If you plan to hang something heavy on your wall, you need to find a stud. If you’re hanging something from the ceiling, it’s called a joist. These studs and joists make up the framework of your house and are much stronger than drywall. Typically they’re wood 2×4’s, 2×6’s or 2×8’s installed 16, 20 or 24″ apart. They run vertically in the wall and across the ceiling they’re usually perpendicular to to the ridge-line of your roof. You can determine that by checking in the attic or a little work with a stud finder. Newer homes and commercial buildings may use metal studs but the principle of finding them is about the same.
A stud finder is an electronic device that can detect differences in density working on the principles of capacitance. It’s not critical to know the science behind how they operate, except to know that they detect mass and not exclusively wood so they can be thrown off by other things in the wall. Like pipes, wires, stashes of money and dormant monsters. Readings can even be changed by the proximity of your hand.
Shown in this series of photos is a mock up of a typical wall. Imagine the glass is drywall. Typically you hold the sensor to the wall, turn it on and it self calibrates. The meter should be reading zero, if not, move to another location and try again. As you move to the right, you can see the meter reading go up as we approach the stud, peak while over it and taper off as we move past it.
Usually the readings will peak when the centerline of the sensor is over the edge of the stud, but as you can see in these photos that can vary slightly. For that reason, as moving across the stud left to right I mark with a pencil the point where the reading first peaks, keep going, then reverse direction, again marking where the reading first peaks. You should wind up with two marks on the wall and the center of your stud should be about halfway in between.
Notice the short horizontal section of wood in the photos. That’s called blocking, and it’s there to add structural integrity to the framework. A stud sensor can only detect that something is back there and has no clue if it’s vertical, horizontal or what it is. That’s why you may get bogus readings. If that happens, try doing your sensor sweep above and below where you need your holes. Make pencil marks and then by using a level in the vertical position, or a plumb bob you can start to get an idea where your framework is and get those shelves mounted securely to the wall.
You’ve probably figured out by now a stud sensor is more of a good guide than an absolutely precise process. You may want to take it a step further and explore with a thin 1″ nail. Gently tap it into the drywall, you’ll know when you find the stud because you’ll feel resistance. If it pushes through easily, pencil mark that hole with a small X and try again 1/2″ to 3/4″ over. Since most drywall is 1/2″ or 5/8″ thick, a 1″ nail isn’t likely to damage anything and the small holes are easy to fill with a dab of paint.
Also note that lumber measurements are specified before shrinkage from kiln drying. So a 2×4 is actually about 1.5″ x 3.5″. Not a huge target to find semi-blind when you’re aiming to find the center! And while it’s not critical to find the exact precise center, you kinda want that screw firmly into wood if it’s bookshelves you’re putting up. Secure wall and ceiling mounting becomes even more critical if you live in an area prone to earthquakes.
When is it OK to use toggle bolts, mollys and anchors in drywall? Basically, when there’ no other choice, the item is relatively light and won’t break or cause injury if it comes down. I’d limit the use of those to curtains, fake plants, paper lanterns and the like.