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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, and are usually repetitive. Joists support the sub-floor (floor deck) directly.
The wider the span between the supporting structures the deeper the joist will need to be if it is not to deflect under load. Lateral support also increases its strength .There are approved formulas for calculating the depth required, however, a rule of thumb for calculating the depth of a floor joist for a residential property is; half the span in feet plus two inches. For example the joist depth 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 cheaper'' referring to the lower quality cost effective joist 14 inches and above.
In the way that the term beam is used, the more proper term is girder. In common language beam is used by carpenters while iron workers refer to a girder.

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See also

See also

★ rafter

★ purlin

★ girder