post at a turn, at a landing, or over the lowest tread.

Handrail ends must return to the wall, terminate into a newel post, or connect to a safety terminal. An exposed end will fail inspection.

Grip Size

The code recognizes two handrail profiles:

Type I: A circular rail with an outside diameter between 1¼ and 2 inches. A non-circular rail must have a perimeter between 4 and 6¼ inches, with a maximum cross-section of 2¼ inches.⁶
Type II: A larger rail with a perimeter greater than 6¼ inches and a cross-section of 2¼ inches or less. Because a full hand wrap is not possible at that size, the rail must have a finger recess on both sides, starting within ¾ inch of the top and at least 5/16 inch deep.⁶

Flat 2×4 or 2×6 boards used as handrails do not meet either profile because a hand cannot close around them.

Guard Requirements

Guards are required on any open-sided walking surface, including stairs, porches, balconies, and landings, where the drop to the floor or ground below is more than 30 inches measured within 36 inches horizontally of the open edge.⁷

Residential guards must stand at least 36 inches high, measured vertically from the walking surface or from the line connecting the nosings on stairs.⁴ On the open side of a stairway, the guard height can drop to a minimum of 34 inches, and where the top of the guard also serves as the handrail, the height must fall between 34 and 38 inches.

Opening Limitations

From the walking surface up to the required guard height, no opening can allow a 4-inch-diameter sphere to pass through.⁸ Two exceptions apply along stairs:

Triangular openings formed by the riser, tread, and bottom rail cannot allow a 6-inch sphere to pass.⁸
Openings along the open side of a stair cannot allow a 4⅜-inch sphere to pass.⁸

Structural Strength

Guards must resist a concentrated 200-pound load applied at any point along the top. Where the guard top does not serve as a handrail, the load is applied both vertically downward and horizontally away from the walking surface. Where it also functions as the handrail, the 200-pound load can be applied in any direction.⁹ Infill components such as balusters and panels must withstand a 50-pound load applied over a 1-square-foot area.

Stairway Lighting

Every interior stairway needs an artificial light source capable of illuminating treads and landings to at least 1 footcandle, measured at the center of each tread and landing. For any stairway with six or more risers, a wall switch must be installed at each floor level.¹⁰ The switch requirement is waived if the lighting is controlled by a remote, central, or automatic system such as a motion sensor.

Structural Loads and Under-Stair Fire Protection

Stairs must support a minimum uniformly distributed live load of 40 pounds per square foot. Each individual tread must also handle a 300-pound concentrated load applied to a 2-inch by 2-inch area, whichever scenario produces greater stress.⁹ Lightweight treads and long unsupported spans that look fine can still fail the load test.

If the space under a stairway is enclosed for storage or any other use, the walls, under-stair surface, and any soffits on the enclosed side must be covered with at least ½-inch gypsum board.¹¹

1
Washington State Building Code Council. 2021 International Residential Code
2
Washington State Legislature. Washington Administrative Code 51-51-0311
3
UpCodes. href=”https://up.codes/s/landings-for-stairways” target=”_blank” rel=”noopener”>R311.7.6 Landings for Stairways
4
Washington State Legislature. Chapter 51-51 WAC – Section R312.1.2
5
International Code Council. 2018 International Residential Code R311.7.8.4 Continuity
6
International Code Council. 2018 International Residential Code R311.7.8.5 Grip Size
7
Washington State Legislature. Chapter 51-51 WAC – Section R312.1.1
8
International Code Council. 2021 International Residential Code R312.1.3 Opening Limitations
9
International Code Council. 2021 International Residential Code R301.5 Live Load
10
UpCodes. R303.7 Interior Stairway Illumination
11
International Code Council. 2021 International Residential Code R302.7 Under-Stair Protection