Shower leaks can occur in three main areas - no doubt other competent posts can add their experiences to this.
- The connection at the tray outlet
- The grout/seal both at the tray and in the tiling
- The connection at the mixer valve/ water supply inlet.
Other than that you can get a settled pipe with a backfall or a failed pipe seal.
And in what can be a very expensive repair, the supply pipework seals can also fail.
I have yet to see the tray itself fail, but it can happen where old age/impact cracks the tray.
This should be the easiest of shower leaks to discover, because the crack will leave a sharp edge.
A very heavy person can deform the tray in use, or a poorly supported lightweight tray can deform or twist.
I have seen a poor rough timber ground substructure cause a significant leak in a new plastic back in a developer spec house.
Load makes the tray pull away from the seal where the tiles meet the tray, making an intermittent gap that's hard to spot when not in use.
Discovering the source of the leak is a matter of trial and error, but an initial remedial action could be to regrout tiles and reseal the junction and check the outlet seal.
A plumber is your first port of call and you could attempt the grouting/sealing yourself - a carpenter might not be needed unless you're pulling up and replacing floorboards, which is a bigger job.
ONQ
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All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.