Leaking coram watergard shower tray in stud alcove

I have had two Coram waterguard shower trays installed as part of an extension. The showers have leaked two or three times. I have sealed everything as best I can and identified the main cause to be the grout above the first row of tiles.
When I bought the trays I phoned coram customer support and was advised that part of the wall should be chased out to allow the tray to fit in my alcove. The alcove consisted of a dry lined wall and two stud partitions with plastered plaster board, the problem seems to be that the tray flexes and cracks the grout above the first row of 4" by 8" tiles which overlap the tray. The water gets inside this crack and runs down behind the tray through the floor and through the ceiling below. I can only reach the front three legs but it would appear that they might loosen (unscrew) with repeated use of the shower.
The tray fits in the alcove between the wood of the studwork but doesn't have plasterboard against each side.
What do you suggest?
I am considering using mastic instead of grout above the first row of tiles, trying to fix the legs so they can't spin with mastic/a screw (??) or using a row of larger tiles above the tray such as the 12 inch floor tiles I have in the bathroom.
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anon wrote:

Can't see how the legs can spin, if they are weight-bearing??
Do you know why there's so much movement? Is the floor underneath not stable; do you need to spread the load more (eg, stand the legs on lengths of 4"x2" across the joists or something?)
Yes, certainly use mastic to seal the gap between tiles and tray though; that's a no-brainer. The usually-quoted advice for when you seal round a bathtub is to half fill the bath with water first, so that you when you apply the sealant, any gap which opens up is about half-way between its minimum and maximum widths, which is the best position for the sealant to set. Don't know how much that helps with a shower tray, but if you have definitely got movement you can't eradicate, it might be worth piling a few bags of sand in there before you apply the sealant!
If you have mastic at the interface between walls and tray, then any movement should be absorbed by the mastic and won't transfer to the area above the first row of tiles, so you should be able to refit those and grout normally, as before.
David
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The legs should be screwed to the floor. This prevents direct movement of the tray and unscrewing of the legs. There are angled holes provided in the feet for this.
Christian.
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Unfortunately the floor is tiled so I can't screw through (not enough space for a drill). I may try mastic around the feet and drill through the legs to stop them unscrewing.
Unless you can suggest anything else?
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I can certainly imagine that a light tray like the Coram sitting on shiny slippery floor tiles would have excessive movement. I think sticking them down with silicone would be a good start.
Christian.
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anon Wrote:

As said the leg shouldn't move once tight unless the floor under i unstable, did the shower tray have upstands?
get the plumber/builder back......
-- taylor.mark1966
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The Coram Waterguard does have upstands.
Christian.
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Well, after pulling tiles off and digging out mastic the problem is water is seeping into the front right corner of the tray. It gathers and trickles over the front edge in that corner.
The tray is level, in fact I have slightly raised the front to allow drainage. I'm going to strip out grout and mastic in this area and re-apply in this area before I get really serious and start pulling the doors and tiles off and doing some major work on the bl**dy thing.
Grrrr
I prefer baths to showers now!
A
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