Is there supposed to be a backer board gap above a bathtub flange?


Is it acceptable to stop the cement board 1/4" above tub flange,caulk the joint between cement board and tub flange and run the tile over the tub flange?


ANSWER - No there should not be a gap between the top of the bathtub flange and the backer board on the wall.

There should be a moisture barrier behind the backer board that is installed in a weather lapping manner starting at the tub flange working up the wall.  The asphalt paper or polyethylene should overlap the tub flange so moisture that collects on the moisture barrier can evacuate into the tub; or more important so it can't get into the wall cavity.

The backer board should overlap the tub flange so the tile has a stable backing to attach to.  This requires shimming the backer board out from the studs so the wall is plumb.  There should be a 1/4" gap between the tub ledge and the backer board.  Technically, after the tile is installed over the backer board that gap between it and the tub ledge should be filled with an ASTM C920 sealant (caulking e.g. silicone).  Although some say you should have weep holes along this gap so any water behind can weep out into the tub.   Others say there should be enough water to need weeping and it can evaporate through the grout joints, which is where it enters.

I always apply a liquid applied waterproof membrane over the  backer board and bond the tile to it, so you keep the water out of the backer board.  Or use a backer board that doesn't absorb.

38 thoughts on “Is there supposed to be a backer board gap above a bathtub flange?

  1. Mark A Newton says:

    I don’t understand. First you say “No there should not be a gap between the top of the bathtub flange and the backer board on the wall.” Then you state “There should be a 1/4″ gap between the tub ledge and the backer board.” Did you really mean to say ” yes, there should be a 1/4″ gap between the bottom edge and the backerboard, but that gap should technically be filled with silicone caulking after the tile is installed”?

    • Donato Pompo says:

      On bathtubs per the plumbing code there should be a 1″ vertical flange (nailing flange) above the bathtub ledge that serves to anchor the tub to the studs and as a way to flash the moisture barrier on the wall to the tub so moisture can’t get into the wall cavity to cause moisture damage.

      If you stop the backer board above the flange then water has a way to get into the wall cavity and the there will be at least a 1″ gap where the tile is not bonded to anything making it susceptible to damage.

      So the backer board and the vapor barrier behind it should overlap the flange leaving no more than a 1/4″ gap, but no less than a 1/8″ gap, above the bathtub ledge. That joint should be filled with a foam backer rod and an ASTM C920 sealant. There is a controversy over whether that joint should be left open to allow water to weep out or to fill it with sealant to keep the joint clean. There are pros and cons for both. The standards currently state to fill the joint with the sealant.

      • Tai Fu says:

        I do not understand that we waterproof the surface of the cement board using hydro ban or similar, why we do not waterproof the bottom of the back board (leaving less than 1/4” to the tub). If we waterproof the bottom of the back board, we may not need to fill the gap with sealant.

        • Donato Pompo says:

          Cementitious backer boards are porous, so water can migrate through it. Waterproofing is always recommended on the positive side of the substrate because the water pressing against the membrane that is supported by the backer board. If the membrane is on the back side of the backer board then it has not support behind it and it can be subjected to hydrostatic pressure that can cause the membrane to fail. This isn’t much of risk on a wall application in a shower as the moisture is primarily vapor.

          Whether you waterproof the backer board on the front or the back, you still have a gap to fill. Per the industry standards you need to fill that gap with sealant to prevent water from entering there and to prevent microbial growth to develop if the joint was left open.

          • Tai Fu says:

            I only see people mention waterproof the positive side (tile side) and leave 1/4” gap at the bottom . I feel we should also waterproof the bottom (the thickness of the board) at the bottom edge of the back board before fill the 1/4” with sealant.

            I am a female homeowner; please forgive me not express my question clearly.

            I have been researching the issue on tiling a no-lip cast iron tub/shower and your answer is the best I can find. Thank you

          • Donato Pompo says:

            You could waterproof the bottom edge of the backer board, but it would not be necessary if you are filling the joint with an ASTM C920 sealant. The silicone or polyurethane sealant will make the joint waterproof and will bond better to the edge of the backer board if installed correctly.

  2. Jim Marxen says:

    What about a cast iron tub I purchased at Home Depot that only has about a 1/4 lip about tub deck? There is not a 1″ flange on it.

    • Donato Pompo says:

      There are kits that the manufacturers of tubs and shower reciprocals sell or give to you that provides the extended flange. Check with the manufacturer of the tub.

