Build up for a tiled tub surround

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highup

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Working on a floor, but the studs behind the tub wall have rotted. This is a 1960's style tile installation above the tub. You know, the kind where they used mastic to adhere the tile to sheetrock.
Well, that's finally failed
Layout c crop.jpg
Layout c 350.jpg
 
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2nd image shows the room. The first image is where I have my questions. It's just the red circled area enlarged.
OK, if I get it down to bare studs, contractor mentioned it is a good idea to shim the studs a hair so tha backer board has a tny gap behind it.
Doing that, the backer board has a slight gap behind it right at the tub flange where it's nailed to the studs. He said fill that gap with silicone before putting the backer board against it.
He also said to leave a 1/8" gap below the backer board........ so the backer board doesn't rest directly on the tub.
He said do the same with the tile, then caulk or grout caulk along the lower edge of the tile.
Not done yet. :D
I found a tile guy to come look at it tomorrow.
He said he's put 15 lb felt on the studs, the backer board............. and I swear he said 1/4"
dat sounds way thin to this carpet dude.

Now, the real questions. Is any of what I described correct?
If not, what's the correct way to seal the wall cavity and build out the wall with backer board?
And is there any "wrong" type of backer board for tub walls?
 
Doing some googling, I read on one site that you install the backer board 1/4" above the tub flange, the fill that large void with thinset when installing the first row of tile.
If you install the board 1/4" above the flange, how does one seal the backer board from the stud in that transition area?
 
But if the backer board has 15"# felt behind it, what about the gab between the backer board and the lip of the tub? That;'s why I'm wondering if the backer ought to overhang the lip of the tub.
 
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Backer rod goes under the cement board, tile goes even tighter, the fill with grout caulk to finish iff.
 
I'm the organizer general. :rolleyes:
I don't know how the seal was done between the tub and wall when the place was built, and I want to know how it's supposed to be done. One web site I saw said to stop the backer board at the top of the tub flange, then to pack the big gap between the tub and the backer board with thinset.
Another guy told me the backer board may need a thin shim on the studs so the backer board slightly overhangs the tub lip............ he also said to let the backer board drop to within 1/4 inch of the tub and totally fill the void with high quality silicone.
My part is the floor......some day. This should have been corrected 5 or 10 years ago when there was no tenant. Now the place is part torn up and there has to be some organization with the plumber and tile guy so the bathroom isn't out of commission for a week.
 
The guy is correct about the shim, I usually never need to shim it I just support the CBU with something to there is about 1/8 in gap or so and stuff the closed cell backer rod in there, then use matching caulk to fill it. The rod keeps you from using to much expensive grout caulk. This is the best image I could find.




Installing a Backer Rod.jpg
 
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