Joist

From Wikipedia, the free encyclopedia

A joist, in architecture and engineering, is one of the horizontal supporting members that run from wall to wall, wall to beam or beam to beam, to support a ceiling, roof (or floor). It may be made of wood, steel or concrete. Typically a beam is bigger than a joist and thus is distinguished from a joist. Joists will often be supported by beams. Joists support the sub-floor (floor deck) directly.

The wider the span between the supporting structures the thicker the joist will need to be if it is not to bend under load. There are approved formulas for calculating the thickness required, however, a rule of thumb for calculating the thickness of a floor joist for a residential property is; half the span in feet plus two inches. For example the joist thickness required for a 14 foot span is 9 inches.

Engineered wood products like an I-Joist gain strength from depth of the floor or the height of each joist. A common saying in this industry is deeper is cheape referring to the lower quality cost effective joist 14 inches and above.

Cf rafter.