Tiling onto cement board with some flex

[Moggaman]

Hi. 
I am installing 2 Grohe wall hung toilets in my new build. Plumbers installed toilet frames first . Then carpenter built frames around toilet frame. Carpenter put 12mm cement board onto frame .Tiler is in now and he is saying that the area shown (roughly) in red is flexing a bit much. The rest of the cement board is fine and not flexing. The reason is that the distance between the vertical timbers either side of the toilet frame  is maybe 100mm wider but also there is no room for a cross timber because of the cistern and pipes etc. 
carpenter is like ‘ what do you expect me to do?’.

help appreciated please…

[nod]

He sounds quite inexperienced 

There is hardly any flex on 12 mil CB

As long as he seals it and uses a BAGGED flexi adhesive 

It will be fine 

[PeterW]

Did they screw the frame to the timber ..? And is the timber bracketed or screwed to the wall..?? If so, it shouldn’t move at all. 

[Conor]

If it were me, I'd be lining with 12mm OSB first.

[Nickfromwales]

The frames have a purpose installed metal rail where the pan tightens back to the wall covering. The idea being all of the compression happens there. 
The cement board should be against that if the studs have been installed correctly, and there should be, ideally, a small bead of Sikaflex adhesive bonding the cement board to the frame to marry the two together.

12mm cement board is bloody solid stuff, but that is a problem as much as an attribute, as it will hold off from the frame until you tile and fit the pan, and then, due to the high point pressure applied, the tiles will either crack on the day or a few weeks down the line. 
 

There should be zero movement, certainly none that a person could detect by physically pressing / pushing the board. 
Take it off, put a straight edge across the horizontal frame members and the studs at each side and locate the issue before moving on from here. 
 

You can install the pan bolts after the bonding has been done and the CB refitted, and use the nuts and washers to put this under compression whilst the adhesive goes off ( 24hrs ). 
 

If it moves after that, put the house up for sale. 

[Gone West]

I installed wall hung toilet (not Grohe) and basin frames and clad using 12mm Hardiebacker board. I pre dilled the Hardiebacker to ensure the board pulled in tight to the frame and didn't have any movement. I had the board Venetian plastered and the plasterer would not have been happy about the board flexing. I would check that the board is tight to the timber.

 

 

 

 

[Moggaman]

14 hours ago, nod said:

He sounds quite inexperienced 

There is hardly any flex on 12 mil CB

As long as he seals it and uses a BAGGED flexi adhesive 

It will be fine 

is the tiler experienced- yes he is.

there is flex in the middle when pushed in !

[Moggaman]

14 hours ago, PeterW said:

Did they screw the frame to the timber ..? And is the timber bracketed or screwed to the wall..?? If so, it shouldn’t move at all. 

Toilet frame fixed to block wall.timber frame screwed to wall. It’s the fact that the spacing between the timber studs is wider due to the cistern and toilet frame and the cement board pushes in when pushed 

[Moggaman]

13 hours ago, Conor said:

If it were me, I'd be lining with 12mm OSB first.

Can the wall hung toilet afford to lose another 12mm of bolt length??

[Moggaman]

12 hours ago, Gone West said:

I installed wall hung toilet (not Grohe) and basin frames and clad using 12mm Hardiebacker board. I pre dilled the Hardiebacker to ensure the board pulled in tight to the frame and didn't have any movement. I had the board Venetian plastered and the plasterer would not have been happy about the board flexing. I would check that the board is tight to the timber.

 

 

 

 

Thank for that. I see the same question here really. I presume u fixed the board along the blue lines and perhaps fixed it into the horizontal metal bracket . What is backing the cement board at the location shown red approximately?

[Moggaman]

13 hours ago, Nickfromwales said:

The frames have a purpose installed metal rail where the pan tightens back to the wall covering. The idea being all of the compression happens there. 
The cement board should be against that if the studs have been installed correctly, and there should be, ideally, a small bead of Sikaflex adhesive bonding the cement board to the frame to marry the two together.

12mm cement board is bloody solid stuff, but that is a problem as much as an attribute, as it will hold off from the frame until you tile and fit the pan, and then, due to the high point pressure applied, the tiles will either crack on the day or a few weeks down the line. 
 

There should be zero movement, certainly none that a person could detect by physically pressing / pushing the board. 
Take it off, put a straight edge across the horizontal frame members and the studs at each side and locate the issue before moving on from here. 
 

You can install the pan bolts after the bonding has been done and the CB refitted, and use the nuts and washers to put this under compression whilst the adhesive goes off ( 24hrs ). 
 

If it moves after that, put the house up for sale. 

Thanks Nick. See again unedited version of what carpenter did before cement board went on . Metal frame flush with timber studs. Can he screw the cement board tight to the metal frame using a metal screw and tighten it around the pan bolts that way?. See green lines below .

I still don’t see how the carpenter could have put in anymore horizontal studs between the toilet frame metal uprights with the cistern In the way etc!.
am I understanding u right in that the area around the bolts in where the compression will occur when the plumbers tightens the pan to the wall and if the cement board is flexible there, it will crack… what about 12 inches above that near the cistern…no compression, so no issue? 
One last question please…what is the order in relation to the work here…tiler tiles the wall and tiles into that white box( the flusher )?. Plumber then takes off the white box after tiling and fits the flush plate. I just worry the plumber will blame the chipper for some issue with the flush plate in the future?. 
thanks 

Thanks

[nod]

1 hour ago, Moggaman said:

16 hours ago, nod said:

He sounds quite inexperienced 

There is hardly any flex on 12 mil CB

As long as he seals it and uses a BAGGED flexi adhesive 

It will be fine 

is the tiler experienced- yes he is.

there is flex in the middle when pushed in !

Expand  

He’s over thinking it 

Tell him to get on with it 

[Conor]

9 hours ago, Moggaman said:

Can the wall hung toilet afford to lose another 12mm of bolt length??

Easily. 

[Gone West]

10 hours ago, Moggaman said:

I presume u fixed the board along the blue lines and perhaps fixed it into the horizontal metal bracket . What is backing the cement board at the location shown red approximately?

It was screwed around the the timber frame and I attached packing pieces, a few mm thick, to the metal bracket to ensure the Hardiebacker was flat and couldn't move when the pan was attached. There was no support for the Hardiebacker in the area marked in red and there was no movement.

[Moggaman]

Thanks all. A simple fix really…. Ye might not agree with it.. drilling a few holes in cement board and sprayed expansion foam into it. Has hardened it up substantially.