I have read several threads where it is not recommended to tile over ply walls. I have only recently started my business after many years diy-ing and, although I am pleased with interest and work so far, I don't like to turn anything down at this stage. A friend of a friend has developed/extended his house and asked me to quote for 2floors and one 16sqm bathroom wall job. However, the bathroom walls are 50/50 backerboard and ply. The ply is machine nailed and I know the owner will not remove it all and replace with backerboard or plasterboard. The wall over the bath is ply and has a little deflection at one join. Would overboarding this wall with backerboard work, ensuring it is well screwed into all possible support noggins and also to the
ply at 200mm intervals?
One of the other walls is rock-solid but is slightly out of level with a 2-4mm hollow a one point. Would it be ok to build it up with rapidset and should I prime it first?
All suggestions of alternatives to ripping it all out and replacing with pb or bb very welcome.............

The ply being nailed is'nt a good idea, I would advise to change the ply for BB. You don't mention the thickness of the ply

Its 18mm ply, owner says ply is machine nailed with what looks like pretty heavy duty nails The wall with deflection is plain wall over the bath, the other wall has window, vanity unit etc and i am happier it will not flex. Would 6mm backerboard over the 18mm ply not be enough if properly screwed through ply to supports?

The 18mm ply is on now, I don't like ply on walls. Prime the ply with sbr, 3-1 and try and make it as water tight as possible, even tank it if you can. Screw the ply to the stud work, nails are a liability. where there is deflection that must be sorted before any tiles are fixed. Use a cement/flexi based adhesive and grout. You would have to put on 12.5mm backer board, not 6mm on the walls. Builders, don't you just love em.
IMO, others may offer you differing advise

same as Whitebeams last post for me too

Thanks guys, although another 12.5 + 8mm tile/adhesive is not going to be good news for him for wall depth over bath.
On the 2nd question can I confirm it's ok to build up hollows (max 5mm) on other wall with rapidset over sbr prime as above??

Should be fine mate

spoke to biscem today about the same problem. they basicly said dont touch it with a barge pole. no amount of sealer will stop the ply from moving & the tile adhesive will eventually come away from the ply. best to check with the tech dept of the adhesive. they are allways helpfull.