In most cases there are three primary types of joist construction, namely I-joists, laminate joists, and truss joints. 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. They’re often somewhat interchangeable, though one usually works best for a specific joist construction project than others and most projects feature various types in different places. For example, the I-joist is normally most common for floor construction where the joist needs to span great distances over basements or enclosed crawl spaces without using a post, and the truss joist typically works best for roof construction where framing members need to span large rooms without placing a wall underneath for support. Laminate joists can normally be used more broadly, but since these tend to be heavier than the rest, plumbing and electrical elements cannot always be installed through them. For strength purposes, though, they are widely regarded as some of the best.
Understanding Joists Generally
Put simply, a joist is a construction feature that lends support to a building or other structure. It’s usually made of two beams that are attached together, sometimes with a hanger or bolt but, more traditionally, with notching. Once attached, the beams together are able to bear weight and distribute loads more efficiently and in a more stable way. Joists are regularly used in framing, and offer stability that can either be plastered over or further reinforced with bigger supporting beams. Most joists take on a shape that resembles an “L” or a “V” once constructed.
Choosing the best construction for a given scenario is often a matter of position and intended use. Different lumber companies and manufacturers sometimes have specific names and trademarked terms for brand-specific joist products, but in general all of the options fall into one of three main types.
The I-joist typically is the lightest joist. 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, which is how this method gets its name. In most cases the entire joist is constructed using glue, which produces a joist that is as close to solid as possible.
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 normally allow for plumbing, electrical, and ductwork to run through to open spaces of the joist, which makes them particularly popular when used in ceiling framing. The unfinished ceilings and sidings of many attics feature this sort of joisting, for instance.
Laminated joists are manufactured in various thickness, usually from 2 to 4 inches (about 5.08 to 10.16 cm), and can range in length anywhere from 24 to 36 feet (about 7.32 to 10.9 m) long. This style of joisting is 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. This is often thought to produce density as well as insulation; the approach mimics that of the I-Joist but normally uses less expensive materials. Laminates can be used broadly and their thickness can sometimes be altered to meet specific construction demands.