Shower tray unstuck

[Malc]

Hi,

I'm fitting a stone resin shower tray, well 3 to be precise. Maker says bed on 5:1 which is what I have done. I had some waterproofer around so I added a bit of that for workability and to make the mortar waterproof which seemed a good idea (given I had the waterproofer). Maker said 8mm minimum so I've aimed for that too and I reckon I have that. I noticed after it said 5mm minimum when tray was fixed so I could have gone less. Underside of tray is NOT smooth so it does need a solid bed.

Floor is green 22mm chipboard on spacejoists at 400mm centre so no flex there. I primed the floor with SBR/Water/OPC mix to seal the floor and stick the mortar. Pipe work is under the floor.

All 3 trays show a crack between tray and mortar around the edge. Fingernail width. One has crack between floor and mortar. The shower tray seems to have a bit of flex. If I push on the edge the crack disappears. The enclosure weighs 48kg but is not in place yet. I reckon the weight of that will close the crack if I can push it closed.

I think that in setting the tray I flexed the edges down and the suction to mortar kept it way until the mortar dried out and then the flexed back. Perhaps I should have primed the tray same as the floor. I did wet the tray with a hand mister to control the suction but I didn't think about priming it for stick as the underside is rough

I think I have 3 options. 1) Take it out and try again but potentially I'll ruin the tray and get the same result. 2) Undercut the mortar at the edge by a few inches, if possible, a inch is maybe as much as could be done, then repoint. Not sure that would work though and could make it worse. 3) Squirt some no nails solvent free (which is thin) or SBR/Water, into the crack then assemble the enclosure on the tray to weight the edge down.

I like 3 the best. Opinions? Other options?

 

[oldbutnotdead]

Those stone resin trays don't flex, they crack.
Your trust in chipboard is much greater than mine, and there will be some deflection in the floor (spacejoists implying long spans).
Have you tried (you'll need a friend or a camera) weighting the opposite side of the tray to see if the gap gets bigger?

 

[Malc]

Good idea I'll try that.

Spacejoists were to allow pipe runs under the floor, not especially long runs. Run is under 4m.

I've also though that the edge I can see is edge where the mortar squeeze out so as I was tapping it level the open edge would less compacted than the rest. I used a trowel to finish the edge of course but the mortar would not have been as compressed at the edge so it could be the edge has shrunk more too. Tray is offset quadrant with 2 sides to walls.

 

[Madrab]

Floor is green 22mm chipboard on spacejoists at 400mm centre so no flex there

You reckon? I would suggest the opposite, unless it was closely screwed and glued and even then. It would also depend on where the edge of the tray sits too and it's size will dictate whether the tray will deflect at the edges.

I always add extra support into a floor if the edges of a tray don't sit exactly on the joists, with space savers that would obviously be more complicated. In this case I would have used CLS and created a plinth with a thick 18mm WBP plywood top.

Where you go now though ... I think you are now into using a flexible gunned solution to allow it to be applied easily as you need to get a solution into and under the gap and fill the space as I find these things just move again over the longer term.
A CT/high grip silicone adhesive may also be an approach but it will always be a repair regardless, unless it is lifted and redone.

 

[Malc]

Yes the chipboard floor is screwed and glued. Spacejoists are 72mm wide so a lot of surface for the glue and the screws were tite type. Joists are 400mm centres, chipboard 22mm. Proper wood glue too. AFAIR there are some noggins too. One tray starts about 0.3m from joist support wall so not in the centre of the span either. Two edges (S and W) show the crack so one of those is across the joists.

The shower tray I wouldn't say was the best but I had designed the pipe work under the floor, and the bathroom layout, around that tray by then. I could find any others with the right size and drain hole position. The bottom is rough. I think it is made by just taking a white plastic shape (which becomes the top surface), pouring in pumice concrete and leaving it to set. The trays you see in the sheds with the flat bottoms I suspect are pressed flat so the concrete is likely better and stiffer.

I have a damaged tray so I could test the concrete and the tray flex. And I'll also load the trays at various points while someone watch the gap, to see if the tray is rocking. I think the tray is flexing but I need to prove that.

 

[oldbutnotdead]

Ah, good plan on the joists for pipework. No use to you now but I've never trusted chipboard in a potentially wet area- plywood or actual timber get my vote every time.
If your tray is flexing then it is not stone resin- if the crack is only under the very edge of the tray (got any feeler gauges? Or a fag paper. Shove them into the crack, see how far they go. 5mm=fine) then throw some silicon at it
If your tray is rocking (straight edge on one side or the other, move weight, see if any gaps appear under the straight edge while the crack appears/disappears) then you have a problem- the tray will eventually crack. Further investigation needed- mainly to find out if the tray is rocking on the mortar or the whole floor is moving. Might need a cheap laser or a tape measure from under the floor to the next level down to check for floor deflection

 

[Malc]

So I've done some tests. Concrete on damaged tray is solid so I'm maligning the trays unfairly. Walked around on the trays, just in socks of course, while wife looked at the crack. No sign of crack getting wider when I go to the back or the middle. Only change is when I load the very front edge (the lip) and crack closes.

Tested the floor. No sign of crack changing as I approach tray. If floor was moving then I reckon as I load the floor it would move under the tray, especially going across joists. I think I will add some noggins under the floor though at the front edge as they wouldn't hurt even if they don't help. Maybe back too. Front/centre/back would be 400 spacing or less which seems extreme but might be an idea.

I'm thinking the issue was the front edge dried out too fast and it is shrinkage crack. I reckon I'll leave it a month to make sure the centre and back are dry. Centre/back may shrink as they dry so close the crack and problem goes away (that would be nice). Failing that I'm leaning towards option 2, rake out an inch or more and repoint. This time put plastic round to slow the drying time. I've no issue walking on the tray centre so I just need to support the very front edge which repointing should do.

Feeler gauge is a good idea though to see how deep the crack is. From the way the tray behaves the crack must be less than the lip depth and loading has no effect until I stand on the lip.