At Home: A Short History of Private Life

by Bill Bryson

Stairs incorporate three pieces of geometry: rise, going, and pitch. The rise is the height between steps, the going is the step itself (technically, the distance between the leading edges, or nosings, of two successive steps measured horizontally), and the pitch is the overall steepness of the stairway. Humans have a fairly narrow tolerance for differing pitches. Anything more than 45 degrees is uncomfortably taxing to walk up, and anything less than 27 degrees is tediously slow. It is surprisingly hard to walk on steps that don’t have much pitch, so our zone of comfort is a small one. An inescapable problem with stairs is that they have to convey people safely in both directions, whereas the mechanics of locomotion require different postures in each direction. (You lean into the stairs when climbing, but hold your center of gravity back in descent, as if applying a brake.) So stairs that are safe and comfortable in the ascent may not be so good for going down, and vice versa. How far the nosing projects outward from the tread, for one thing, can materially affect the likelihood of a mishap. In a perfect world, stairs would change shape slightly depending on whether a user was going up or down them. In practice, every staircase is a compromise.