I'm trying to mount a TV in my basement. House was built in the 60's, and the basement used to be a bomb shelter, from what I've been told, but most of the cinderblock walls were taken out. I'm trying to mount my TV on the wall, and when I drill in, I drill through the drywall, before hitting a hard spot. With a wood drill bit, I can apply extra force, and it will force in, but soon after, I hit a hollow area. I can't get lag bolts to tighten, as they start to go in somewhat tight but eventually I start feeling a grinding sensation and they end up getting loose before they are fully in.

Am I drilling into old Cinder block? It doesn't feel like the hard area I'm drilling in is very thick... maybe 1/2-3/4 of an inch, which seems too thin to be a cinder block. TV is ~70 pounds, am I safe to mount it there? Should I use anchors? One side of the TV will be mounted into a standard wood stud, so will that likely provide enough support?

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    Have you got a metal detector on your stud finder? Knowing if the hard thing is metal or not would be useful, because your description matches that of metal studs (which you can anchor to). If not - you should probably get one and use it before you do any more drilling in your wall to be sure that you're not drilling into any wires or pipes. – UKMonkey Jul 26 '18 at 12:16

Cinderblocks, as you call them, are also known as concrete masonry units (CMU). The “wall” thickness of CMU’s is 1 1/4”, so it’s probably not CMU’s if you think it’s 1/2” - 3/4” thick.

Could be plaster...it’s about 3/4” - 7/8” thick. Or, it could be thin coat plaster over wallboard. Usually the wallboard is 1/2” thick and very hard (dense) plaster about 1/16” thick is applied directly to the wallboard.

Plaster, thin coat plaster, wallboard, etc. are not structural and would not be suitable to support your TV.

I’d look for another stud and follow the manufacturer’s recommendation about connecting into studs. (Do they give an alternate installation layout using a piece of plywood that spans from two studs?)

  • Would plaster give that grinding feeling when screwing through it? Could I drill through old cinder block with a wood drill bit, or would that be impossible? Also, my stud finder is showing the whole section of wall as a stud. Would this be the case with plaster and wall board? – Weeter Jul 26 '18 at 6:43
  • @Weeter Hmmm...sounds like it’s plaster on metal mesh...if your stud finder “is showing the whole section of wall as a stud”. (The stud finder is just a magnet and locates nail heads in most walls.) Here the stud finder is locating mesh that is nailed to the studs or CMU wall. The grinding sound is probably the metal mesh. If you can mount into one stud on one side AND drill through the mesh and anchor to it, I think you’ll be fine for a 70 lbs.TV. – Lee Sam Jul 26 '18 at 7:03
  • @LeeSam A stud finder most definitely doesn't work like that; it measures the capacitance (which is indirectly the density) of the material behind it; and will absolutely not find the nail heads. Some will even use radar. – UKMonkey Jul 26 '18 at 12:12
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    @Weeter no idea. The last time I had to drill into a stud and couldn't work out what was actually behind the wall I ended up cutting a small slit into the plasterboard so that I could see it (I was drilling into a bathroom wall - I was either going into a stud, or a pipe!) Wall filler is cheap. – UKMonkey Jul 26 '18 at 14:40
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    Is there an electrical outlet or switch on the wall? If so, you could take the cover off and look at the edges. – Lee Sam Jul 26 '18 at 15:14

get an endoscope and drill a small hole so you can see what you are doing. other wise you are just guessing what is behind there. drywall repair costs very little.

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