My buddy is doing some work on his bathroom and he's running into a jam with his tub. When his 1-piece tub was installed 20 years ago, the floor was tiled underneath the tub and there is a gap between the floor and the base of the tub. The owners put one of those cheap rubber adhesive strips down to cover it up. (see pic)

My friend will be tiling right on top of the existing tile but he's not sure if that will be enough to hide the gap. Well, between the mud and the tile the gap will be covered. But if he can't clean up the gunk that was left by the adhesive rubber strip and years of dirt buildup, the bottom will look like crap.

He doesn't want to break up and remove the old tile because that won't help the problem with the gap. He doesn't want to put base trim or any type of wood at the bottom.
He has thought about laying Durock before the new tile and that would cover the gap... but then his bathroom floor would be an inch higher than his hallway floor.

Do you guys have any suggestions?

While I disagree with installing new tile over ld tile, the gap problem is easily resolved with caulking which should have been used in the first installation. The "old crap" can be removed with a razor blade.

Check to see if the tub floor is out of level towards the tub in relation to the door opening. If it is lower that's great.
Second you don't put tile over tile in a bathroom floor.Rip up old tile Install cement board for tile or ply if that the case for new CLEAN sub floor. Use a small piece of 1/4 luan to build up lower part of floor under the cement board or the ply. If you need to feather/slope the imperfections there is a cemtious floor patch that can be feathered and done right on top of the sub floor that the tile can set on or under the floor. It sticks to wood as well as cement board. This would really help solve the problem. Even if you needed to do a 1/4 to 3/8 inch overall slope to the tub. Again that could be accomplished with the "henry" floor patch/leveling that you can get at Lowe's. I do tons of rehab and there are TONS on floors that are out of level. The most important thing is getting the floor FLAT. And that what it noticed.
From the picture it already looks like a dip in the tile in front of the tub. I would tear up the tile alone just to check to see if the tub Trap is not leaking there and that would explain the separation you have there now from the joist/floor getting slightly wet over years or the time it was leaking at one time.
Don't try to go at this half way and think caulk and trim etc fix the sub floor dips and old tile gone

Signed 21 Boat

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Ive done a lot of tile and I promise if you lay tile over tile you will not be happy with a year from now. 21boat is absolutley right about the tear out it gives you a chance to fix sub floor problems before they get out of hand. I have never ripped out a tile bathroom floor that didn't have some water damage underneath and the tub is one of the most problem prone areas. If I've missed anything or if you have more questions just post back . GOOD LUCK, AC

Thanks guys. I just sent a link for this thread to my buddy. I'm sure he will follow suit.