Strengthen old plaster wall for heavier tiles

[Tatyhead]

I'm renovating my bathroom. Completely stripped down to joists old fitments out all old tiles and adhesive cleaned down left with good surfaces. The area around the bath / shower is existing exterior ply which had been primed. The tiles had been on for 25 year but the bottom edge of the ply had suffered some decay as a result of water ingress so have cut that out. In his area I want to improve the water tightness so thinking screw waterproof backerboard over the ply into the studs.

The tiles are 600 x 300 Porcelain floor to ceiling.

Q. Any thoughts to support this idea or alternatives please. What thickness board/make
Q. What are the prefered methods for sealing between ply,backerboard and tiles at the junction with the bath.

The rest of the bathroom is 1930's plaster ( I think lime ) over breeze block. The previous tiles went to dado but were light by comparison to porc as a result I am not totally confident regarding the plaster integrity to support the weight .Doorways and architraves etc restrict options for battening off so. Any thoughts please. Considered aquapanel but can't as it's not suitable to be fixed direct to plaster with no air gap behind.
I knocked down a wall in the hall yesterday and the plaster just seemed to fall off with a tap.

Q. I think we can mechanically fix with thunderbolt into the breeze block but what board for loading. ?

It's never simple for meeeee or is it.

 

[martin1c]

If you want to do the job properly you might want to remove the existing plaster. As you say, it will probably fall off given a bit of help. You can use ordinary 12.mm plasterboard dot and dabbed onto the walls outside of the bath/shower. Tile straight onto the plasterboard as this will support 32 kg sqm.

Around the bath/shower area you can screw 10mm marmox boards to the joists. Seal the joint line with silicone to the bath. Once you tile down to the bath lip silicone again.

Good luck

 

[Ceramico tiling]

I would prefer to use Wedi(fibre glassed) boards direct to the studs and then tape and waterproof all junctions.I may be wrong but there may be mould issues with screwing onto ply in a bath area.I would then use hardy backer boards for the rendered part and screw them all the way into the blocks.I would also stick them with wettish mix of good addy.

 

[Tatyhead]

Great quote spot on. Thanks for the input.