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A roof made with a concrete slab with concrete joists

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, and is thus distinguished from, a joist. Joists are often supported by beams and are usually repetitive.

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 and reducing the depth as needed; however, a rule of thumb for calculating the depth of a wooden 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. Many steel joist manufacturers supply load tables in order to allow designers to select the proper joist sizes for their projects.

Engineered wood products such as I-joists gain strength from the depth of the floor or the height of each joist. A common saying in the industry is that deeper is cheaper, referring to the lower-quality cost-effective joists 14 inches and above.

Bandsill is another term for joist used by construction workers and home inspectors in the southeast U.S.

See also


1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

JOIST, in building, one of a row or tier of beams set edgewise from one wall or partition to another and carrying the flooring boards on the upper edge and the laths of the ceiling on the lower. In double flooring there are three series of joists, binding, bridging, and ceiling joists. The binding joists are the real support of the floor, running from wall to wall, and carrying the bridging joists above and the ceiling joists below (see Carpentry), The Mid. Eng. form of the word was giste or gyste, and was adapted from O. Fr. giste, modern gite, a beam supporting the platform of a gun. By origin the word meant that on which anything lies or rests (gesir, to lie; Lat. jacere). The English word "gist," in such phrases as "the gist of the matter," the main or central point in an argument, is a doublet of joist. According to Skeat, the origin of this meaning is an O. Fr. proverbial expression, Je scay bien ou gist le lievre, I know well where the hare lies, i.e. I know the real point of the matter.


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