Cracked Substrate

[Boondocker]

Afternoon all. Have recently said hello to theforum and finally got around to posing some questions on a bathroom refurb.
Firstof all, having removed the 30 year old cappuccino coloured bathroom suite withmatching tiles I am left with plastered walls that have a nice selection ofcracks across them.

In oneof the photos where the wall has been channelled out to accept the edge of abath you can see the vertical crack goes through the plaster and in to theblock work (it does not go all the way through to the other side). You can alsosee that the bonding and plaster has not adhered to the wooden plate upon whichthe block work has been built.

Myintention is to put a low level (40mm) shower tray with pumped shower where thebath used to be, tile walls and floors with 60 cm x 30 cm glazed procelaintiles (21kg/m2).

Firstly,any opinions on tiling this wall? Any backer board, Orbry, Marmox, Kerdi etcwould have to be dot and dab. As the max load for plastered walls is 20kg/m2 itcouldn't bear the weight of backerboard plus tile/grout/adhesive. So, alsobearing in mind the wall's condition would mechanical fixing through to theblock work be the only option?

Thefloor appears to be sound and level. All original flooring has already beenripped out and is to be replaced with 22mm WPB marine ply,then Ditra Heat matand then tiled.

Soundslike a simple plan after all the thinking and if I don't get a move on then myother half will be restricting bedroom access until it's finished!

Allcomments and criticism gladly accepted.

​https://www.dropbox.com/s/pjfv0ogp4wy3x1d/bath wall3.jpg?dl=0
https://www.dropbox.com/s/x1tgdaa26cdqj1i/bath wall 2.jpg?dl=0
https://www.dropbox.com/s/53wubeihtl1mhol/bath wall 1.jpg?dl=0

 

[widler]

Knock the plaster off, PVA the walls, dryline it in plasterboard/backer board ect ect ect and tile away .
It's plastered onto thermalite block, it will fall off

 

[Boondocker]

Thanks, Widler.
I'm happy to do that. But, it ain't gonna fall off. What's on there is well and truly stuck on.
What I don't want to do is start knocking it off and take the block work with it.

 

[widler]

Ok no worries, do the same over the plaster but stick on with adhesive and plug and screw , looking at it it looks flatfish so minimal adhesive, level it, let it set and plug and screw.
Personally I'd use plasterboard, easier to handle, comes in bigger sheets and cheaper

 

[White Room]

The wall blocks are Thermalite, they should never have plaster/render applied to them..cracking is standard.
Might be worth tapping to see how hollow or solid they are before fixing anything to them or removing.

 

[widler]

He has already said "what's on there is well and truly stuck on " mark , I've already said " it's plastered onto thermalite, it will fall off"

I don't get what your saying mate [emoji15]

 

[Boondocker]

I've just given an area of plaster on the wall a hefty whack with a hammer and chisel and I can say that it ain't coming off unless the blocks come with it.

What i've noticed is that some of the finish plaster has a residual substance on it. If you wipe the wall with a damp sponge then some areas darken as the water soaks in and other areas don't, could that be residual PVA or something else and will it prevent a good adhesive bond? If it is PVA how can I get rid of it?

Does anyone have any comment re 22mm WPB ply and then laying Ditra heat mat on top then tiling. Is it better to glue and screw 6mm board on top of the ply then continue as planned? I am trying not to raise floor level too much.

Cheers for any further advice.