Wall Hung toilet movement? What is too much?

[iamajc]

My sister bought a new build property which has wall hung toilets, NK by Noke Porcelona
The pans are all moving enough for the silicone sealant to fail across the top of the pan and if you put a bit of weight on it it opens up by about 5mm. They've been in use (light use 4 toilets in a holiday home) for 3 years.
I'm Gas Safe, don't do a great deal of bathroom work, not fitted a wall hung toilet before so not sure of what the cause is.
It seems to be the pan flexing away from the wall as the movement is at the top.
Haven't had a chance to do the obvious 1st check and see if they did the nuts up tight enough, in the hope that the wall frame is not flexing.
Is it a case of a small amount of movement is expected when an adult sits on it and it should be sealed on the same principle as a bath? Do I chuck 75kg of bagged sand on it and re-silicone, which would hopefully prevent further failure of the silicone?
Will there be enough access through the flush panel to inspect and tighten the wall fixings at the top of the frame? Any advice will be appreciated.

 

[ShaunCorbs]

Chances are there’s plasterboard behind which is wrong as this can compress best option is ply or cement board

 

[iamajc]

It's tiled with tiles that are at least 600x300, I'll have a good look at the grout lines to see if they're cracking and showing any signs of movement, I'll also check to see what they boarded it with.
All four rooms they're in are fully tiled, I can't see them being too happy if they have to tear them out to replace plasterboard.
Thanks for your reply.

 

[fowlerboi]

Theres a couple of tricks to getting them to not flex.

Set the studwork 1mm back from the toilet frame so its pulled tight against it and then use cement board, use an adhesive on the toilet frame so the board is fixed to it.

Plaster and paint is rubbish and should be avoided at all costs. It gets dirty is hard to clean, the silicone used to seal round the pan means you can never re-paint it without it looking awful and the small amount of movement in the pan is enough to erode the plaster at its contact points until the pan gets looser and looser and gets worse and worse.

When second fixing if using the indented brackets that you wind a grub screw into use the measurement they specify and then do one full turn more so that the pan naturally pulls into the wall with 4 blobs of silicone behind the pan on each corner. when tightening the allen key you should see the pan forcibly pulled towards the wall, if you don't give the fixing bracket one more full turn


I would look at the tiles around the pan first and make sure none are damaged and the grout lines aren't cracking. If they are all OK I would suggest the framing is fine and that its likely the second fix issues I've listed above.

 

[iamajc]

That's brilliant mate, thank you. I didn't see any signs of cracked grout on the floor so don't think I'll find much wrong when I look closer.