Measuring before you calculate
These devices are almost always shoehorned into a fixed opening, so the space you have — not just the height you need to climb — drives the design.
Find your total rise
Measure plumb from the lower finished floor to the upper finished floor, landing, roof deck, or platform surface — the full vertical distance the device has to cover.
Measure the floor opening
Unlike a normal stair, these devices are usually built to fit a fixed hatch, mezzanine cutout, or wall recess. Measure the horizontal space available at the base and enter it as Available Floor Opening to check the fit.
Pick the device type and the code you're building to
Alternating tread devices have offset zig-zag treads and are climbed face-forward; ship's ladders have full-width open-riser treads and are often descended backward like a ladder. Then choose IBC (occupied buildings) or OSHA (workplace access) in Advanced — the two use different minimums.
Read both tread-depth numbers
Physical Tread Depth and Projected Depth are two different checks. A tread that's deep enough to stand on can still fail the projected/foot-clearance minimum if the nosing overlap between treads is too small — the compliance chips flag this separately.
The five calculations behind every result
Nothing here is a black box. This is exactly what runs when you change a number or switch device type / code standard.
Step Count & Riser Height
R = Total Rise ÷ n
Same rounding logic as a normal stair — you can't build a fractional step, so the target riser is only a starting point; the real, uniform riser is the total rise re-divided by the whole step count.
Physical Tread Depth & Total Run
The physical tread — where a foot actually lands — sets the horizontal advance per step. This is deliberately not the same number as the projected depth below; conflating the two is the single most common design error on these devices.
Projected Tread Depth
A separate foot-clearance check: the physical tread plus how far it overlaps the tread below. IBC requires this combined figure to reach 8.5" even when the physical tread itself is allowed to be as shallow as 5".
Angle of Ascent & Support Length
Angle from the riser-to-tread ratio, same trig as any stair — just steeper. Support length is the hypotenuse formed by total rise and total run, the board length a center stringer or side rail is cut from before notching.
Threshold Selection
Every compliance chip on this page pulls its pass/fail threshold from a lookup keyed on your Device Type and Code Standard choice — IBC and OSHA genuinely use different numbers for the same dimension, detailed in the code reference table below.
Walking through a 96" loft access rise
Same five calculations, filled in for an alternating tread device climbing 8 feet (96") to a sleeping loft, checked against IBC — the default this calculator loads with.
Step count
96" total rise, targeting a 9" riser: 96 ÷ 9 = 10.67, which rounds to 11 steps. The calculator always rounds to a whole step count — you can't cut a fractional step.
Actual riser height
Re-divide total rise by the whole step count: 96 ÷ 11 = 8.73" per riser — comfortably under the 9.5" IBC maximum for both device types.
Physical tread depth, run & angle
Auto mode uses the IBC physical-tread minimum for an alternating tread device: 5". That gives a total run of 10 treads × 5" = 50", and an angle of atan(8.73 ÷ 5) ≈ 60.2° — solidly inside the 50–70° range required by both IBC and OSHA.
Where it fails: projected depth
With the default 2.5" nosing overlap, projected depth is only 5 + 2.5 = 7.5" — short of the 8.5" IBC minimum. Passing the physical-tread check does not automatically satisfy the separate foot-clearance check. Fixing this means increasing nosing overlap to at least 3.5", not increasing the tread itself.
Support length
Stringer length is √(96² + 50²) ≈ 108.2" — the straight-line board length a center stringer needs before notching for treads.
This is exactly why the Projected Depth chip can fail even when riser, physical tread, and angle all pass individually — the physical-depth minimum and the projected-depth minimum are two separate IBC requirements, and a design that clears one by using minimum tread depth can miss the other.
IBC vs. OSHA, side by side
This calculator checks your result against whichever column you select in Advanced. The two codes serve different purposes — IBC governs devices built into occupied buildings, OSHA governs workplace access — and they genuinely disagree on several numbers.
| Dimension | IBC — Alternating Tread §1011.14.2 | IBC — Ship's Ladder §1011.15.2 | OSHA — Alternating Tread §1910.25(f) | OSHA — Ship Stairs §1910.25(e) |
|---|---|---|---|---|
| Angle of ascent | 50°–70° | 50°–70° | 50°–70° | 50°–70° |
| Riser height | 9.5" max | 9.5" max | not specified | 6.5"–12" rise between treads |
| Physical tread depth | 5" min | 5" min | 8.5" min | 4" min |
| Projected / nosing depth | 8.5" min (incl. nosing) | 8.5" min (incl. nosing) | not separately specified | not separately specified |
| Tread / clear width | 7" min per tread | 20" min clear width | 17"–24" between handrails | 18" min tread width |
| Open risers | not required | not required | required if tread < 9.5" | required |
| Headroom / vertical clearance | 80" min | 80" min | 80" min (general, §1910.25(b)(2)) | 80" min (general, §1910.25(b)(2)) |
| Handrail height | 30"–34" | 30"–34" | per §1910.28 [VERIFY: exact OSHA workplace handrail/guardrail height figure for stairs under 29 CFR 1910.28, not stated numerically in 1910.25] | per §1910.28 [VERIFY: exact OSHA workplace handrail/guardrail height figure for stairs under 29 CFR 1910.28, not stated numerically in 1910.25] |
Reference only — always confirm against your local jurisdiction's adopted code edition, or your workplace's governing OSHA general-industry standard, before building.
