tile backer on top of dot and dabbed plasterboard ? - Tile Advice

[Rob713]

Hi all.
I'm looking for advice regarding the tile becker fitted on the plasterboard.
It's about load capacity. How to increase the one on the plasterboard? Is it feasible without removing the plasterboard??
At the beginning of the job the customer wanted to go for 100x300 tiles so we've installed the MTR plasterboard across the bathroom but he's changed his mind at some point, and now wants porcelain tiles 800x800 (16.7 kg per tile which is 26.09 per sq m if my maths is correct Would it be ok to install e.g. 6mm marmox/hardie/kerdi on top of existing boards on a full bed of tile adhesive and additionally install screws into the solid wall underneath with washers every 300mm?
Thanks in advance

 

[Dave]

Pb can hold 32kg per sqr mtr.

 

[Rob713]

Adhesive is 1.05 kg per mm per sqm plus water so - let's say 1.5kg. At 6mm it's 9kg per sqm . With tile - 35 kg per sq m. Slightly above the limit. Can I ignore it?

 

[Trigger]

If I am reading this right, you want to fix tiling board onto plasterboard. If so I would prime PB, fix tiling board with tile adhesive and then fix through into brick/blockwork with screws and washers.

 

[Rob713]

If I am reading this right, you want to fix tiling board onto plasterboard. If so I would prime PB, fix tiling board with tile adhesive and then fix through into brick/blockwork with screws and washers.

Yes, exactly. That was my plan. The point is when I've spoken with a marmox guy he said that I can't do this like this as there's no full bed of adhesive between the pb and solid wall and there will be no warranty whatsoever on such an installation. And installing screws with washers won't help. That way I went here to seek advice from tilers. I'd like to use Marmox' The Tilebacker - the one with mesh only on one side. Based on info from their leaflet that should work.

 

[Trigger]

Interesting, I'd like to know the reason for that. If you think about it there is not a solid bed of PB adhesive behind PB on a stud wall either, you obviously have the voids between the studs.

So with your dot and dab PB if you fixed rows of screws and washers at 400mm centres and at the tiling board edges is it any different to fixing to a stud wall. If the PB has been installed correctly in the first place it should have rows of dabs at 400 centres and around the perimeter anyway.

 

[Rob713]

Trigger, thanks for your time.
That was my way of thinking either! When I asked marmox guy what will be the weakest point of the sandwich created on the wall he said ... "screws". Because part of the screw won't be fully embedded in any material (he meant that gap between solid wall and plasterboard). I'm not too sure what to do next.

 

[Trigger]

Every day is a school day as they say. I’ve done it many times albeit not with Marmox boards.

The only other option I can suggest if you don’t want to go down the current route is to remove the dot and dabbed boards and baton the wall out.

Someone else may be able to offer other suggestions.

Keep us updated.