What are alternating tread-type stairs?
Alternating tread-type stairs are steep, space-efficient stairways with offset treads designed for one foot at a time. In OSHA-regulated general industry, they are a distinct type of stair—not a ship stair or fixed ladder—and may be used only when an employer can demonstrate that standard stairs are not feasible.
| Design criterion | Alternating tread-type stairs | Ship stairs |
|---|---|---|
| Slope from horizontal | 50–70 degrees | 50–70 degrees |
| Tread arrangement | Alternates for left and right feet | Full-width treads with open risers |
| Minimum tread depth | 8.5 inches | 4 inches |
| Minimum tread width | 7 inches at the nosing | 18 inches |
| Riser condition | Open if tread depth is under 9.5 inches | Open; vertical rise of 6.5–12 inches |
| Handrail spacing | 17–24 inches between handrails | Governed by applicable stair and rail provisions |
What are alternating tread-type stairs?
Alternating tread-type stairs are stairways whose treads alternate from side to side, often around a central support. Each tread provides a deeper stepping surface for one foot than a conventional full-width tread could provide within the same steep footprint. OSHA defines the system more precisely: the alternating arrangement means an employee typically does not have both feet on the same level while using the stairway. This stepping pattern is the feature that distinguishes the system—not simply its steep angle or compact dimensions. Alternating tread-type stairs belong to the same broad family of space-efficient access systems as ship stairs. Both can connect levels where a standard stair would require too much horizontal run. Under OSHA’s general-industry rules, however, they remain separate stairway types with separate dimensional criteria. Calling an alternating tread assembly a “ship ladder” does not change which criteria apply. The geometry and intended use determine the relevant requirements.
How do alternating treads work?
A person uses an alternating tread stair by placing each foot on the wider portion of the corresponding left or right tread. The unused side of each tread is cut back or offset, allowing the next tread to overlap vertically without taking up the horizontal run associated with conventional stairs. The result is a stair that can provide substantial tread depth for the active foot while rising at a 50- to 70-degree slope. That compactness comes with a different movement pattern. Users must establish the correct lead foot and continue the alternating sequence while ascending or descending. Handrails are therefore integral to the access system, not an optional accessory. OSHA specifies both a tread configuration and a 17- to 24-inch distance between handrails. The design must be evaluated as a complete route: approach space, rail continuity, overhead clearance, landings, openings and the destination all affect whether employees can use it safely.
How do alternating tread-type stairs differ from ship stairs?
Alternating tread-type stairs and ship stairs occupy the same 50- to 70-degree slope range under OSHA, but they manage that steepness differently. Alternating tread stairs use offset stepping surfaces, so each foot lands on a different level and side. Ship stairs use full-width treads, open risers and stair rails. OSHA requires ship-stair treads to be at least 18 inches wide and 4 inches deep, with 6.5 to 12 inches of vertical rise between tread surfaces. For alternating tread stairs, OSHA instead requires at least 8.5 inches of tread depth, at least 7 inches of width at the nosing, and 17 to 24 inches between handrails. Open risers are required when tread depth is less than 9.5 inches. These are not interchangeable specifications. A designer should first identify the stair type, then apply the complete set of requirements for that type. A hybrid assembled from favorable measurements in both categories is not a sound compliance strategy.
How are they different from fixed ladders?
A fixed ladder is a ladder permanently attached to a structure, building or equipment. Its user climbs on rungs, steps or cleats and often faces the structure. An alternating tread system is a stairway: it has treads connecting levels, incorporates handrails and is traveled using an alternating-foot pattern. OSHA’s angle illustration places alternating tread-type stairs at 50 to 70 degrees and ladders at 60 to 90 degrees, so the angle ranges can overlap. Angle alone therefore does not classify an access system. The form of the walking surface, the rail arrangement and the way an employee uses the system also matter. This distinction has practical consequences. Fixed ladders are governed principally by §1910.23 and have ladder-specific clearance, rung, side-rail and fall-protection provisions. Alternating tread-type stairs fall within §1910.25 and the stairway-related provisions of §§1910.28 and 1910.29. Selecting a label based on convenience can send a project down the wrong design path.
When does OSHA allow alternating tread-type stairs?
