Hi, it's a shame it was skimmed in the first instance as plastered wall has a weight limit of 20kg/m2 but the plasterboard itself has a weight limit of 32kg/m2, so it would have been stronger without the skim.
If you are just using plasterboard then it should really be tanked out to prevent moisture getting to the plasterboard and failing.
I would prefer either tanking it or using a backerboard rather than the plasterboard in the first place as the money that has already been spent on plasterboard, plaster and labour skimming it could have been put towards the backerboard or a tanking system.
Nomorenails and hardibacker are just two backerboards at the lower price end of the market that do a 6mm board, but as they are meant for flooring at this thickness they would have to be put against something to stop them flexing and cracking the joints or the tile itself as time passes.
There is a thicker hardibackerboard at 12mm that is meant for walls and would have been better used than the plasterboard and should be used if you are thinking of taking down the existing plasterboard and replacing.
I would also check the manufacturers instructions as I think most backerboards are water resistant rather than waterproof, meaning they are resistant to water so wont degrade and fail from contact with it but moisture can/does pass through some of them to what ever is at the other side. I spoke with customer services a few months back at Knauf re aquapanel and they advised this is why they recommend stud behind their boards, so if or when moisture passes through them it has an air gap to dissipate naturally.
The tanking system waterproofs the area to be tiled and I think one of the cheaper ones widely available to anyone is the Dunlop Shower Waterproofing Kit from Screwfix at £43.00, but you might get a similar/better trade deal on a bal, weber etc. kit from your local trade supplier, as their kit may contain more product for the money, which if left over can be put toward your next job, saving a bit in the long run.
If you are going by industry standard for the weight of tile on the wall it may also be worth noting that it’s recommended that new plaster is left a minimum of 4 weeks before tiling commences.
It’s ok if the client wants to keep their costs down, but it will be you who is the first port of call if it fails, wanting money back or the job putting right. The adhesive/tanking/backerboard manufacturer may back you if their product has failed, but not if you haven’t followed industry standard or manufacturers guidelines.
Sorry I’ve written a lot, of which you may already know or may not even agree with but hopefully some info may help.
Cheers
