Steps are either open or solid. Their horizontal surface is known as a “tread,” and the vertical as a ”riser.” External steps generally have a low riser of 4 or 5 in compared with 6 in for steps indoors, and the tread should be determined by the following equation:
2R + T = 22–30 in
(where R is riser and T is tread).
For example, a riser of 4 in gives a permissible tread of between 14 and 22 in (8 + 14 = 22 in; 8 + 22 = 30 in). High risers, of 6 in or higher, are exhausting to walk up if there are many of them, but serve well as sitting steps.
Another key to good design is to always group steps in a minimum of three risers. An isolated single riser may not be noticed (road curbs are the exception). Long runs of steps should be broken by ...