Installed shower tray on mortar - still movement

14 Dec 2007
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United Kingdom
I am really ****ed off! Following the manufacturers instructions, I installed a stone resin walk-in shower tray onto a SBR primed marine ply base on joists, laid tray onto 5:1 mortar mix with 8-10mm thickness and weighed down tray with 200kg weight.

48 hours later, I'm seeing 1-2mm movement at the edges when walking about on the tray!

How can I fix it? Am I going to have to lift the tray (which I've already siliconed to wall along two edges) and stick it down with something else?

Really don't understand what I did wrong...
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The tray is like this underneath (not my pic). I'm thinking that the machined flat parts are bedded into the sand and cement, but as a result the edges are 'floating' a little and that's where I see the movement.


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What make is the tray? I'm sorry to say but if I received a tray like that for one of my customers I'd be telling them to return it straight away. As far as I'm concerned that's really bad/cheap finishing.
The whole base should be flat and level, or at least flat and level over large portion of the underside and certainly at all the edges, that's probably why they ask for it to be set into sand and cement so they don't have to finish it properly.

Did you run a level over the base and see what size of gaps there were to the low points? That would determine how much more mortar mix you might need. You maybe need more mortar mix to fill the larger gaps.
It's an MX Elements tray:

If you watch this video at the 2'50" mark it shows the tray being ground down so the bottom is flat (in certain areas)

Maybe my mortar bed wasn't thick enough? Some people say you need an inch of mortar! I didn't do that as I don't want the tray to be way higher than my finished floor. I made the mortar so it was a bit runnier than brick laying consistency, maybe too dry? It's also very hard to wiggle the tray into the mortar as it's sitting in a corner, so I just walked on it then weighed it down.
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Our company fit 3-4 MX trays per week, and we always use flexible floor tile adhesive, I’ve never fitted a tray on a sand and cement and I never will
So are you fitting trays like my one, i.e. not flat on the bottom but full of undulations and mouldings?

I have taken the tray up and removed all of the sand and cement, ready to start again.

Which flex adhesive do you use, and what thickness do you put down? My tray is 1800 x 760, do you know how many bags of adhesive you would use for that?

I was actually wondering about trying to 'level' the bottom, with self levelling compound, then using silicone to glue it down. Would make tray incredibly heavy!

Another idea was to level the bottom with low expansion foam, turn tray over, squirt foam in, then lay plywood on top and weigh it down until the foam is set. Then silicone that down.
We fit exactly the same, Stone resin MX shower trays

If I was doing yours

I would secure all floorboards/chipboard under the tray, cut 12mm ply 1800x760, the Mx trays are actually 5 to 10mm bigger than the size stated, so the ply just sits away from the edge of the tray, put a screw every 6” in the 12mm ply, so it’s secured well to the floor

We use flexible rapid set from topps tile, I would expect 1 bag to do that tray, but as we buy it in bulk, I’d take 2 just incase, make a nice thick mixture as the weight of the tray will squash a runny mix, we normally leave it over night to make sure it’s set solid, but normally your good to walk on the tray in 2/3 hours
I already have 18mm marine ply screwed down. I cut it to fit the tray exactly.

I've taken a couple of pics - the underside of the tray, and what the mortar looked like when I lifted the tray. I can see that the 'flush' parts have squished the mortar down to about 6mm, but there hasn't been enough mortar to fill all the sunken parts. (I laid about 10-12mm).

Running a level across the bottom of the tray, some of the 'atomic model' ground down mouldings are flush but some are nowhere near flush. How deep should I have the tile adhesive to make sure everything gets supported? and how do I get the flush areas to be sitting on a thin bed of adhesive and the 'not flush' areas to be supported? It's such a big heavy tray it's hard to shoogle it around when levelling it.
It would great to figure out a way to turn the tray over, fill with tile cement, then put a thin lid on it. Turn it over and place on the plywood, then slide the lid away. Ta da! all mouldings filled and tray secured.
We just use blobs on the floor where the circles are going to be and then whatever’s left in the mixing bucket gets blobbed all over

The moulding doesn’t have to be filled, just make sure you get a good bed of rapid set all over under the tray, and it’ll be fine
Ah ok, those high spots are there by design and not just a bad pour, the mould's made specifically to create those and the ridges by design and then the mortar will be laid for those pads/ribs to sit on .... fair enough.

Having a read through their MI's, It's very specific about using mortar and nothing else really though, might be worth a call just to check what happens if something else is used ..... only thinking about the warranty.

Just a thought, what kind of floor is the ply/shower sitting on?
Your mistake was shoogling it- those trays are too big & heavy. Trick is to get your slightly wet bed level then drop the tray on it as square as poss, check for level. If not level, lift tray, redo bed, repeat til level then chuck yr big weight in the middle & leave it.
Walking on it will have squeezed mortar away from the edges & created your wobble.
Baths'n'Gas - do you also make sure there is adhesive around all the edges? That is where I got the slight flexing. And is Mapei flexible adhesive ok?
I’ve never used Mapei, but it’s quality brand, so I’d assume it will be good enough

I do sometimes put adhesive around the edges, depending on how far out the floor is

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