The new Subpart D standard defines when Alternating Tread-Type, Spiral or Ship Stairs can be used in the workplace in 29 CFR 1910.25 (b) (8): “Spiral, ship or alternating tread-type stairs are used only when the employer can demonstrate that it is not feasible to provide standard stairs”. If a spiral, ship or alternating tread-type stairs is allowed by this section, they must be installed, used and maintained in accordance with manufacturer’s instructions (29 CFR 2910.25 (b)(9).
- Spiral Stairs
- 29 CFR 1910.25 (d)
- Must have a clear width of 26 inches.
- Must have a maximum riser height of 9.5 inches.
- Must have a minimum headroom above spiral stair treads of at least 6 feet, 6 inches (measured from the leading edge of the tread).
- Must have a minimum tread depth of 7.5 inches, measured at a point 12 inches from the narrower edge.
- Must have a uniform tread size.
- Ship Stairs
- 29 CFR 1910.25 (e)
- Are installed at a slope between 50 to 70 degrees from the horizontal.
- Must have open risers with a vertical rise between tread surfaces of 6.5 to 12 inches.
- Must have a minimum tread depth of 4 inches.
- Must have a minimum tread width of 18 inches.
- Alternating tread-type stairs
- 29 CFR 1910.25 (f)
- Must have a series of treads installed at a slope of 50 to 70 degrees from the horizontal.
- Must have a distance between handrails of 17 to 24 inches.
- Must have a minimum tread depth of 8.5 inches.
- Must have open risers if the tread depth is less than 9.5 inches.
- Must have a minimum tread width of 7 inches, measured at the leading edge of the tread (i.e. nosing).
Listed below is Figure D-10 “Angles for Stairs, Ramps and Ladders” that is a good reference for determining which walking-working surface standards apply to your ladders and stairs based on their slope angle.