A truss joist is basically a rectangle made from two-by-four lumber that is fastened together with metal strap fasteners. The framing members of the truss joist are attached together so that the frame of the truss is 3.5 inches (about 8.89 cm) thick, which is the thickness of a standard two-by-four. After the basic rectangle of the joist has been built, cross bracing is then installed inside the rectangle for extra support.
Truss joist technology has allowed the building industry to generally overcome a few problems. When framing any roof or floor, the main issue is whether the framing will span distances greater than 16 feet (about 4.88 m) without sagging. Also taken into consideration is whether the structural framing for a roof or floor will support the weight of walls or live loads, such as snow or rain. A truss joist can span distances as great as 24 to 36 feet (about 7.3 to 10.9 m) without the support of bearing walls or posts. Typical truss joist widths may range from 16 to 24 inches (about 40.6 to 60.9 cm) wide, depending on the structural strength needed by the builder.
Mainly, truss joists serve as a means for structural support for floors and roofs; however, since the joist is not solid and is framed from lumber, wiring, plumbing, and ductwork can be installed through the space in the trusses without the need to cut the joists. Since the trusses are so wide, the ceiling or floor space that is created by theses framing members generally is wide enough to insulate heavier than a space created by conventional joists framing, which usually are two-by-tens or two-by-twelves that are 9 to 12 inches (about 22.9 to 30.5 cm) wide.
Because truss joists are prefabricated and built using special fasteners, they are able to withstand extraordinary amounts of weight, which allows for them to be spaced up to 24 inches (about 60.9 cm) apart rather than the more conventional 12 to 16 inches (about 22.9 to 40.6 cm). The trusses typically come from the factory with notches in the ends so that they will fit over the sill plate of a foundation; however, the ends of the truss joists typically need to be closed in using 3/4-inch (about 1.9 cm) plywood. Addition of plywood usually is necessary to tie the joists together to the floor framing, as well as provide a sealed truss system that will be airtight. Manufacturers of truss joists may, however, recommend that a builder use two sheets of 3/4-inch (about 1.9 cm) plywood instead of one.