There seems to be a split between those who say the backer board should go over the tub flange and those who say it should go above. Those who say over say so for drainage. Those who say above seem to suggest because of the surrounding drywall plane.

The tile expert forums go with it over the flange; seemingly the general contractor forums suggest above. Most forums suggests a vapor barrier even with Hardi Backer.

How does a person make the right decision with so many conflicting viewpoints? I know in other posts, the two options are covered; however, little is discussed with regard to the problem of going over the tub flange causing the drywall plane to be uneven. Shimming for example over 1/4" would seem to be a problem blending the CPU board with the drywall. How is that done successfully?

  • If you want to know how to deal with the potential problems caused by one installation method, you should ask a question about that. This question (in my opinion) was asking which of the two installation methods is correct. If that was not your intended question, please ask a new question, and be clear about what your looking for in an answer. Trying to refocus an existing question (especially one with answers) is difficult, so it's easier just to abandon the question and start again.
    – Tester101
    Feb 26 '15 at 11:29

If the bathtub flange is too proud of the studs to allow the backer board to fit acceptably, the studs should be shimmed out so they are even with the flange. If the manufacturer of the tub says so (and they probably do), the backer board should go over the flange. Find (or make) some wood strips of suitable thickness, and glue or staple them to the studs above the tub flange.


I found that the bathtub was not close enough to the studs for the backer board to go below the flange without bending one or the other.

So I put backer board down to 1/8 inch above the flange with vapor barrier behind and down into the tub enough to protect it from dripping thinset, and placed the tile down to 1/8 inch above the tub. That way everything worked out great. After the tile and grout set, I cut the vapor barrier, cleaned, and caulked.

If the flange had been closer to the studs, I might well have placed the backer board lower to the tub. In the end, it is only about a 1/2 inch difference. Not enough to lose any sleep over.

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