Ceiling joists provide support for more than one surface in your home. The first surface is, unsurprisingly, the ceiling, the materials of which attach directly to the joists. The other surfaces are the walls, which the joists span the distance between, providing a source of support to keep the walls from shifting.
Standard Size of Joists
Though there is no standard joist size for the ceilings in all buildings, most average homes use ceiling joists in a board size of 2-by-6 inches. The range of joist sizes used in homes varies though, between 2-by-4 inches and 2-by-12 inches. The difference in the joist sizes applies to the width of the boards, which determines the height of the ceiling. A 2-by-4-inch joist, for instance, hangs into the room by 4 inches, which may leave the ceiling high, while a 2-by-12-inch joist in the same position hangs 12 inches into the room, which decreases the ceiling height by 8 inches.
Length of Joists
For a joist to have the most strength, it should stretch all the way across a room from the top of one wall to the top of the opposite wall. This means that the size of the room determines the necessary length of ceiling joists. Since this is the case, ceiling joists have no standard length. Instead, the length of the ceiling joists varies from room to room, depending on each room's measurements.
Regardless of the size or length of joists, they do retain a standard spacing of 16 inches. The ability of the joists to support weight depends upon this spacing, and 16 inches provides sufficient support for most home ceiling types. If you have a room that needs to hold a particularly heavy type of ceiling, such as marble, joists may need to be spaced more closely together, such as in 12-inch increments instead of 16-inch.
Once joists are in place, you can install numerous types of ceiling materials, though in most instances you must lay plywood across the joists to create a flat surface over which to install the ceiling materials. You also have the option of leaving ceiling joists exposed and finishing them. This mimics the exposed beam look and can be accomplished more effectively by using joists thicker than 2 inches. The danger of leaving joists exposed and applying no finished ceiling is that you have no secondary source of coverage. So, if the roof springs a leak, it's going to come right into your home.