Ship ladders are a hybrid. Part stair and part ladder, they make vertical circulation more comfortable in mechanical or service spaces. Ship ladders are generally preferred to normal ladders since they are more like stairs. They are typically installed in mechanical spaces or on rooftops, and they are also sometimes used to access roof hatches.
The dimensions of ship ladders vary based on available space so they are always custom fabricated. The angle of the ladder is usually 60°-70° and the design should use the shallowest angle possible within this range. Vertical rise between treads is generally around 9". The width of ship ladders is usually 24" and the tread depth is 6". Handrails are always provided. Of course, you should also check the OSHA codes as well as local building codes when designing a ship ladder.
OSHA Ship Stairs Requirements
OSHA provides ship ladders requirements (they call them ship stairs) in 29 CFR section 1910.25 (Stairways) under paragraph (e). OSHA is very clear that ship stairs should only be used where you can demonstrate that standard stairs will not work.
General Requirements: the general requirements of section 1910.25 must be met, including the need for guardrails, handrails, vertical clearance of 6'-8", uniform treads/risers, etc. Be sure you review all of the general requirements when designing a ship ladder.
Live Load: at least 5x the anticipated live load, but not less than 1,000 pounds point load.
Slope: 50 to 70 degrees from horizontal.
Risers: open risers with a vertical rise of 6.5" to 12". Note IBC code for I-3 spaces calls for maximum riser height of 9.5".
Tread: minimum depth of 4". Note that IBC code for I-3 spaces calls for 5" minimum.
Width: the clear width should be at least 18" wide.
International Building Code Ship's Ladder Requirements
According to the International Building Code 2018, ship ladders cannot be used as a means of egress (with one exception.) The exception, per IBC 2018 1011.15 is:
Ship's ladders are permitted to be used in Group I-3 as a component of a means of egress to and from control rooms or elevated facility observation stations not more than 250 square feet with not more than three occupants and for access to unoccupied roofs.
Per IBC 2018 (1011.15), ship's ladders serving Use Group I-3 must meet the following:
Handrails: must be provided on both sides.
Treads: minimum depth of 5". The total of the tread depth plus the nosing projection must be at least 8.5".
Risers: maximum height of 9.5".
Ship Ladder Dimensions Diagram
The following diagram is a recommendation that combines the IBC and OSHA requirements. This diagram is general in nature so it may not apply to every possible situation. Use your professional judgement when finalizing your design.