Ship Stairs and Alternating Tread Devices: When Code Allows Them

Many loft projects die on one question: how do you get up there without giving away half the room to a staircase? A conventional stair climbing a 9-foot rise needs roughly 10 to 14 feet of run, plus headroom clearance and landings. In a loft over a garage, a mezzanine nook, or a tiny-home sleeping platform, that footprint can swallow the whole point of the space.

Two code-recognized devices solve this: ship's ladders and alternating tread devices. Both climb far steeper than a normal stair and appear directly in the International Residential Code. This guide covers what each device is, when the IRC allows it, the required dimensions, and the math that shows how much floor space you save.

The Two Devices, Explained

A ship's ladder is a steep, fixed stair with conventional parallel treads, like a ladder given real steps instead of rungs, usually at 60 to 75 degrees from horizontal. In homes they typically serve attics or lofts where a full stair will not fit. Climbers use both handrails, but each step is a proper horizontal tread.

An alternating tread device is different. The IRC defines it as a device with a series of steps between 50 and 70 degrees from horizontal, attached to a center support rail in an alternating manner so that the user does not have both feet on the same level at the same time. Each paddle-shaped tread takes one foot; left and right treads alternate, shifting your body side to side, so every step lands on a full-depth surface.

Both serve secondary access, never a home's main circulation. That distinction is the heart of the code rules.

When the Code Allows Them

The IRC treats both as supplements to real stairs, not replacements. Under R311.7.11, alternating tread devices shall not be used as an element of a means of egress; they are allowed where a required egress stair or ramp already serves the same space at each level, or where egress is not required. R311.7.12 applies the same rule to ship's ladders.

One exception matters for loft designs: the IRC allows an alternating tread device to serve as an element of a means of egress for lofts, mezzanines, and similar areas of 200 gross square feet or less, as long as it is not the exclusive access to a kitchen or a bathroom. Ship's ladders get no equivalent exception, so treat them strictly as non-egress access: attic storage, a mechanical platform, a loft reached by choice.

One caution: the IRC is a model code adopted with local amendments. Some jurisdictions have modified these sections, and local sleeping-loft provisions can add rules. Check with your building official before you build.

The Dimension Rules You Must Meet

Alternating tread devices carry the most detailed dimensional rules of any stair-like device in the IRC (R311.7.11.1). Every item on this list must be satisfied:

• Tread depth: not less than 5 inches.
• Projected tread depth: not less than 8-1/2 inches.
• Tread width: not less than 7 inches.
• Riser height: not more than 9-1/2 inches, measured vertically between the leading edges of adjacent treads.
• Angle of ascent: between 50 and 70 degrees from horizontal.
• Clear width at and below the handrails: not less than 20 inches.
• The first tread must begin at the same elevation as the floor, landing, or platform it serves.

Handrails are required on both sides under R311.7.11.2, at 30 to 34 inches above the tread nosings, lower than the 34 to 38 inch band for normal stairs because your hands ride the rails continuously. Ship's ladders also need handrails on both sides under R311.7.12.2, with tread and riser dimensions under R311.7.12.1 and the same 20-inch clear width. Open risers are permitted as long as the gap does not pass a 4-inch sphere.

How Much Floor Space Each Device Really Saves

The horizontal run of any stair-like device is the total rise divided by the tangent of its angle. For a 9-foot rise (108 inches), a common loft height:

• Alternating tread device at 70 degrees: 108 / 2.75 = about 39 inches, or 3.3 feet
• Alternating tread device at 60 degrees: 108 / 1.73 = about 62 inches, or 5.2 feet
• Alternating tread device at 50 degrees: 108 / 1.19 = about 91 inches, or 7.6 feet
• Ship's ladder at 65 degrees: 108 / 2.14 = about 50 inches, or 4.2 feet
• Conventional stair at 32.5 degrees (7-inch rise, 11-inch run): 108 / 0.64 = about 170 inches, or 14.2 feet

That is the entire argument in one table. A 60-degree alternating tread device needs roughly 5 feet of run where a code-compliant stair needs 14. Sanity-check your geometry with a ship ladder calculator before committing to an angle. Steeper means less run but more two-handed, attentive climbing, so treat the angle as a comfort choice too.

Where They Work Best, and Where They Do Not

These devices shine for secondary access: lofts and mezzanines that already have a code-compliant stair elsewhere; sleeping lofts and attic conversions where a full stair would dominate the room; mechanical platforms, storage lofts, and roof hatches; and tiny homes or accessory dwelling units where every square foot matters, as long as local small-dwelling provisions are satisfied.

They do not belong as the only way up to a regularly used bedroom, especially in a rental or a home with children, elderly occupants, or anyone with mobility limits. They cannot carry large furniture or appliances. They suffer outdoors or in damp, unheated spaces, where treads get slippery and wood rots. And they are off the table wherever the local jurisdiction has amended these sections out of its code.

If the space is a primary living area, a full stair or a compact spiral stair is the honest answer, even when it costs more floor space. Steep devices are a compromise for secondary spaces.

Climbing Safely: What the Numbers Do Not Tell You

Code dimensions make a device legal; safe use still depends on the climber. Four habits separate daily confidence from a device everyone avoids.

First, learn the foot sequence. Each tread takes one foot, and the first tread meets the floor. Descend with a hand on each rail and no rushing.

Second, keep both handrails clear and light it like a real stair, top and bottom. Smooth socks on varnished treads at 65 degrees are a bad combination; add slip-resistant strips.

Third, plan the headroom. Measure vertically from the sloped nosing line, as on a conventional stair. Under a low joist this is a head-strike hazard, and 20 inches leaves no room to duck sideways.

Fourth, keep children and pets out of the plan. Treat either device like an open balcony: gated or locked when adults are not supervising.

Frequently Asked Questions

Can a ship's ladder be the main stair in a house?

No. Under the IRC a ship's ladder cannot be an element of a means of egress. It is allowed only where a required egress stair or ramp already serves the same space at each level, or where egress is not required.

How steep can I make a ship's ladder?

The IRC sets no maximum angle in R311.7.12; it is a design and comfort call. Most residential ship's ladders fall between 60 and 75 degrees. Past 75 the treads are too narrow to stand on, and you have a ladder with extra steps.

Do I need handrails on both sides?

Yes, for both. Alternating tread devices need handrails on both sides at 30 to 34 inches above the tread nosings; ship's ladders need them on both sides too. One-sided rails do not satisfy the code.

How do these compare to a spiral stair for saving space?

A spiral staircase needs about 5 feet of diameter for the IRC's 26-inch clear walking path, and it counts as a real stair with normal egress status. An alternating tread device at 60 degrees needs similar width but far less run depth. Choose the spiral when the space needs proper egress; the steep device when the space is secondary and the exception applies. Winder stairs offer a middle ground with a compact turning footprint.

The Bottom Line

Ship's ladders and alternating tread devices earn their place in one kind of project: reaching a small, secondary space where a full stair cannot fit. Answer the egress question first, because everything follows from it. If the space is a primary living area, a loft over 200 square feet, or the only route to a kitchen or bathroom, neither device qualifies. If the space is secondary, the exception applies, and your local official agrees, the rules are precise: 50 to 70 degrees, 9-1/2 inch maximum risers, 20 inches of clear width, handrails on both sides.

Measure twice, check the adopted code edition once, and treat the angle as a geometry decision and a daily-use decision. A well-planned steep device gets used with confidence; a poorly planned one gets quietly avoided, and an avoided access point was a wasted build.