Hello. I wish to tile my bathroom walls with 600*300*9mm glazed porcelain tiles. I weighed them and they are 20kg per sq metre excluding adhesive. After painstakingly removing all the plaster from the brick walls in the bathroom and cleaning the wall thouroughly with polydisc I had it rendered with cement render. It was mixed outside in a drum mixer using plastering sand, especially formulated for dampproofing applications. Portland cement and was mixed with sovereigh waterproofer and sbr.
6 weeks on the walls are totally dry but the colour is very uneven. Some areas are a mid tone grey and others are dark like cement. If I brush the dark areas, cement powder brushes off. Areas where the coat is thinner, e.g. over a pipe chase are lighter grey. I cut out two small samples to see how solid they are and compared with a lump of plaster the dark piece was about the same and the light piece a little tougher.

Have they messed up the mix or is this normal? Is it likely to be strong enough to hold the tiles I want to use? Should I do anything to it, e.g. board with "no more ply" or will it be ok to go onto as is. I may try sticking a tile on under where the bath will go as then see how easy it is to remove. Will any damp penetrating though the grout later from the shower cause any problems with this substrate?

Any advice from you experience would be gratefully recieved.

Originally Posted by bureaucrazy

Hello. I wish to tile my bathroom walls with 600*300*9mm glazed porcelain tiles. I weighed them and they are 20kg per sq metre excluding adhesive. After painstakingly removing all the plaster from the brick walls in the bathroom and cleaning the wall thouroughly with polydisc I had it rendered with cement render. It was mixed outside in a drum mixer using plastering sand, especially formulated for dampproofing applications. Portland cement and was mixed with sovereigh waterproofer and sbr.
6 weeks on the walls are totally dry but the colour is very uneven. Some areas are a mid tone grey and others are dark like cement. If I brush the dark areas, cement powder brushes off. Areas where the coat is thinner, e.g. over a pipe chase are lighter grey. I cut out two small samples to see how solid they are and compared with a lump of plaster the dark piece was about the same and the light piece a little tougher.

Have they messed up the mix or is this normal? Is it likely to be strong enough to hold the tiles I want to use? Should I do anything to it, e.g. board with "no more ply" or will it be ok to go onto as is. I may try sticking a tile on under where the bath will go as then see how easy it is to remove. Will any damp penetrating though the grout later from the shower cause any problems with this substrate?

Any advice from you experience would be gratefully recieved.

Cement based render should be pretty damned hard after 6 weeks. SOunds to me like it was not mixed properly - My guess is that if they mixed the SBR with the cement forst and then put the sand in it has balled up in the mixer and not mixed through properly - I say get them back to sort it out cos it don't sound right to me.

Ajax is spot on with his post, what concerns me is the plastering sand and all the admix. Waterproofing and sbr, I've used sbr in a mix and it's like using glue but would'nt add the two together. If you are rendering for a shower you would need a good quaility sand which would be well graded and well mixed. The choice you may have have is use a sbr mixed 5-1 with water and let soak into the wall to bind the loose areas of cement.

hi not good causes to much addy .poor mix ratio 3 to1 walls rec .walls may of been to dry on application and dried render out to quick. no real patch for it apart from removal and re do if its not structualy sound now dont risk it with tiles .(any cracking )

yeah, sounds like a balled mix to me too (as Whitebeam, come from a plastering/tiling background). The problem being the stage at which water was added during the mix. To late it balls up and you can't get a homogenous mix and its made up of bits of sand here and bits of cement there, and nothing uniform in its' consistency