The ceiling beam, or ceiling joist, has two prime functions. The first is to tie the walls of a structure or a room together, effectively completing a box configuration. The second is, as the name implies, to support the ceiling of the structure, or room. Ceiling joist framing is undertaken after the walls are erected.

Ceiling joists are a number of horizontal beams, running parallel from one wall to the opposite wall of the structure. The beams are attached to the top of the wall at what’s known as the wall plate. A ceiling joist at the top floor of a structure may also support a flat roof, as well as the ceiling. If the roof of the structure is sloped, or pitched, each ceiling joist support is attached to a roof rafter, the angled roof support beams. This joist-and-rafter configuration prevents the sloped roof construction from pushing the walls of the structure outward.

Roof trusses are triangular forms used in construction of sloped roofs, and basically consist of rafters and ceiling joist pre-constructed as a unit. Obviously, this saves the extra labor, and expense of erecting each rafter and joist individually. The triangular shape of the truss has long been proven the strongest roofing configuration as far as structural load-bearing capability is concerned.

Typical ceiling joists used in home construction are made of wood, and often called stud ceiling joists. There are, however, metal ceiling joists, usually manufactured from steel or iron, for use in larger, sturdier structures, such as manufacturing plants, or skyscrapers. Ceiling joist sizes and construction are determined by the load the joist is to carry. In most cases, joists, like framed walls, are spaced 16-inches (40.64 cm) apart, or on center.

Ordinarily, the ceiling joist will run in the same direction as the roof rafters. There are, of course, circumstances where parallel construction of joists and roof rafters is not feasible or practical, either due to the design of a structure, or for reasons of expense or aesthetics. In such cases, the joists will run at a 90 degree angle to the rafters, and ties are used, in a manner similar to the parallel joist construction, to strengthen the room or building by binding the tops of the walls.

Ceiling joists are very often not long enough to span an entire structure. In such cases, the joists must be overlapped by some four to six inches (10.16 cm to 15.24 cm), at the point of interior support, such as an interior wall or column. Additionally, ceiling joists may be attached to the roof rafters directly, or butted against the wall plate using hangers firmly attached to both joist and wall plate.