Alternating Tread (Ship Ladder) Stairs: Where They’re Used

Alternating tread stairs — also called ship ladders — run at 56 to 70 degrees, roughly twice as steep as a standard 30–37 degree stair, and work by offsetting each tread so only half the tread width faces forward on any given step. That offset lets a foot land fully on a narrow tread that a conventional stair couldn’t fit at that angle, which is the entire point of the design: it fits a full stair-climbing motion into a footprint closer to a steep ladder.

How alternating tread geometry works

On a conventional stair, every tread is the same width and a person’s foot lands centered on it. On an alternating tread stair, treads split left-right in a repeating pattern — the right foot lands on a tread that only occupies the right two-thirds of the stair width, the left foot on the next tread up occupying the left two-thirds, and so on. Because each foot only needs to clear a narrow strip rather than the full stair width, tread depth in the direction of travel can be as little as 7–8 in (180–200 mm) instead of the 10 in (254 mm) minimum a standard stair needs, while riser height climbs to 9.5–12 in (240–305 mm) per step.

That combination compresses total run dramatically. A conventional stair covering a 9-ft (2,745 mm) rise at 37 degrees needs roughly 12 ft (3,660 mm) of horizontal run; an alternating tread stair covering the same rise at 60 degrees needs closer to 5 ft (1,525 mm). Run the specific angle and rise through the Stair Angle/Pitch Calculator to confirm riser and tread numbers before fabrication — the offset geometry means standard rise/run formulas from a conventional stair don’t transfer directly.

Where alternating tread stairs are actually permitted

Alternating tread devices are explicitly addressed in IBC 1011.14 and IRC R311.7.10.1, and both codes restrict where they can be used rather than treating them as a general-purpose stair type. They’re accepted as a secondary means of access to spaces like mechanical mezzanines, rooftop equipment, and lofts — but in most jurisdictions they are not permitted as a required means of egress for occupied spaces, meaning a room that needs an alternating tread stair to exit also needs a second, conventional means of exit.

Application	Typically permitted?	Governing code reference	Notes
Mechanical/rooftop equipment access	Yes	IBC 1011.14	Common use case — non-occupied, infrequent access
Loft or mezzanine in a single-family home	Often, with restrictions	IRC R311.7.10.1	Usually capped at a defined floor area and occupant load
Required egress stair	No, in most jurisdictions	IBC 1011.14 exceptions	Needs a conventional second exit
Marine or industrial access (origin of the name)	Yes, under different standards	OSHA / maritime codes	Not governed by IRC/IBC residential provisions

Because acceptance varies by jurisdiction and by what the space is used for, confirm local amendments before designing around one — a mezzanine access stair that’s fine in one county can require a full conventional stair two counties over. Cross-check the rise and angle against the IRC Stair Code Checker before committing to the layout.

Common mistakes

Installing one as the only access to a habitable room. This is the most common code violation with alternating tread stairs — they’re frequently installed to save space to a finished loft bedroom or office, which then fails inspection because habitable space needs a compliant means of egress, not just a means of access.

Getting the handrail height wrong for the steeper angle. Standard handrail height (34–38 in / 865–965 mm measured vertically from the nosing) still applies, but because the stair is so much steeper, a rail sized by eye rather than measured from each individual nosing often ends up too low relative to how a person’s hand naturally falls on a near-ladder-angle climb. Confirm with the Handrail Height Calculator rather than matching it visually to a conventional stair nearby.

Undersizing tread depth on the offset side. Because only part of each tread is usable per step, some fabricators shrink the entire tread to save material — but the usable portion still needs to meet the minimum the device standard requires (typically 8.5 in / 216 mm at the walk line for alternating tread devices under IBC 1011.14.2), not just look proportionally similar to a smaller version of a normal stair.

Related calculators you might need

Alternating tread stairs and attic ladders solve overlapping problems — if the application is specifically attic or loft access rather than mezzanine equipment access, compare against the Attic/Loft Ladder Calculator, which covers folding and disappearing options that don’t require the offset-tread fabrication. For the egress question specifically, run the space’s occupant load through the Egress Stair Width Calculator to confirm whether a second conventional exit is required alongside the alternating tread device. Once the angle is fixed, the Stair Angle/Pitch Calculator and Stair Nosing Calculator refine the individual tread dimensions.

Frequently asked questions

What is a ship ladder stair used for?

It’s used anywhere a conventional stair won’t fit but a straight vertical ladder is too steep or unsafe for regular use — rooftop mechanical access, mezzanines, lofts, and originally shipboard access, which is where the name comes from. The offset-tread design lets it climb at 56–70 degrees while still giving each foot a full tread to land on.

Are alternating tread stairs legal in a house?

Generally yes for access to a non-habitable space like storage loft or mechanical access, under IRC R311.7.10.1, but not as the sole means of egress from a habitable room in most jurisdictions. Always confirm with the local building department, since acceptance varies more on this stair type than almost any other.

How steep is an alternating tread stair?

Typically 56 to 70 degrees from horizontal, compared to 30–37 degrees for a standard residential stair. That’s close to the angle of a steep ladder, which is why the offset tread pattern exists — a conventional flat tread at that angle wouldn’t be wide enough for a full foot placement.

Can alternating tread stairs be used as a fire escape?

Rarely — most fire and building codes require a required egress stair to be a conventional configuration, and alternating tread devices are specifically excluded from that role in IBC 1011.14 except in narrow, defined exceptions. Treat them as secondary access only unless a code official has explicitly approved otherwise for the specific space.

What’s the difference between an alternating tread stair and a spiral staircase for tight spaces?

An alternating tread stair is steeper and takes even less footprint than a spiral stair of the same rise, but it’s harder to carry objects up and generally excluded from egress use — a spiral stair, sized with the Spiral Staircase Calculator, is more often accepted as a habitable-space solution because it behaves closer to a conventional stair.