Ceiling joists, or ceiling beams, are wooden members with two prime functions. They connect the walls of a room together, completing the "box" of a building, and they support the ceiling of the room. Ceiling joists shouldn't be confused with floor joists, which also support the first-level ceiling. The only place ceiling joists are installed is where there is no living space above. Ceiling joists are installed after the walls are erect.
Calculating Size and Installation Specs
The size and spacing of the ceiling joist members are based upon the span, or length, from wall to wall and the weight of the ceiling finish. The ceiling will be drywall, plaster or other fire-rated assembly. A qualified and licensed design professional--architect or engineer--should be contracted to calculate the size and installation specifics of the ceiling joists based upon span and ceiling type, along with bridging specifications and attachment details. Calculation of size of ceiling joists takes into consideration the structural design, load vs. span calculations, attachment specifications and building code requirements. Ceiling joists in small span areas, like a closet, can be as small as 2-by-6 dimensional lumber, spaced 2 feet apart. In long span areas, ceiling joists may be 2-by-12 dimensional lumber, spaced 12 inches apart. A design professional may also specify engineered lumber, like I-joists, which are wooden I-beams assembled by a manufacturer. With all of these variables, designing and selecting materials for ceiling joists without professional qualifications can lead to dangerous failures in the installed ceiling joist system.
Installing Ceiling Joists
Ceiling joists are installed on the top of the wall at each side of the room. Ceiling joists must sit with full bearing atop each wall. If the wall has a 2-by-4 top member, the ceiling joist must sit fully on top of it and bear 3 1/2 inches on the same (a 2-by-4 member is 3 5/8 inches wide). Likewise, with a 2-by-6 wall, the ceiling joist must sit totally on top of it, at 5 1/2 inches. Ceiling joists are toenailed to the top wall member, with 16-penny nails, or manufactured metal accessory wall ties as designed. Ceiling joists are not nailed to the wall members. If the roof and attic construction is above the ceiling joist installation, the ceiling joists must run parallel to the rafter construction and be attached to the roof rafters and wall top to create a tie condition.
Stiffening Ceiling Joists
Solid bridging is recommended to stiffen ceiling joists. These are solid wooden blocks placed and nailed between the ceiling joists in a straight line, continuous from wall to wall. Most installations call for bridging to be made of the same size lumber as the ceiling joist members and spaced at a maximum of 8 feet on centre. The bridging can be placed closer together if specified by a design professional. Bridging transfers weight from one joist to another to aid in deflection and sagging. Bridging also helps to avoid drywall cracking due to deflection.