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Building elements

Floor joist

A floor joist is one of the parallel horizontal beams that carry a floor and span between walls or beams.

Also known as joist, joists, floor joists, timber joist, I-joist

Joists are laid side by side at regular centres, commonly 400 or 600 mm, and together form the floor structure. The floor deck is fixed on top and the ceiling below, and the space between the joists carries insulation, pipes and cables.

Joists can be solid sawn timber, glulam or engineered I-joists with timber flanges and a board web. Their depth and spacing are set by the span and the load, and by the stiffness needed to keep the floor from feeling bouncy.

Related terms
Floor cassetteA floor cassette is a prefabricated floor section: joists with sheeting and often insulation and services. It is built off site and craned into place as a unit. Tongue and grooveTongue and groove is an edge profile where a ridge on one board fits into a slot on the next, locking boards together into a continuous surface. GlulamGlulam (glued laminated timber) is a structural timber product made by gluing layers of timber together with the grain running in the same direction.
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