Late and plaster walls and ceilings were used in home construction through the late 1950s until drywall appeared on the scene. Lathe strips are horizontally nailed across vertical studs; sometimes wire mesh is added and two layers of plaster are applied over the lathe. The lathe-and-plaster technique makes it difficult to locate the studs when you want to hang pictures or need a stud to hold the weight of cabinets or electronics. Traditional stud finders don't work because of the lathe strips or the wire, but a simple wire probe or other methods can help you locate the studs and joists in plaster ceiling and walls.
To verify joist and stud location, climb into the attic and walk across the rafters -- not between them -- to measure the first rafter or joist's location from the wall that is shared in the room below. Also measure the distance between joists from the center of each joist. Transfer the measurement to the room's wall, working from the same wall where you measured in the attic. Start at the corner and mark the wall across the room in 16-inch or 24-inch increments as noted in the attic. Standard balloon-framed homes install studs or ceiling joists 16 inches or 24 inches on center. Joists are usually installed directly above studs on the wall's top plate. Two-by-four lumber is actually 1 1/2 inches wide, leaving 3/4 of an inch on either side of its center. Test stud or joist location using the hammer-and-nail or wire-probe method.
Wire probes offer the oldest method of locating studs behind plaster walls and joists in ceilings. To make a probe, cut and straighten a coat hanger with wire cutters. Measure approximately 3 1/2-inches from one end to bend the wire at a 90-degree angle into a L-shape. Inspect your baseboard or crown molding for visible nails for a possible stud or ceiling joist location, or tap on the wall until you hear a thud, not a hollow sound. Drill a small hole through the lathe in your wall or ceiling on these locations using a stepladder to reach the ceiling. Insert the bent end of the coat hanger into the hole, working the wire in past the 90-degree angle. Spin the wire clockwise. When the hanger hits a solid object, it is likely a stud or joist. If it does not, measure to either side of that location by 3 1/2 inches, insert the probe and spin it clockwise. Repeat this until you find a stud or joist.
Hammer and Nail
You can also use the hammer-and-nail method to find studs and ceiling joists in plaster. Lightly hammer the nail through the plaster wall or ceiling where you expect a stud or joist; a nail will bounce when it strikes lathe strips behind the plaster because of their flexibility, but won't it if strikes a stud. You can also locate studs by finding an electrical outlet, which is usually attached to either side of the stud. Test each side of the outlet by driving a nail into the wall above or below the outlet. You can find ceiling joists in the same manner because lighting fixtures have an electrical box attached to either side of the joist.
Simple Stud Finders
Simple stud finders are not a reliable source for locating studs in plaster walls. Stud finders work by picking up the difference in density through the wall. Lathe and plaster walls mask studs by simulating their density to create a false signal. Metal stud finders are better at locating studs in plaster as long as wire mesh was not used over the lathe strips. Wire mesh causes a stud finder to emit a positive signal all across the wall. Metal stud finders work by locating the nails in studs. Slide the metal stud finder horizontally across the wall when it's on. When the signal is continuous, this indicates wire mesh inside -- it won't work for finding the stud. When the finder emits a signal, slide the metal stud finder vertically up the wall at the same point the stud was found horizontally. The stud finder emits a signal for each nail it finds.