I'm installing a new bath tub and shower. I've removed the old tile down to the studs.

Should the backer board be in front of or behind the tub flange? I put it on the rear tub wall from floor to ceiling at first, but I can easily remove it if it should go OVER the tub flange.

  • It is really hard to tell what you are describing. Is your tub installed? Are you saying the backerboard is hanging over the flange a little?
    – DMoore
    Sep 23, 2013 at 4:18
  • 1
    Please don't shout - and I really can't edit your username Sep 23, 2013 at 4:33

4 Answers 4


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Let's look at the three pictures above.

#1 - This is probably the most common install. You cement board is on the same plane as the flange. It is thicker so it ends up hanging over.

Tip - If you are using cement board by itself I would strongly suggest using 1/2 inch. Also I am a strong supporter of hardibacker since it doesn't allow water through or hold water. Durarock is a great product but I use hardibacker in wet areas.

#2 - Some people are going to look at this and say never do it. Well it is done. Two main reasons. First is the tub tub deck isn't exactly flat. Some bow towards the middle. This allows the backer board to almost sit it on some spots and maybe be 1/4-1/2 inch up in others. The second is smaller tiles. If you are going small glass mosaic all the way up not only should you do this but it is a must. If you stagger the backer the bottom tiles could move if hit near their bottoms (especially if you are caulking the deck to tile transition).

Tip - In a lot of my tub surround installs I start with 1/4" drywall, plastic sheet (moisture barrier) then 1/4 hardiboard. This is a fast install, allows you to get something like picture #2 minus the furring strips (which are not fast), the 1/4 inch backer is much easier to get screws in right, and with a total of a half inch you can meet the rest of the room. I read about this install over at the johnbridge forums probably 8-9 years ago and have done at least 20 this way without one issue (2 in my home).

#3 - Same deal as the first picture but with a goofy flange. Again you could use furring strip or drywall.

Tip - Once you fur out the back or the front of your tub you have a dilemma. How do I meet up with the rest of the room. Let's first of all say you need to fur out a 1/2 inch. Wow if you fur out and put up 1/2 inch backer you are out an inch. So that is why I say to keep the drywall+backer method in your back pocket. You can go 1/2 inch drywall then 1/4 inch backer. Also how do you meet up with the rest of the room? Well if it is a small bathroom I have double layered drywall along a wall. This is very easy to do and maybe $20 in materials vs furring out the whole wall - which is harder to match things up. An alternative in this case is to use 5/8 for the rest of your room and transition under tile work. Very easy to build up 1/8 inch using extra thinset.

