Alternating tread stairs, also known as ‘ship’s stairs’ or sometimes ‘ship’s ladders’ are a special form of stair designed to halve the horizontal run space needed for a staircase without reducing the tread width. They are not usually used for long runs of stairs and are more commonly found for accessing lofts, decks, or attic spaces. They are also not usually more than 1m wide. Building codes may limit where these can be used.
This form of stairs is designed with alternating series of treads for each foot, offset by the usual step height but with double the step height in the individual tread run and a slope twice as steep as a usual staircase. The same staircase structures described for the standard stairs are used, but with double the slope and the addition of a third central stringer or with each run of treads on a separate supporting stringer beam with a divider plate between the treads.