Where can I legally use a ship's ladder instead of a normal stair?

Model codes generally permit alternating tread devices for secondary access to limited spaces such as lofts, mezzanines, equipment platforms, attics, and roof hatches. They are not allowed as the primary means of egress or as the main stair to habitable rooms. Because local amendments vary widely, confirm your specific use with the building official before designing around one.

What angle should a ship's ladder be?

Most ship's ladders are designed between 60 and 70 degrees from horizontal. Below 60 degrees you lose the space-saving advantage that justifies the device, and above 70 degrees it climbs so steeply that most people find it uncomfortable and unsafe for regular use. Pick your angle from the available footprint, then let the calculator solve the tread layout.

How do alternating treads actually work?

Each tread is only half the device's width, and treads alternate sides as you climb: left foot, right foot, left foot. Because you only ever place one foot per tread, each tread can be deep enough for a full footfall even though the overall run is very short. The side-to-side offset between successive treads is what makes the steep angle walkable.

Do ship's ladders need handrails?

Yes. At 60 to 70 degrees a continuous graspable handrail on both sides is essential, and model code provisions for alternating tread devices address handrails specifically. Rails should run the full length of the device at a consistent height so a climber always has something solid to hold, especially when descending facing the ladder.

Can a ship's ladder serve as the main stair to a bedroom loft?

In most jurisdictions, no. Sleeping lofts and bedrooms are habitable spaces that require compliant stairs or other approved egress, and alternating tread devices are restricted to secondary access. Some tiny-house and loft-specific provisions exist in certain codes, but you must verify with your local building department rather than assuming the IBC allowance covers your project.

How much floor space does a ship's ladder save compared to a normal stair?

A conventional stair at a comfortable angle needs roughly 10 to 14 feet of horizontal run for a 9-foot story. A ship's ladder at 65 to 70 degrees can cover the same rise in about 3 to 4 feet of run. That is the entire point of the device, but the tradeoff is a much steeper, more demanding climb that suits occasional access, not daily living.

Are ship's ladders safe for children or older adults?

Generally they are a poor fit. Ship's ladders require good balance, leg strength, and the coordination to alternate feet correctly, and descending facing the device is unfamiliar to most people. For households with children, older adults, or anyone with mobility limits, a compliant stair, even a steep code-minimum one, is the safer choice.

Ship Ladder Calculator

What Is the Ship Ladder Calculator?

A ship's ladder, also called an alternating tread stair, is the steepest stair-like device building codes allow, and this calculator helps you lay one out correctly. Instead of full-width treads, a ship's ladder uses alternating half-treads: you step with your left foot on one set of treads and your right foot on the other, so each foot lands on a deep, flat surface even though the overall device climbs at 60 to 70 degrees. That geometry lets a full flight fit into a footprint where a conventional stair would never work, which is why ship's ladders show up in lofts, mezzanines, tiny homes, sleeping lofts, roof hatches, and equipment platforms. This calculator takes your total rise and available floor space and returns a workable layout: the stair angle, the number of alternating treads, tread depth and offset, and handrail guidance. One honest caveat up front: most jurisdictions restrict ship's ladders to secondary access for non-habitable spaces, not primary egress from bedrooms or living areas, so always confirm with your local building official before you build.

How to Use This Calculator

Measure the total rise from the finished lower floor to the finished upper floor, exactly as you would for a normal stair.
Measure the horizontal footprint you can spare, from the face of the upper opening to where the bottom tread will land.
Enter your target angle. Most ship's ladders land between 60 and 70 degrees; steeper than 70 degrees starts to feel like a ladder.
Set the tread depth and the alternating tread offset so each foot gets a full, flat landing surface on its own side.
Review the calculated tread count per side, the rise per tread, and the projected handrail line on both sides.
Check the headroom and landing clearance at the top, where the upper opening or hatch must clear a climbing person.
Confirm that your planned use, such as loft or mezzanine access, is one your local code actually permits for alternating tread devices.
Export the results as a PDF to keep with your permit paperwork or to show an inspector.

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