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I'm trying to mount a TV, but the only studs are on the outside walls of this niche in my fireplace. Meaning I could build a little wooden box and lag into the narrow face of the studs (the 1.5" thickness, instead of the normal 3.5" thickness).

But I've never lagged into the side of a stud before. Is it strong enough to hold hundreds of pounds? This will be an extension arm mount for a heavy 70" HDTV, so it really needs to be strong.

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    I recommend trying to shop for some lag bolts online that are less than 1.5" long and look at the manufacturers specifications for installation of those fasteners in wood (since that is the installation depth you have to work with). That should give you the information you need and help identify suitable fasteners for purchase. – statueuphemism Oct 18 '17 at 21:41
  • Wait your last comment was the most important how heavy we talking? Whats the reach of the arm measure fully extended at 90 degrees from wall to pivot on end? If this is a full extension arm with a 70 pound tv with the pivet im guessing around 35 inches thats tons of force my friend. In my opinion open the wall and add studs and supports there are tons of articles online. – David Moritz Oct 20 '17 at 4:16
  • How many bolts and what size? – isherwood Nov 25 '17 at 0:48
  • It depends on the diameter of the bolt a 1/4" bolt 1" deep 225 lbs of pull out strength, increase the depth by 1/2 and now the 1/4 pullout strength is 337 lbs. A 1/2" lag 378 lbs per inch. 3/4"= 513 lbs per inch deep. This is based on Douglas fir. Most tv stands have a limited arm length because the frame work would be huge and heave if more than a few inches out for tilt. – Ed Beal Dec 27 '17 at 20:23
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An alternative is to use the flat stud in conjunction with one of the new style toggle bolts. toggler

Now you have a 2+ inch piece of metal placed behind the shallow side of the stud. It would take many hundreds of pounds of force to pull that little steel bar through the stud.

You would need to drill a slightly larger hole to accommodate the toggle.

All of this assumes that the flat stud is not backed by another stud. If that were the case, standard lag bolt approach would have plenty to grab and a very rigid structure.

Images and links are illustrative only, not an endorsement of goods or sources.

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Attaching a short block of 2xN (12", 16", double 8", whatever works) across 2 or 3 of those flat studs should provide a rigid enough support with 3" of depth for a 4-corner lag attached TV mount. The 2xN is screwed to the studs every few inches with enough smaller fasteners with enough bite depth, spreading a heavy load well despite the short bite depth. It could even work overtop the drywall. (Random sized 3/4"+ stud-width and bigger remnants of plywood on garage walls provide versatile attachment areas for chargers and tools and shelves, etc. and a TV is just a heavier version of the same concept.)

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