Alternating-Tread Stair
Staircase with alternately halved treads for minimal space.
In an alternating-tread stair, the treads are alternately shortened on the left and right so that only one foot uses the full tread surface at a time. This design permits a very steep stair configuration with minimal floor area. Alternating-tread stairs are only permitted as non-required stairs and serve as access to attics, mezzanines or as space-saving solutions. The best-known variant is the paddle stair.