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What Are the Different Types of Joist Construction?

Floor joists on new construction.
View of second story floor joists from below.
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  • Written By: BJ McCarley
  • Edited By: J.T. Gale
  • Images By: Gord Webster, Howard Sandler
  • Last Modified Date: 22 June 2014
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    Conjecture Corporation
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I-joists, laminate joists, and truss joists are some of the types of joist construction that typically are available. All are engineered lumber products that are exceptionally strong and have the ability to span greater distances than conventional lumber joists, such as two-by-tens or two-by-twelves. Each one usually works best for a specific joist construction project. For example, the I-joist typically works best for floor joist construction where the joist needs to span great distances over basements without using a post; the truss joist typically works best for roof construction where framing members need to span large rooms without placing a wall under it for support; and laminated joists can be used for either project. The laminated joist, however, is heavier than the rest, and plumbing and electrical elements cannot be installed through them; however, for strength purposes, the laminated joist is the strongest.

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The I-joist typically is the lightest joist of the joist construction group. It usually is constructed of half-inch (about 12.7 mm) Oriented Strand Board (OSB) and two-by-two inch (about 5.08 cm) pine strips. Basically, the OSB is cut into 14-inch-wide (about 35.6 cm) strips that can range from 24 to 36 feet (about 7.32 to 10.9 m) long with the strips placed on the top and bottom edge of the OSB. A final product will resemble the letter "I" if a person looks straight down the length of the joist. The entire joist is constructed using glue, which produces a joist that is as close to solid as possible.

Laminated joists are manufactured in various thickness, usually from 2 to 4 inches (about 5.08 to 10.16 cm), and range in lengths from 24 to 36 feet (about 7.32 to 10.9 m) long. The laminated joists are basically thin sheets of lumber products — usually plywood — that are no more than 1/4 inch (about 6.35 mm) thick and are sandwiched together to form a thick beam. Laminated joists are different from I-joists in that I-joists are not sandwiched together to form a thick beam.

Truss joists are constructed out of two-by-four lumber that is framed to form a 16- to 24-inch-wide (about 40.6 to 60.9 cm) rectangle. The center of the rectangle is cross-braced to add strength to the entire joist, and the framing is fastened together using special clamp fasteners that tie the joists into one unit. Truss joists will also allow for plumbing, electrical, and ductwork to run through to open spaces of the joist, as well.

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