A grab bar is a simple object, but the hard part is invisible. The bar does its job only when its screws bite into solid framing, blocking, or a backing rated for that load. Tile, drywall, and a thin layer of cement board look firm and will hold a towel, but they will not reliably hold a person who leans her full weight on the bar as she stands up.
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What makes it hold
Three things decide whether the bar is safe. The first is what is behind the wall at the exact spot she will reach. The second is the anchor system that matches that wall. The third is the direction of the pull. A bar that she pushes down on and pulls sideways on needs more than a surface screw, and the bar's packaging or instructions should say what load it was rated for and what fixings it requires.
What to check before you pick up a drill
Find the studs or blocking with a stud finder, then confirm by tapping and listening, since a finder can read pipes or wiring as framing. Mark the wall where she will actually grip, not where the bar looks neat. Note whether the wall is drywall, plaster, tile over cement board, or something you cannot identify. Look at the grout lines and the tile for hairline cracks, because drilling a cracked tile can spread the damage. Check what runs behind the spot, such as supply lines or a drain.
When you can do it yourself
The job is reasonable to do yourself when you can find solid framing or blocking at the spot she needs, you have the fixings the bar's instructions call for, you have a drill with a depth stop, and the wall is dry and not in the path of plumbing. Test the finished bar by pulling on it hard from the direction she will use. If it moves at all, take it down and start again with a better anchor.
When to hire it out
Call a professional when you cannot find framing, when the wall is plaster or an unknown material, when the spot is in a shower where waterproofing around the holes matters, when the tile is cracked, or when the best position is several inches away from where a simple mount would go. A licensed installer who finds blocking may also be able to add it, which is worth asking about before you settle for a bar that only looks secure.
When the question belongs to the care team
Where the bar should go and how high it should sit depends on how she stands, turns, and steps. If she has balance concerns, ask her doctor or the therapist who has seen her move whether the placement fits her routine. That decision is theirs to shape with you, and it is worth asking before anyone drills.