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So I have a strange wall construction in part of my house. I'm mounting an articulating mount, up to 55", in a stud.

I took out a cable outlet and box to see the construction because I wasn't finding any studs.

Starting from the room it's 1/2" drywall, 3/8" drywall, 1" foam, 2.25" actual length stud, then gap, then I think furring, then masonry exterior.

I'm thinking some 4.25" lag bolts into the stud will hold, but it's so bizarre I wanted to double check.

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Trying to mount an articulating arm that is designed to mount to a single stud will not stand up mounted to that type of wall construction. Even with long lag bolts there is just too much chance that mount will move around and cut into the drywall, become loose and make a mess of things.

What you should be doing is to mount a panel of good quality 3/4" plywood that is large enough to span across at least two studs with three preferred. Make the height of this panel at least three times greater than the arm's mounting bracket. Use multiple large screws to mount the plywood into place into each stud. Now you can mount the articulating arm to the plywood surface and be assured of a secure attachment that will stay tight and firm against the wall.

You can sand and paint the plywood to match the wall color and it will hardly be noticable behind the TV. If you use flat head screws to mount the plywood they can be countersunk and then spackled over to make a nice flat surface.

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  • Agreed. The problem with lag bolts is that, with all that soft material between the studs and the base of the arm, you're depending on the bending strength of the lag bolts, which isn't how they're meant to be used.
    – Daniel Griscom
    Apr 20, 2019 at 16:30
  • Not clear from your answer -- do you suggest removing the drywall (and shimming) so that the plywood is flush with the finished wall rather than surface mounted?
    – amI
    Apr 21, 2019 at 2:40
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    @amI - Not at all. Mount the plywood right on the existing surface of the drywall. Multiple good sized screws through the plywood and into each stud will clamp the plywood panel tightly to the wall. The large size of the panel will assure that it does not move around at all.
    – Michael Karas ♦
    Apr 21, 2019 at 5:10
  • Agreed -- although a flush installation seems more professional to me -- and there's already a good sized hole started (but keep the foam layer).
    – amI
    Apr 21, 2019 at 5:31
  • @amI - I disagree with you. Trying to get a flush surface is just not worth the effort. Besides that the wall integrity will be sacrificed a lot in the areas where you try to split the mounting across just a half of the stud width. And if you were able to achieve it the surface textures will be different that will make the area be just as apparent as the plywood affixed to the wall surface.
    – Michael Karas ♦
    Apr 21, 2019 at 5:56
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Noooo cut the wall open to desired size back to stud install 50mm x 50mm timber across studs u can put some rock wool or kingspan between the timber to replace the thermal board youv removed then screw 15mm ply to the new framing now screw 9mm plasterboard over th ply that will leave you with 5mm for a skimcoat of finnish plaster once done you will have a safe structure for your tv and a wall that looks as it should 👌

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Frame a square using gauged metal studs. The hollow of the studs must be facing each other. Measure the square box from the outside, then cut plywood (1/2"- 3/4") half an inch shorter than the actual size. Use drywall screws to install plywood.

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Mounting between 2 studs is a good idea. Use a piece of thick plywood or MDF. It needs to be at least as wide as the mounting bracket (obviously), and wider would be better.

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    Welcome to Home Improvement! Much of your answer was really a comment, not an answer; I've edited it out. With a bit more rep, you will be able to post comments. And, please take our tour so you'll know how better to contribute here.
    – Daniel Griscom
    Apr 21, 2019 at 11:07
  • And, someone has proposed removing the "MDF" suggestion from your answer. They're right in that MDF isn't a good idea, but it's your answer, so you'll have to change it.
    – Daniel Griscom
    Apr 21, 2019 at 16:59

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