Ceiling joists are the lateral beams within an attic space or between floors of a home to which the plaster or drywall of the ceiling is attached with metal screws. Within a room, the joists extend from one wall to the other. In most homes, ceiling joists are set either 40 or 60 cm (16 or 24 inches) apart. Knowing this can help to make an approximate judgment of where a specific ceiling joist is located. A simple device called a stud finder can then pinpoint the location of the joist in a ceiling.
Measure diagonally at a 45-degree angle, either 40 or 60 cm (16 or 24 inches) from the corner of the room. Tap around that point with the end of the screwdriver handle. If the sound is hollow, continue to tap around the area until it changes to a solid sound. This is the approximate location of a ceiling joist.
Turn on the stud finder and pass the device over the approximate area of the joist to pinpoint it in the ceiling. When the stud finder alerts that it has located a stud, move the device around that spot to determine the direction at which the stud lays.
Measure either 40 or 60 cm (16 or 24 inches), depending on the joist spacing, perpendicular to the line of the first joist to locate the next joist. Use the stud finder to pinpoint its location. Continue this process until you locate the joist you are looking for.