Hi I am in the process of repairing a stall shower where originally ceramic tile was installed on water resistant sheetrock.The shower has a fiberglass base which I am going to leave in place.Due to leaks in the bottom part of the shower,the sheetrock behind the tiles rotted and disintagrated and needs to be replaced. I am considering replacing the bottom two feet of damaged sheetrock with new ceramic tiles installed on hardie backer board.If what I read is correct I should set the hardie backerboard 1/4 inch above the top of the shower base flange and put some form of waterproof material in back of the hardie board. If the above is correct my questions are as follows:
1)should the waterproof material overlap the shower flange in any way or should it just be cut off at the end of the hardie board?
2)should I apply either mastic or silicone between gap of shower base top of flange and where hardie board meet
3)when I bridge the bottom ceramic tile across the gap beteen the hardie board and top of shower base should I apply mastic behind the lower portion of the tile where it presses against the inside of the fiberglass base or silicone ? I know you have to leave a small gap of maybe 1/8 inch between the fiberglass base and bottom edge of tile for application of the silicone.
Any help would be much appreciated jim

It's true that most of the water in the shower will hit in the bottom two or three feet of the walls and on the floor. However, there's a reason that pros will waterproof up to the height of the shower head.

Water vapor will penetrate the grout up much higher than three feet, just not to the degree that it does in the lower half. How sure are you that there is no mold or damage in the upper part of the shower that you intend to leave untouched? If you knew right now that in two or three years you would have to tear out the walls all the way up, would you proceed as you plan to right now?

To answer your questions, though:

1)If you put 6 mil plastic on the studs it needs to extend over the flange.

2) Use silicone to fill the 1/4" gap between the shower base and the Hardibacker

3) Since this is only a 1/4" gap it shouldn't be an issue, unless the tile are very small. However, you shouldn't be using mastic in any shower application. Use thinset mortar that comes in a bag and must be mixed with water.

Kman is correct, nothing can beat the old way, kraft paper wire and motar float would be the best way, if thats not an option than I would replace the entire shower with hardboard

No one was suggesting an old fashioned mud job here.

I would gut the entire shower, at least all the walls. The difficult part in a completely tiled shower, floor & walls, is the pan. Since this has a plastic base why patch it up? It's gonna look bad. For one thing the tiles are not gonna match even if you had enough left from the original batch. I do NOT repair showers that are built with wallboard.

Jaz