Leak developed 16 months after a full bathroom refit.

Hi there, Firstly, there should be absolutely NO movement of the tray whatsoever, from your description it sounds like your tray has been installed using a riser kit (which means it sits off of the floor up on little legs) probably, in this instance, sitting directly on top of flexing floorboards. It is general good practice in the rare event you have to use a riser kit to overboard the area under the tray with at least 18mm marine plywood, being secured to the existing timber floor with both adhesive and screws, to eliminate any possible movement from the underlying substrate. Further, I also bond any part(s) of the tray‘s edges To a sealed and primed wall that it may sit against, again, to additionally dampen out any possible movement, and help provide a more competent seal. This reminds me of one very similar repair I had to diagnose a couple of years ago in where the original installer had not used any sealant around the waste to the underside of the tray, but additionally not installed a centre leg of a 1m2 tray. When the client stood in the tray it flexed and distorted the aperture of the waste just enough to allow water to escape around it; in that instance I was lucky enough to be able to get under it, install a centre brace, seal the waste, and all was good. Unfortunately until any movement is eradicated, permanently, this just won’t go away, and an absolute diagnosis of where the leak is originating from must be attained. As mentioned by others it could be failed sealant, a flexing subfloor, a cracked pipe, cracked waste, an incorrectly fitted waste, a broken or cracked leg, even a distorted compression fitting opening when flexed, Or all seven. Best wishes, Martin.

Possible cracked shower tray or waste

I think that when you are in the shower the is a drop causing the silicone to be separated from the tiles. Allowing water in. Supporting the underside will stop the drop perhaps or putting weight into the tray first to cause the drop and then re sealing will over come the problem

If you have movement in your shower tray base, then there’s nothing other then taking the tray out, rebuilding or fixing the base so you don’t have any more problems in the future, personally wouldn’t chance it. As for it leaking when your in it and not in it. Generally water goes everywhere when your in it as opposed to when your not in it, it’ll generally go straight down the trap. So points towards the seal, I’d ask if fitter sealed behind shower enclosure wall profile where it meets the tray. You should be able to see if the seals broken between wall and tray so I’m guessing it’s not obvious.