OSHA does not treat compactness by itself as permission to use an alternating tread stair. Section 1910.25(b)(7) requires standard stairs when operations necessitate regular and routine travel between walking-working surfaces, including access to equipment operating platforms. Section 1910.25(b)(8) then limits spiral, ship and alternating tread-type stairs to situations in which the employer can demonstrate that standard stairs are not feasible. This feasibility condition should be addressed before dimensions are optimized. Employers should document the physical or operational constraint that prevents a standard stair, preserve the site measurements and design assumptions behind the decision, and confirm that the selected route suits its expected users and tasks. Where the exception applies, §1910.25(b)(9) also requires the stair to be installed, used and maintained according to the manufacturer’s instructions. Meeting the dimensional minimums does not replace either the feasibility demonstration or the manufacturer’s requirements.
What dimensions and structural conditions apply?
The alternating-tread provisions in §1910.25(f) work together with the general stairway provisions in §1910.25(b). In addition to the 50- to 70-degree slope, handrail spacing and tread geometry, the employer must ensure at least 6 feet 8 inches of vertical clearance above the leading edge of each tread. Riser heights and tread depths must be uniform between landings. Landings and platforms must be at least as wide as the stair and at least 30 inches deep in the direction of travel. Each stair must support at least five times the normal anticipated live load and never less than a 1,000-pound concentrated load at any point. A door or gate opening directly onto the stair requires a platform, and its swing cannot reduce the effective usable depth below the applicable limit. These provisions show why a catalog width or angle cannot establish compliance on its own: structural capacity, transitions and the surrounding installation remain part of the assessment.
What handrail and fall-protection rules matter?
Section 1910.25 requires stairway handrails, stair rail systems and guardrail systems to be provided in accordance with §1910.28. Table D-2 in §1910.28(b)(11) determines the required arrangement based on the stair’s number of risers, width and whether its sides are open. Section 1910.29(f) then establishes criteria for the systems themselves, including height, graspability, clearance, smooth surfaces, openings, projection hazards and strength. OSHA distinguishes a handrail, which gives an employee a handhold for support, from a stair rail system, which creates a barrier along an exposed side. One component does not automatically perform both functions. Current rail rules also contain installation-date distinctions, and OSHA issued interim enforcement guidance in 2026 addressing handrail and stair-rail height requirements. For a real project, the employer and designer should review the current regulatory text and applicable guidance rather than relying on a simplified diagram or an older product detail.
Where are alternating tread-type stairs used?
Alternating tread-type stairs can be considered for constrained general-industry access routes where standard stairs are demonstrably infeasible. Potential settings include access to elevated service platforms, mechanical areas, rooftop equipment or other restricted locations with limited horizontal run. Suitability still depends on how the route will be used. Travel frequency, tools or materials carried, environmental exposure, footwear, lighting, contaminants and emergency-planning needs can all affect the choice. OSHA’s general-industry Subpart D is not a universal building code, and §1910.25 excludes certain stairways, including stairs on scaffolds, stairs designed into machines or equipment and stairs on self-propelled motorized equipment. State-plan OSHA requirements, fire and building codes, accessibility rules, owner standards and local permitting requirements may also apply. A qualified professional should reconcile those authorities for the jurisdiction and occupancy before fabrication. This article explains the federal general-industry framework; it is not a site-specific compliance determination.
How can the Ship Stair Builder support early planning?
If the feasibility review points to ship stairs rather than an alternating tread system, the Ship Stair Builder can help turn site measurements into a configurable ship-stair concept. It allows a project team to explore the relationship among rise, angle, tread geometry and available space, then identify selected dimensional conditions that need attention before a quote or detailed review. The builder models ship stairs; it does not design, check or validate alternating tread-type stairs. It also does not certify OSHA compliance or replace engineering, fall-protection, installation, manufacturer or site-specific review. Used within those limits, the tool can make early decisions more visible, reduce avoidable specification errors and help teams address selected dimensional issues before fabrication. That preparation can reduce exposure to preventable redesign or compliance problems, but no configurator can guarantee that an installation will avoid an OSHA citation or fine. Final responsibility remains with the employer and the qualified parties evaluating the complete application.
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This article summarizes selected federal requirements and is not a site-specific compliance determination, engineering approval or legal opinion.
- OSHA 29 CFR 1910.21 — Scope and definitions
- OSHA 29 CFR 1910.25 — Stairways
- OSHA 29 CFR 1910.28 — Duty to have fall protection and falling object protection
- OSHA 29 CFR 1910.29 — Fall protection systems and falling object protection—criteria and practices
- OSHA interim enforcement guidance for handrail and stair rail systems