Alternating tread device vs. ship's ladder
Both trade comfort for footprint. The difference is in the tread itself, and it changes how the device is actually climbed.
| Feature | Alternating Tread Device | Ship's Ladder |
|---|---|---|
| Tread layout | Offset zig-zag — each tread serves one foot, alternating left/right | Full-width tread — both feet can land on the same step |
| Typical descent | Face-forward | Often backward, ladder-style |
| Support structure | Single center stringer, common | Two side stringers/rails, common |
| Best for | Frequent access where face-forward visibility matters — loft bedrooms, mezzanine offices | Infrequent/maintenance access — roof hatches, equipment platforms, tank tops |
| Typical IBC egress use | Mezzanine ≤250 sq ft, ≤5 occupants | Group I-3 control room ≤250 sq ft, ≤3 occupants; unoccupied roof access |
Egress limits, floor openings & load
A device can pass every riser, tread and angle check above and still be the wrong choice for the space — these four things decide whether it's even permitted.
Egress restrictions
Both device types are limited exceptions, not general-purpose stairs. IBC allows an alternating tread device from a mezzanine ≤250 sq ft serving ≤5 occupants, and a ship's ladder from a Group I-3 control room ≤250 sq ft serving ≤3 occupants, or for unoccupied roof access. IRC separately allows either device for a loft or mezzanine ≤200 sq ft that doesn't provide the sole access to a kitchen or bathroom.
Roof hatch / floor opening sizing
A steeper device needs a shorter — but still code-minimum-headroom — opening at the top. As a reference point, a 56° alternating tread device typically needs roughly a 2'6" × 6'0" hatch to clear 80" of headroom, while a 68° device typically needs only about 2'6" × 4'6" for the same clearance. Confirm your specific opening against the manufacturer's layout or your local AHJ — this isn't a fixed formula this calculator computes live.
Descent orientation & handrails
Because the tread surface is narrow, a ship's ladder is often descended backward, facing the treads, the same way OSHA treats a fixed ladder. An alternating tread device's full-width-per-side tread is usually climbed face-forward instead. Either way, both device types require continuous handrails on both sides — a single handrail, standard on a normal stair, isn't sufficient here.
Load rating
For workplace installations, OSHA 1910.25(b)(6) requires the structure to support at least five times the normal anticipated live load, and never less than a 1,000 lb concentrated load at any point. This is a structural sizing question for the stringer/support material and fasteners, separate from the geometry this calculator checks.
Common mistakes when planning one of these devices
Most failed inspections on alternating tread devices and ship's ladders trace back to one of these six planning errors.
Conflating physical and projected tread depth
A 5" physical tread satisfies the IBC minimum tread depth on its own, but the separate 8.5" projected-depth (foot clearance) check depends on nosing overlap, not tread size — see the worked example above.
Assuming it can be the main stairs
Both device types are narrow, code-limited exceptions to a standard means of egress, not a substitute for one. Confirm your occupancy, area and occupant-count fall inside the specific exception before designing around it.
Mixing IBC and OSHA numbers
A residential loft ladder built to IBC minimums and a workplace platform access built to OSHA minimums use genuinely different tread and width figures for the same-looking device — pick the standard that actually governs your project.
Sizing the headroom but not the opening
An 80" headroom check confirms vertical clearance at one point. It doesn't confirm the hatch, mezzanine cutout, or wall recess is long enough at the actual angle you're building — check both.
Installing a handrail on one side only
Standard stairs often get by with a single handrail. Both alternating tread devices and ship's ladders require continuous handrails on both sides because of how steep and narrow the tread surface is.
Undersizing the support for the actual load
Stringer/support length from this calculator is a straight-line geometric figure. Workplace installations still need the structure sized to the 5×-live-load / 1,000 lb concentrated minimum from OSHA 1910.25(b)(6), which this page doesn't compute.
When to use an alternating tread device or ship's ladder
These are space-saving tools first — reach for one only once a standard stair genuinely doesn't fit.
Sleeping Loft Access
Compact, face-forward access to a bedroom loft in a tiny home or cabin.
Mezzanine Office Access
Reaching a small mezzanine work area without giving up floor space to a full stair.
Roof Hatch / Equipment Access
Infrequent maintenance access to rooftop mechanical units or unoccupied roofs.
Marine & Vessel Access
Engine room, deck and control-room access aboard ships, matching the OSHA ship-stair standard directly.
Industrial Platform Access
Tank tops, catwalks and elevated observation stations where a standard stair isn't feasible.
Attic / Storage Loft Access
Occasional-use access to overhead storage where floor space below is at a premium.