I have an old house built in the early 1930s. The conventional stud finders are useless on plaster. The more sophisticated ones are more expensive in the $50 range.
What features would a stud finder need to have to work well in my situation?
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This stud finder with magnet and level is what I keep on me at all times. It is the only stud finder that I have and works excellently. It is strong enough to pick up nails used for crosses most of the time too.
I have no affiliation with the product - I am just a customer that has bought 6-7. The magnet is super strong and the pointer + level help. The only downside is the plastic tip breaks on every one if I drop it, but it still does the trick. A bonus is that it finds screws, nails, tools, whatever... in hard to see/reach places. I don't know how many times I was up in an attic and dropped something and pulled out the stud finder.
If the house construction is lath and plaster (with no metal mesh), you may be able to use a stud finder with metal scan capabilities or a dedicated metal finder. The reason why these work, over basic stud finders is because metal finders work by finding the nail heads that attach the wood lath to the studs.
If metal mesh was also used to help secure the plaster to the lath in some instances, even metal finders may not help you find the stud.
A strong magnet on a string! It works like magic on plaster and costs less than $5!
Get a small, strong magnet Be careful! Especially with neodymium magnets and children/pets and around other metal/magnets/credit cards!
Tie the magnet (or stack of magnets) to a thin string, roughly 1.5-2 feet long.
Slowly drag the magnet sideways across the wall, so that it's hanging freely by the string. Eventually, it will stick to a hidden nail behind the plaster. These are the nails that hold the lath to the studs.
Once you find a nail, go up/down about 1.5-2", and you should find another nail. If you find a whole vertical line of them, then you found a stud!
There can be other metal objects hidden in the wall (electrical wire sheathing/boxes, metal mesh/repair screens, corner reinforcements, etc.) so you want to make sure you find a full line of discrete/separate nails. It will not detect copper wires or pipes so be careful drilling!
I use little bits of blue painter's tape to mark the nails, then I go about 16" or standard stud spacing to the side, to find the next stud, and mark the nails in that one too. Then I use a level to match 2 nails horizontally, to identify the horizontal lath, and I drive screws into the lath to hang small pictures, etc.
I kept my magnets in the plastic package they came in, so I can just tie the string to the hole in the package. It also keeps it from scratching the paint or twisting. I do have to take the magnets out of the package to center on faint nails now and then.
Source (answer #3): https://thecraftsmanblog.com/how-to-find-studs-in-a-plaster-wall/