Large gap between tiles and shower tray 15mm

I brought a new house recently and it came with a downstairs bathroom with a shower.

Unfortunately all the waste pipes come of the ground however they are all about 15mm to far ahead of the wall the shower tray/toiler etc go. The pipes are set in concrete, its not in the budget at the moment to rip it all apart so will have to 'repair' all that needs fixing.

The main issue is the shower tray which has a 15mm gap all the way around, previously they filled it with grout however that has cracked leading to a leak after which they still continued to use the shower. As the gap is around 15mm and the wall isn't straight either, what can be done to fill the gap and then seal it so its waterproof at least.

 

without posted photo's of the shower and the WC area its difficult to visualise what could be done.

its no big deal to dig out some concrete or a sand and cement screed, and extend the tray waste.
but if you have 15mm to make up on, say 3 sides, then it appears the tray is far too small for the compartment, and you will be left short no matter what you do?
i doubt that its a 15mm filler of grout?

 

It can't be 15mm all the way around otherwise the tray would just needs pushed in tighter.

I presume it's because the walls are out of square and the tray touches the wall at one or 2 points? Only way you could do it would be to dig all the grout out, make it super clean, use a hairdryer to completely dry out the gap and then fill the gap with a high quality silicone, over 2 or three passes and/or fit a high quality seal.

 

i had a gap down the side of a shower tray , simply because i made the room larger and just used a piece of white plastic trim - this was fitted under the boarding we had instead of tiles.
But without a picture its difficult to tell

would you be able to simply add the plastic trim all the way round and then mastic the join

 

it could be an all round 15mm gap in a, say 3-wall compartment.
it might be that the tray cannot be pushed back "tighter"to the rear wall because it has to line up in plane at the front?

until we can see a pic its hard to tell what, if anything, the tray has to be in plane with - eg return walls, double studs for fixing any enclosure jambs to etc.
then there's the business of the drain position.

 

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