Anchoring through shower tile & beyond

Question

The attempted shower seat

I recently tried to install a shower seat in my shower. It came with an aluminum wall bracket, and is anchored with 5 approximately 3'' screws inserted into typical plastic wall anchors.
I had my doubts this would hold, so to minimize overall damage, I managed to limit drilling to only one tile.

Attaching the seat, it held well, but after further testing, the tile cracked in the middle, horizontally. Oddly, the adjacent tiles, left and right also have a hairline crack aligned with the middle crack. I did not expect this type of failure.

Questions

What caused this? Did the backing (whatever it is) break and transfer to attached tiles?
How do I proceed?
Any guidance for attaching things to unknown walls?

I'd like this seat to work, but can't proceed without understanding why it failed. I assumed anchors would pull out or the tile would crack.

extra details

this is a rental, so some techniques may be limited.
the mortar at bottom of tiles is loose, I might be able to remove to spy on backing material only tub edge visible
while hammering in plastic anchors, the wall reverberated quite a bit, and seemed very hollow.
this wall is oddly thick and I'm unsure what is behind it.

batsplatsterson · Answer

The screws hold the plate to the tile;  when the plate flexed under load, it bends the tile with it, but tile is not flexible like the metal, so it cracked.

As far as guidance for attaching to unknown walls:  you really have to get to where you know what you're anchoring into before you can do so safely.

With bigger holes you could use stronger anchors such as toggles that get past the tile into what may be stronger material behind, but that could really be a can of worms.  The  backing material may be stiff enough that things won't flex and crack the tile but it's very hard to say for sure, you could make this much worse.

So how much was your security deposit? :)

Joe Fala · Answer

The tiles cracked because anything supporting the weight of a human has to be stronger than a tile and cement board. The adjacent tiles cracked because the middle tile is pulling the entire wall with it off the studs. Those shower benches need to be fastened very securely. Bathrooms are the most dangerous place in the house. If you are going to replace the tile make a small hole through the backing material and locate the studs.

Answered by Joe Fala on November 30, 2020

Anchoring through shower tile & beyond

The attempted shower seat

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