shower tray bedding

[Class of '55]

Hi Guys
I recently fitted a low profile shower tray. Its plastic, which i've always steered clear of but because it was low profile, 40mm, I figured that it shouldn't flex. It was bedded on s&c 5:1, tapped down, levelled and left to set. All fine so far. However I tested it to check the hi-flo waste etc and after 5 mins of running it lifted at the front quadrant edge. its now out and there are no obvious reasons as to why it's gone this way. The render has set and there are no low/high spots. For some reason there does appear to be some condensation? It appears as if the hot water has flexed the tray and broke the seal (which would explain the condense) but am I wrong in expecting a shower tray to withstand a bit of hot water!!!
I have fitted dozens of trays but never a low profile. The reason for my query is that tbh i'm a bit stumped. Its confirmed my hatred of plastic but i need to ask if theres anything obvious ive missed before i consign it to the skip and get a resin tray, bearing in mind the floor and waste is all set and i really dont want to have to 'start again'. I really dont have the heart to have another go, it should have been perfect. Can anyone offer any advice? Anyone experience the same problem?
Cheers Guys

 

[Egga]

Yes Ive had a similar problem with plastic trays. The M.I says use s & c but it dosent key to the plastic tray. It lifted and made me look stupid in front of customer.
Rang customer services who were useless and could offer no help.
In the end i used a high strength silicone which did the job. Sat it on a plywood base.
I always recommend resin trays. Loads better than plastic. IMO.

 

[Class of '55]

Yeah cheers Egga. And im with you re resin trays. I half figured that silicone's gonna be the only option but ive never come across this and youre exactly right, the s&c doesnt key to the plastic. Tbh when you sit down in hindsight and think about it, it never will. Been in this game for eons, perhaps i should have realised...... but there you go.

 

[Class of '55]

Hi Egga, what silicone did you use. I tend not to get too involved with it for heavy fixing. Im guessing that a Dow Corning sealant is not what we're talking about here? When you say you mounted it on a plywood base was that prior to fixing to the floor. I have covered the floor in marine ply as prep for a tiled floor and figure on dropping it straight on that.
Feel like a rookie here so my apologies but I'm strictly too old school for my own good. Did you prep the shower base with anything?
Steve

 

[CBF]

i never bed any trays on s&c, trays flex so does a wooden floor s&c doesn't it will just start breaking up, i silicon it down if it's flat on the floor or if it's raised i use celcon blocks (thermolites) silicon both under them & on top & sit the tray on that, they don't move & no frame to move or fall apart.

 

[Egga]

I used 'Sticks like Sh!t' I think Evo stik make it.

I had to build a base for this one cause there was a joist in the way of the waste.

Yes straight onto plywood floor is best if its dead level and flat. Ive also used floor tile adhesive to good effect.

I didnt treat the ply with anything, but a joiner i work with uses 'SBR' which primes and seals it. Can get it from builders merchants.