hi all first ever post on here but i need advice regarding a job were on,

scenario : preparing a wetroom floor and tanking walls in shower area customer has her own builder applying the marble slab to the walls,
slabs are 20mm thick and 1800mm x 1200mm,(very big walls solid brick) the original background was lath and lime plaster daub. this has been removed and i suggested wedi board dabbed and doweled onto the brickwork as i believe this is probably the strongest bond available i also suggested they mechanically fix the slabs with pegs and wire, thursday got to the job and the builder has screwed marine ply to the brick work at 600mm centres so about 8 fixings in each board.he swears blind that this will be sufficient to hold the weight and wouldn't trust wedi boards, so tommorrow i'm going to tell the customer i don't think this guy is doing the job correctly and raise my concerns! all i want to know if anyone on here has had to fit marble of this size and am i correct with my recommendations

Plywood will only hold about 30kg per sqr mtr....Deffo no good for those slabs...

A cement board is the way to go...or direct fix to the brickwork..

How high is the tiling to be..?

hi dave, 1.2 mtrs high and 2.4 mtrs where the shower is so two pieces high, the shower area is a walkthrough, in carrera marble which i think is 50kg per mtr2

You are dead right to raise these concerns - builders usually know next to nothing about fitting tile, stone or anything in our trade correctly, and use the word ' just ' as soon as they start on it. I would want a stone fitter to do it, not just change the method!
If it is a whole wall covering, the slab work could be adhesive and mechanically fixed direct into the brickwork - set off to make its own plumb face plane. The Wedi route would almost be irrelevant, and a render coat to even the brick face would be the sound choice if required.

Hey we're all over this one for you tubsy....and its a shower too! - see what I mean about builders! I think in those areas it should be rendered over, with a waterproofing additive or a DPC prior to tiling for best practice.