Designing Kitchens and Bathrooms - Skim coat with thin set in shower?....

We have a ceramic tile shower surround that was installed in the late 50's. It seems they used plaster board with a skim coat of "something" over it (seems plaster like) and then the tile installer used a mastic to set the tiles.

I have a spot where the grout came loose (original grout, that I am going to re-grout) and the moisture that got behind it rendered the mastic useless. I took some of the area that was real bad down to the studs and re-installed with cement board. The outlying areas are still solid except for the mastic which I removed. I don't really want to get into removing large amounts of tile and replacing with cement board. I was wondering if putting a layer of thin set over the plaster board would be of any benefit before then resetting the tile with thin set mortar?

Thanks.

Plaster board, plaster and drywall - whatever it is you have there is not a good backer for tile in a shower. Additionally, mastic should not be used. Tile and grout are not waterproof, and neither is thinset so skim coating the wallboard with thinset wont help. What you have there are tell tale signs that this shower is failing and needs to be replaced. I know its not what you want to hear but thats whats goin on. The best chance for the patch to last would be to use a paint on type waterproofing membrane like redgard over the exposed wallboard. It will keep the water from reaching the wall board in the area you have repaired. You can tile directly to the redgard, its made specifically for this. The bigger problem will be the old unprotected areas of the shower that will probably not be to far behind. I hope Im wrong. Good Luck.