Stud finders help locate the solid wood boards that make the solid frame of a wall. When hanging shelves or mounting a television, these 2-by-4s provide significantly more strength and stability than drywall or plaster alone. However, locating these studs requires different methods and tools than when dealing with other building materials.
How Stud Finders Work
To understand why some stud finders work better on plaster or lathe, it is important to understand how these devices locate wall studs. When most any stud finder is laid against a wall and activated, it measures the density of roughly 6 inches into the wall. Once it has that measure, move it along the wall. When the density increases, it beeps to notify a stud (or something similar) has been located. If the stud finder is activated on a stud, and dragged to a less dense area, it gives an error.
How Plaster Differs from Drywall
Drywall is made from the mineral gypsum, compressed between sheets of paper. This provides a very even, consistent level of density when a stud finder takes its readings. Plaster, an older method of creating walls, relies on a series of wood slats nailed perpendicular to the studs, from floor to ceiling, referred to as "lathe." Small openings are left between the slats. Two applications of plaster are applied to the lathe, with the first squeezing between those openings. Because of the thick nature of the plaster and the small amounts of plaster that squeeze through the lathe, the density readings vary constantly. This often defeats many modern stud finder methods.
The Best Stud Finder for Plaster and Lath
Any stud finder with a "metal scanning" option is the best for plaster walls. The slats behind plaster walls must be nailed to each stud, which requires a number of nails along the entire vertical length of the stud. Using the metal scanner to find these nails allows you to best determine where the stud is located on a plaster wall.
Alternatives to Commercial Stud Finders
Without a stud finder, a variety of methods have been suggested to help locate studs or the metal that holds studs in place. For plaster walls, a rare earth magnet, available from hardware stores or an old computer hard drive, is perhaps one of the most reliable. When tied to a string, this powerful magnets can be moved along a wall. It is attracted to any screw or nail over which it passes, serving as a rudimentary metal scanner.