shower grout failure ?

[John.Stitch]

Well I have been removing tiles and tidying up.
Questions about joints.
On the blockwork wall
First I will glue and screw the Marmox to the wall.
The tray (which has a lip on the base to prevent overspill) abuts it and is 750 long.
I will use 3 tiles to cover that distance (250x330).

On the last tile will go the enclosure frame up the wall which I will seal up the wall with mastic

Going away from the tray along the wall plasterboard starts the lower half of which is tiled and the upper half is painted which was not skimmed.

So can I get away with butting the Marmox against the plasterboard and filling any unsightly gaps or do I need to tape it and if so the tape will show on the painted half so is there a way of concealing the tape perhaps by scraping away some of the top surface of the plasterboard and then applying the tape as the plasterboard is not tapered at that edge.
It does not sound very sensible to scrape away as the plaster underneath will be revealed and not a good idea to tape that.

The water should be contained by the enclosure frame and the Marmox.

On the stud wall similarly the Marmox will abut the plasterboard so do I need to tape it or not as the whole wall is tiled?
Marmox extends beyond the tray by some 3".

 

[Richard C]

Make sure you use plenty of fixings every 300mm, the fixings must go through the adhesive dabs or you’ll distort or even crack the board.
Have a look at the video clips here;
http://www.pureadhesion.co.uk/marmox-1250x600x10mm-insulated-tile-backer-boards.html
but I use more fixings than they show.

On the last tile will go the enclosure frame up the wall which I will seal up the wall with mastic

Don’t use mastic; use a bead of decent sanitary silicone. A bead of silicone should also be used down any internal corners of the shower enclosure rather than grout; is tends to crack where the two walls meet & will let water in.

Going away from the tray along the wall plasterboard starts the lower half of which is tiled and the upper half is painted which was not skimmed. So can I get away with butting the Marmox against the plasterboard and filling any unsightly gaps or do I need to tape it and if so the tape will show on the painted half so is there a way of concealing the tape perhaps by scraping away some of the top surface of the plasterboard and then applying the tape as the plasterboard is not tapered at that edge.
It does not sound very sensible to scrape away as the plaster underneath will be revealed and not a good idea to tape that.

Butt the Marmox fill & tape (you can use cement powder adhesive for this). Then you really need to skim the PB beyond the tile line; unplastered PB is definitely not a good idea in a bathroom &, again, it should really be MR boards (needs priming before skim) not standard wallboard. This can be done before tiling, draw a line on the board where the tiles start & get the spread to skim down & waste in just beyond the line, leaves a finished plaster wall ready for paint after tiling.

On the stud wall similarly the Marmox will abut the plasterboard so do I need to tape it or not as the whole wall is tiled?

You should always fill (again use cement adhesive) & tape board joints.

 

[John.Stitch]

Thanks Richard

The tray is mounted on WPB and 4 lengths of 6x2 but not secured to the floor.

It is a concrete ground floor then DPC and the 6x2 rests on the DPC.

No fixing of the 6x2 to the concrete floor were made when it was installed as the tray and wooden support were thought to be heavy enough not to move.

The tray will move slightly if pushed as no tiles or board limit it currently but it does require some force due to the weight.

Is this a sensible approach?

I could add some wedges between the 18mm chipboard floor and the 6x2 but the floor is floated so I'm not convinced that would do much.

 

[Richard C]

Is it a new or are you re-using the old one? what’s it made of? If it’s stone resin, it must be supported over the entire base using a weak mortar mix (usually stipulated by the manufacturer as a warranty requirement) or there is a big risk of it cracking. GRP trays are more usually bedded on silicone or have support legs. It’s important the tray doesn’t move as this will cause continual failure of the silicone bead, allowing water in & then your back to where you started your first post.

 

[John.Stitch]

Thanks again Richard
It is a cast resin tray and the same one I helped to install 5 years ago.
I separated the plinth from the tray (brown mastic was used).
The plinth does flop somewhat on its end with the 6x2 hanging off it.

I'll redo the plinth so that one length screws against the studding and another is screwed to the blockwork wall and I'll use thicker ply as it was only 9mm and I'll use a weak cement mix.
Presumably just to give it a bed to sit on so the muck can set around it and thus hold it well.

What ratio is used 7:1?

I'll have to reroute one 15mm pipe for the cold basin tap as it sits against the studding and then to check it is level before adding the mortar.

 

[Richard C]

What ratio is used 7:1?

That’ll do or even a little weaker; don’t mix it too wet, just enough to allow you to bed the whole underside of the tray down onto the ply. The idea is to provide full support across the base & it doesn’t need to be that deep; don’t use sharp sand. Stone resin trays are basically cement with fibres added & a polyester resin face that you stand on, they are very brittle & won’t suffer distortion or point loads at all well.