I'm mounting a 38lb TV (with a ~10lb mounting bracket).
I located a stud, but the drywall floats an inch above it, plus the drywall itself is another 1/2 inch. So a couple 3-inch screws (and not a micron longer or they'll protude into my nice old neighbor's kitchen!) is required.
Pretending the drywall won't hold any weight, that means the 3-inch screws will be half-in and half-out of the wood. With a TV hanging off the end of them.
Recipe for disaster?
+++++++++++++++++++
So, three days hence, here's what I did: instead of using the living room wall I share with my neighbor, I used the living wall room that I share with my own hall closet (which wasn't my original choice due to window/sun glare).
I now have several 6" bolts going through the drywall, plaster & lathe, and studs in the closet, with big-ass washers for a little extra "spread". The Cheetah mount is wide & flat enough that it isnt pulling into or denting the drywall, and amazingly there is ZERO glare/reflection coming off the window.
Thanks everyone for your help, you gave me the info, perspectives, vocabulary and confidence to tackle this problem intelligently. 🙂
Best Answer
Case 1: Bridge Air Gap By Fastening to Building Structure With Overhung-load Bearing Fasteners
The air gap problem is quite common: The sheetrock on many walls I have encountered doesn't contact the studs everywhere. So, when you tighten down a TV mount to the wall, the drywall deflects toward the studs. You can easily cause cracking of the drywall, particularly when there is a seam in the area.
The fastener and the building structure have to be strong enough to withstand the torque created by the downward force of TV and Mount acting over the distance between the drywall surface and the building structure.
Case 1.1: Use Hanger Bolts
Assuming you find wood building structure in there (furring doesn't count here because it will twist under the torque), I recommend hanger bolts.
Use a spade bit to clear a hole in the drywall big enough to accommodate a nut flush with the surface of the drywall. Then put the mount over the hanger bolt and tighten another nut down on that. The first nut will keep the drywall from deflecting toward the wall. Size the hanger bolt larger than normal because of the overhung load. From your description I would guess that 4x 1/4" hanger bolts would be adequate.
Make sure you pilot drill for your hanger bolt or you risk splitting whatever wooden structure you're screwing into.
Case 1.2: Variation for Concrete Construction
I've also encountered furred out concrete clad in drywall (common in highrise condominiums). In that case, the same principal as Case 1.1 applies, but use "drop-in anchors" and a threaded stud instead of hanger bolts.
Case 2: Fasten to Furring with No Air Gap
Sometimes drywall is attached to and is in contact with (i.e. no air gap) furring.
Case 2.1 Furring Attached to Building Structure
Where furring is attached directly to the building structure, drive multiple wood screws directly into the furring. If there is some hard material behind the furring, ensure that the screws are short enough. Otherwise they can jack the furring from the building structure when their points impact cinderblock or concrete.
Case 2.2 Furring Dubiously Attached to Building Structure
Where furring and drywall are creating their own structure, drive multiple wood screws into the furring to fasten your TV mount.
Drywall itself is rather heavy, so if the wall is able to hold itself up, there's a good chance it can hold up your TV to provided you distribute the load over a large enough area of drywall and furring.