Normal Install Instructions

Normal Install Instructions

  1. If you need to push out the install add furring strips or drywall.
  2. Staple moisture barrier (plastic sheathing) to the studs (or furring strips or drywall). The plastic should go a few inches into tub deck and above your highest backer board. It if fine to take it to the ceiling too.
  3. Screw in backer board using roofing nails and approved screws. First every piece should have 6-8 screws. They are a pain to get flush so use roofing nails for the rest (need at least 1 1/4 inch).
  4. Put up your alkali resistant mesh tape on all backer to backer seams (gray mesh sold near backer boards at the big box). If it doesn't say alkali resistant it isn't so don't buy the drywall mesh.
  5. I strongly suggest that you hide backer to drywall transitions under tile. When I do a normal tub surround I buy 4 pieces of backer. One goes up vertical on each side (5 feet), one horizontal on the back, and another cut at 2 feet horizontal so that we have 5 feet above tub deck all the way around. Drywall above that . I even do the drywall if tiling to the ceiling. Then the tile is going to at least 5 1/2 feet above the deck and 3 1/2 feet from the wall. No transitions for me.
  6. First step of tiling day is to mix thinset a little on the soft side. Go over the joints that have your mesh tape and fill them in.
  7. Plan your tiling pattern. If you have to make cuts, hide them in the corner. Plan to never cut your tiles height-wise - just tile a little higher (or lower).
  8. Start at the bottom. With bigger tiles you may need to start at the second row and have them sit on a strip until the thinset is strong enough to not sag. Make sure your bottom row is 1/8 inch from tub deck. Note that on curved tub deck you might have to be 1/16 of an inch on some parts and much bigger on others.
  9. After you are done tiling cut the plastic right at the tile level all the way around. Push plastic under tile gap.
  10. With the new cheap plastic tub you pretty much have to caulk the deck to tile transition. For heavier more solid tubs you can grout this area. I like grouting the bottom because you can always go over it with caulk in the future. People will say to never grout because it will crack. As I mention above it depends on the type of tub.
  11. Caulk your corners and caulk the top area of tile.
  • I didn't understand the Mapelastic, Kerdi band AND Kerdi Fix in solution #3.
    – HerrBag
    Sep 23, 2013 at 20:21
  • @HerrBag - I will edit that out. I used this picture for a specialized project - the picture shows a way to deal with an issue, not necessarily the way I would pick.
    – DMoore
    Sep 24, 2013 at 0:02
  • 4
    I just want to compliment DMoore on his answer. Your description is extremely comprehensive and well illustrated. I could have used it when I did my shower pan install a few months ago; but, I did it right, IMHO. The only thing I questioned was caulking the tile to the pan: seemed to me this would trap any moisture coming down the "barrier". But I did it anyway.
    – getterdun
    Sep 25, 2013 at 14:21
  • @JohnGleason - Thanks for the good words. I hate caulking the bottom of the tile too. With the cheap plastic (fiberglass) pans it is almost a must because when they get filled with water or a couple of heavy adults stand in them they will shift down well enough to give your grout a crack. Moisture should be able to escape out your other grout lines - I wouldn't be concerned about it escaping, getting in is the problem. I just like the grout bottoms because it looks better and easier maintenance. Takes about 5 mins to regrout the bottom every 2-3 years.
    – DMoore
    Sep 25, 2013 at 16:37
  • If you keep current with grout sealer (once a year), very little moisture will penetrate.
    – HerrBag
    Oct 2, 2013 at 3:00

The backer should go OVER the flange, for better water drainage.

Because the backer board tends to bow out when 'lipped' over the tub flange, you should:

  1. Install the tub
  2. Install shims on the studs to 'pad out' the backer so that it just clears the tub flange and overhangs it a bit (but doesn't 'land' on the rim of the tub) but leaves a 1/4 to 3/8 inch gap to the tub. Do this on all backerboard walls, so that the backerboard is plumb.
  3. Use a waterproof caulk between the tub flange and the back of the backer board.
  4. Use an alkali resistant mesh tape and seal all backer board vertical seams with portland based thinset, mudded smooth like drywall joint compound.

The bottom edge of the backer should terminate above the upper most edge of the pan. The tile should come further down over the rim and occlude the rim flange and terminate about 1/4" above the pan shoulder. That 1/4" gap should be caulked with waterproof caulking.


being furloughed I have time to attack my 1978 year old bathroom, ripped out the original shower one piece install. Had a professional install a heavy duty fiberglass shower pan with center drain. Originally we were going to install a Schluter shower system(way to expensive)so opted to go with the cement board install.Shower pan is exactly a 48"W x 36"L and it is tight up against the original studs with clips that hold the shower pan down and level in place as the cement underneath dries. Answer: At first my first piece of cement board (I purchased the 3'x5'boards)I had installed the board over the lip of the pan then as I researched more I removed this board completely because common sense told me it will absorb moisture. So I researched more and found a solution: I cut found a staight edge thick enough to hold my cement board just high enough to miss the screw and clip that holds my pan in place, I now have my cement board approx 1/4" above the flangers lip of the pan. Next step: using alkaline self adhesive fibermesh tape I will tape all seams and corners as well as the bottom of the cement board to the pan (Pan surface has painter tape to protect the pan as well as an old sheet in the middle covering all areas)next to mix Modified thin set( consistency of pudding)and apply over the fiber mesh, once this dries I will apply Kerdi waterproof 5" wide tape to the corner (Creasing the fiber tapein half so it has equal coverage on each side of the inside corners)All this has ample drying time. Last phase: apply Three (3) coats of Aquaq defense waterproofing membrane solution to all surfaces applying heavily paying close attention to the bottom of the cement board and the shower pan. Once all three coats are dry. Lay out my tile design and calculate and install tile letting the last row of tile come within 1/4" of pan.

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