Yesterday, I hung two 4ft 1in x 8in red oak bookshelves on my plaster and lath walls and I want to make sure they’re anchored enough since I wasn’t able to hit studs. I tried using a magnet to find the studs after my stud finders couldn’t do the job. I found the spots where the magnet clearly stuck to the wall and marked them but when I drilled in there was no resistance after making it through the plaster and lath. I have no idea what the magnet was attracted to at those 16 inch intervals but I had no resistance with any of my drill holes. Each of the two shelves is held up with 3 heavy duty shelf brackets tested to hold over 100lbs for every two brackets. Each bracket has 4 screw holes to use for fixing to the wall so each shelf has 12 screws and wall anchors. Each individual wall anchor and screw combo is tested to hold up to 55lbs. The total amount of weight per shelf is about 40-45 pounds and each of the 6 total shelf brackets is 13 inches away from the nearest one both horizontally and vertically. The shelves seem perfectly sturdy but I want to know if this is going to hold long term?
Those anchors are not the best for hollow spaces, they need meat to hold on. But if you are lucky it might work for a while.
Finding studs the old way. Start at the corner (there is always a stud there) measure 16 inch to find next stud (or 24 inches in some cases)
Check for anchors with wings that work very well with hollow walls.
Something like this
Some might recommend different type hollow wall anchors.
Those are good but have a limitation.
You must know the thickness of the wall for them to work.
To find the studs mark the level of the shelf on the wall, then drill a hole at that height, then put a bent wire in there and feel around for the stud. once you find the stud screw the bracket into that.
When you install the shelf it covers the holes you drilled.
when to no longer want the shelf use the same plaster you're filling the screw holes with to fill the exploration holes.