I am a DIY newbie. I want to mount a TV on the wall. The wall is drywall, but the builder of the apartment told me there is concrete behind it, and the fixings should go in the concrete.
What kind of anchors and technique should I use in that situation if I want to make sure the TV stays on the wall?
You need to attach the mount to the concrete, not the drywall.
There are special anchors for concrete. Most are made of metal and expand into the hole drilled in the concrete with a hammer drill and a masonry bit.
Most masonry anchors are sleeve anchors and use bolts (also called machine screws) to expand the anchor.
There are also metal lag shields that use lag bolts (heavy duty screws) to push outward on the hole and hold the device. These are fairly reliable, but are less forgiving if you need to remove and reinsert the bolt.
There are also masonry screws that can hold in concrete without a separate anchor, but these need precise drilling and care when tightening to prevent stripping the hole. While they are fine for things that use numerous attachment points, such as studs in a concrete floor or wall, the risk of failure on a device that only uses a few mounting points make these a bit more risky (in my view - contrary points of view welcome!).
The instructions packed with the mount often recommend which size of bolt or screw to use, and many come packed with mounting hardware for masonry attachment. Drill the size hole called for by the particular anchor, and take care to drill straight and not oversize the holes.
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What kind of gap ? If quite a small gap it is usually fine to go straight through plasterboard and install anchors in concrete behind.
There are many options, which would vary depending on size and weight of TV.
Nylon wall/fischer plugs for light/small TVs. May have issues with depth in this case.
These then come in larger versions with coach bolts.
As you get bigger you can use expanding anchors, some are stud type, some are threaded sleaves.
I personally like screw in anchors which is another option.
Lastly chemical set anchors.
If masonry you will likely need an SDS masonry bit and a rotary hammer drill. Then depending on anchor type, may need a hammer and screwdriver or spanner/ratchet.
If there is a large gap with a stud ideally just try and pick up the stud and screw to that, if your TV is small enough that it's between studs it may be small and light enough to only pickup one stud or use expanding anchors/toggles.
If a large TV try and straddle 2 or more studs, if need be you can install say for example a small wood or MDF panel painted or with coating/laminate to suit your wall that goes across the studs and spreads the load, this then allows the mount to be installed anywhere on the panel and with large TVs this panel is covered/not really noticeable.
These options generally require a drill and screwdriver.
