Stairs may be classified into two basic structural types: continuously supported and freespanning.
Continuously supported stairs—Continuously supported stairs are commonly used as interior stairs. Both sides of the stairway are supported by wall framing, so calculations of spanning capacities are not necessary. These stairs are site-built in some regions, but are predominantly prefabricated in others.
Freespanning stairs—Freespanning stairs have the structural capacity to span from the bottom stair to the top stair without intermediate support. The freespanning stair is commonly used as an exterior stair between floors or landing levels or in conjunction with porches and decks. It is often also seen as an access stair to basements and attics. The strength of a freespanning stair is usually in the carriages (stringers) that support the treads, although the handrail may also contribute to the strength of the stair. Freespanning stairs, like continuously supported stairs, may be site-built or prefabricated. Some freespanning stairs have only a single central support.