Bathtub and wall transition and waterproofing.

[DIY345]

My daughter recently bought a home built in about the 1950s. I have been helping with the renovations to help her save money. My daughter has back trouble which required changing the bathtub to one with better back support.

The walls in the house are plaster composed of gyproc wall board overlayen with brown coat and white putty coat plaster that is about one inch thick. I have broken away the plaster up to about 6 to 12 inches above the tub to enable its removal.

At first, I was going to break out the walls around the tub and sheetrock them, but then, its hard to stop as the wall at the head of the tub has a window in it designed to fit the 1 inch thick plaster walls. And I wouldn't know how to fix that. So, I have decided to try and clean up the plaster I've broken out by making nice straight cuts with a sawsall and then putting in a double thickness of cement board to fill in the gap between the plaster and the flange of the bathtub. Then I'd like to install a plastic surround.

Question is how do I make the transition from the cement board that will be about 1 inch thick to the tub flange that's about 1/4 inch thick? How do I water proof it as it was apparent that there was leakage and mould between the old metal tub and the plaster?

[havasu]

Without pictures, this is going to be tough. My recommendation is to bust out all the old plaster on the wall, reinforce the studs, and apply a cement board with Kerdi down that wall to the lip of the tub. Kerdi is a complete waterproofing layer, if done correctly.