Joists carry the structural load of floors and ceilings. The ends of the joists are held up by support beams, foundations, or load-bearing walls. Rooms used as living areas must be supported by floor joists that are at least 2 × 8 in size. Floors with smaller joists can be reinforced with "sister" joists.

Floors with 2 × 6 joists, like those sometimes found in attics, cannot support living areas unless "sister" joists are attached alongside each original joist to strengthen it. This often is necessary when an attic is converted to a living area. Sister joists also are used to help support a header when ceiling joists must be cut, such as when framing a skylight shaft.