Once. The professionals as you call them is the reason I had to redo it. I would love to be able to hire someone. I believe I am about $40,000 in the hole because professionals were hired to build my house.

Grout lines will never be waterproof regardless of how thin they are. You should use proper underlayment and tarpaper is not it. How about checking one of those tiling guide books out of the library?

If water is leaking at the tub drain and out to floor beyond that tub,,, nothing you do to bath floor will prevent rot. Certainly not GROUT.

Sounds to me like the bath is a do OVER.

Greg use 1/2” cement board make sure you thinset it down as well as screw it. Then along the floor next to the tub gets some schluter Kirdi strips make sure you caulk it along the tub edge, those strips are like 6” wide.
Of course water does go through grout but not much just make sure after tiles done you also caulk the edge along the tub!! Over 40 years in the business that will work

If you are going to attempt a complete bathroom gut, as needs to happen, at least read the industry bible to see how things should be done. Hacks from hacks never work.

https://www.tcnatile.com/products-and-services/publications/218-english-publications/188-handbook.html

IMO you don’t know what you don’t know . Get a pro before the house rots out from under you.

"Once. The professionals as you call them is the reason I had to redo it. "

No greg you may think you hired professionals but you did not. Now you are gathering pieces of misinformation to confuse the matter even more.

1) 1/8" plywood does not exist. perhaps you meant 1/4" luan under the vinyl.

2) Shluter Kerdi or Ditra? seeing you mentioned Kerdi that would mean you are applying to cement board as Kerdi can't be applied to plywood.

3) Tar paper / thinset mortar /backer board / thinset mortar / tile is a recipe for disaster.

Sorry two attempts and still full of bad info. Stating a method that is 100% wrong is great confirms what others stated. Hire a professional tile setter.

Just a thought. Perhaps correcting the actual cause of the water getting outside the tub would be a good idea:)

Thanks for input. NOT 2 attempts. One attempt. I was repairing damage that a 5 star highly rated professional did. I believe in the theory that that more answers you get, right or wrong, the closer you get to the correct answer. The last time I hire a professional to install AC unit in the attic. I had to repair two bedroom wall and a ceiling. I would love to hire and pay good money to a competent professional. Sorry to say the is very little recourse on bad workmanship.

https://www.ceramictilefoundation.org/homeowners-guide-to-hiring-qualified-tile-installer

• Tiling 2nd Edition
• Tiling Complete: 2nd Edition (second edition of Tiling Complete)

More useful than insults.