Joist

The Full Picture

Joists run in parallel, typically at a fixed spacing such as 12, 16 or 24 inches on center, and carry the deck or subfloor above and a ceiling below. They bear on beams, girders, walls or foundation ledges. Wood joists include solid sawn dimensional lumber and engineered wood joists, such as I-joists. Steel joists are generally open web, like a small truss, and are covered in the open web joist entry. Joist spans use the clear span, which AWC describes as the length from support to support.

For wood, the layout drives the count. The number of joists is the length of the run divided by the spacing, plus one for the end. Extras are added for doubled joists under parallel walls, around stair and floor openings, and at cantilevers, along with rim or band joists at the ends, blocking and bridging. The plan and the framing details show spacing, size and these extra pieces, and the structural notes give the species and grade. Lumber is priced by the piece, by linear foot or by MBF using nominal sizes, so a 2x10 counts as 2 x 10 / 12 = 1.667 BF per LF.

For steel joists the unit is each, listed by mark or designation on the structural plan or joist schedule, with the span and the depth. Suppliers often price joists by weight, and bridging and accessories are separate lines. Erection brings code requirements: OSHA states that for spans at or above the thresholds in its tables, a row of bolted diagonal erection bridging is installed near midspan, and the hoisting lines stay attached until that bridging is installed. Erection of joists is therefore tied to the steel erector's scope.

Pitfalls include missing the doubles and rim joists, mixing nominal and actual sizes, and omitting the bridging, blocking or hangers. Joists in floor systems also have depth that decides ceiling heights, so a change in joist depth during value engineering can ripple into the mechanical and finish scopes.