Installed shower tray on mortar - still movement

I think for sure what's happened is, by putting the weight around the edges, I've bowed the tray a tiny bit while the mortar set. Once I took the weight off, it left a bit of spring at the edges.

All stripped now and ready to try again. I contacted the tray manufacturer and they say I can use tile adhesive as long as it contains portland cement. I assume Mapei flexible adhesive does, that's what I'm about to use.

They also say don't weight the tray, just settle it in.

Fingers crossed I get it right this time!
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So I have now bedded the tray using flexible tile adhesive. I used 1 x 20kg bag plus a bit more and 5 litres of water to make a thick mix and maybe made way too much - the tray is now sitting a good 10mm above the plywood base on a bed of adhesive. Think it might have been better to put the adhesive where the recessed bits are and not the circle pattern. It's level though and I can use a bit of quadrant to hide the gap once the flooring is in.
Nah. The circles probably match with a plinth kit- as long as they and the edges are supported the thing will be fine. 10mm isn't the end of the world- make sure you leave it well alone for a day just to make sure the goo goes off properly.
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Mortar mix can only be used down to a certain thickness, even 10mm is a bit thin as it just biscuits and delaminates.
That's why tile adhesive is better as it's designed for thin bedding.
As for the edges, I usually pack in some sand and cement/tile adhesive/grout and then apply a clear silicone bead when all is tiled and ready for use.
what kind of floor is the ply/shower sitting on?


Guess I'm not going to get an answer. The reason I ask about the floor type that it's all going down onto is that it may not be the tray or bedding that's moving, rather what's underneath it all supporting it. You can apply all the material in the world but if it's boards sitting in between joists at the edges, it will still deflect and will need supports added.
A plain mortar mix will just shrink and break up when too could use a 50/50 mix of water and PVA or SBR and that will bind it together (together with a sharp sand/SBR bonding coat on the floor).
Tile adhesive etc would seem a better solution...
Madrab, sorry, floor was mentioned earlier. It's floor joists with extra noggins then 18mm marine ply screwed down on top. Totally solid. Inspected tray this morning and it looks rock solid now, ready to tank the walls and get tiling.
@mitchino I appreciate this might be a long shot, but was wondering how your tray as held up? I’ve got a similar tray that I’ve just taken up from a first shot of mortar. Mix was too wet, and just didn’t feel right after a day, so looking at other options. Mines a very similar looking base, so wondering about tile adhesive too.

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