Proper Measurements for the Construction of Stairs

Basics

• Stairs must rise at 12 feet or less between floors or landings. They must be at least 36 inches wide starting from the handrail and continue at least 80 inches above the stair step, floor or landing. Below the handrails, the minimum clear width must be at least 31.5 inches if only one handrail is installed, and 27 inches if handrails are placed on both sides. Each flight of stairs must start and end with a floor or landing. (This is not required at the top of interior stairs, but only if a door does not swing over the stairs.) The minimum width of this space must match that of the stairs. If the landing or floor area is not square, its minimum depth must be at least 36 inches.

Steps

• The tread is the horizontal step, which must be at least 10 inches deep, and have uniform measurements across an entire flight, with variations of under 0.375 inches. The riser is the vertical area between steps. Its maximum height is 7.75 inches and must have similarly uniform dimensions. Risers may slope from the underside of the step edge by 30 degrees or less from the vertical. They may be solid, have a pattern or be open. However, any openings must not allow a four-inch-diameter sphere to pass. This prevents small children from getting stuck in these spaces.

Handrails

• Each continuous flight of stairs with four or more risers needs a handrail on at least one side. The rail must have a height of between 34 and 38 inches, as measured from the top of the rail to the plane next to the step edge, or finished surface of a ramp. Handrails with circular cross-sections must have an outside diameter of 1.25 to 2 inches. Those that are not circular need a perimeter dimension of 4 to 6.25 inches, with a maximum cross section of 2.25 inches. If a handrail has a perimeter over 6.25 inches, it needs graspable recesses for fingers on both sides of the profile. These recessed parts must measure at least 0.31 inches deep and continue from 0.75 inches from the top of the rail to at least 0.87 inches below the top.

Spiral

• Spiral stairways do not take up as much room as standard incline stairways. They must have a minimum clear width of 26 inches from the handrail to the step below. Each tread must measure at least 7.5 inches as measured one foot away from the narrower edge. Risers must be a maximum of 9.5 inches. Spirals need minimum headroom of 78 inches.