  3. Everardo Rodriguez says:

    Donato, I have a similar situation as Jim Marxen, regarding the cast iron tub it’s a Kohler villager and we called Kohler directly and they did not have an extension kit for the tub surround flange/lip. We called lowes and did they not have anything, we called 2 of our plumbing distributors and they also did not have any. Can you give us a name brand. I was just planing on having my moisture barrier on the back of the cement board panels and letting it come through to the surrounding tub leaving a 1/4″ gap filling it with a backer rod and then sealing it with silicone. What’s your opinion. Thanks for your time, really appreciate it.

    • Mike Montgomery says:

      the tile industry has adopted a system known as paper tail for tops that do not have a flange. It incorporates running the vapor barrier down to the tub deck with a slight overhang. The backside of the overhang is sealed to the tub with either polyurethane or silicone sealant . Then the cement board and then the tile

      • Donato Pompo says:

        I am not aware of any industry standard that endorses that method for flashing your moisture barrier to a bathtub that doesn’t have a 1″ flange as required by the plumbing code standards. In theory it could work, but it still would not meet the plumbing code standards.

        There are kits that you can buy from the tub manufacturers that adhere a 1″ flange to the top edge of the tub with a peal and stick strip, but it is kind of flimsy.

        • Mike Montgomery says:

          I was a litigation attorney specializing in defense of contractors in construction defect lawsuits in so Cal for 30 years. During my career I represented countless plumbing and tile contractors. The tile experts in the industry, ( both on the defense and plaintiffs side) were in agreement on the Paper tail installation for cast iron tubs that did not have a built in tile flange.

          • Donato Pompo says:

            Any legitimate tile expert should know that the International Plumbing Code requirement is to have a 1″ flange along the bathtub edge. The TCNA Handbook or the ANSI A108 installation standards for tile do not recognize anything less than complying with the plumbing code and they do not recommend or recognize the term “paper tail” or that method for shower/tub applications. In theory if done correctly it could work, but there is no protocol for it nor does it fall under the standard of care for that application.

  4. Bob Teal says:

    When do I need cement boards above a shower or tub? Can I not use the purple gyp board anymore? I had an inspector say no more gyp board, then some say it is ok. I use full onyx panels over the purple board usually. Is this allowed for full solid panels and cement boards are needed for tile? That would make sense. Thanks.

    • Donato Pompo says:

      Water resistant gypsum board can’t be used in a shower unless it is fully waterproofed first by applying a waterproof membrane over it. This is true whether it is tile or a slab stone. There is DensShield Tile Backer board that has a gypsum silicone core and a waterproof surface that can be used. Or you can use cementitious backer board that you need to waterproof too. Or you can use the foam backer boards that normally come with a waterproof surface.

  5. Gary Oneill says:

    If I furr out the 1/2” backer board to make it cover the tub flange. How will it be flush with the 1/2” sheet rock wall

    • Donato Pompo says:

      You can use furring strips over the studs prior to installing the sheet rock.

      If the sheet rock is already up you can apply enough thin-set mortar over the sheet rock and on back of backer board to bring it out about 1/8″ or so and then screw backer board into studs. Applying thin-set on back of backer board is more for adjusting and supporting the backer board for continuous contact than for attachment.

      If the sheet rock isn’t DensShield or some other water resistant material then you need to waterproof the face of the back board preferably with a liquid applied waterproof membrane.

  6. Janet says:

    My query is similar to an earlier one, but enough different I need to ask. I am tiling a shower wall above an old cast iron tub that has a very small flange. I removed a 1.5 inch wire and mud/cement base with tile from the wall and subsequently, my studs are 5/8 – 3/4 inch from the tub. Two questions: 1. Do I use two sheets of Hardie Board ( one 1/2 inch and one 1/4 inch) to meet the 5/8-3/4 inch Sheetrock on the wall above the tub or should I shim the studs and use 1/2 inch Hardie Board? 2. The Hardie Board should overlap the tub flange; right? How do I do that in this situation.? Thanks. I appreciate your expertise and time.

    • Donato Pompo says:

      You do have to bring the wall out so water doesn’t get behind the tub.

      It would be best to again put in moisture paper, metal lath and the float it out will wall mortar. This way you can compensate for any irregularities in the walls and make sure the walls are plumb and the walls are square to the tub.

      If you aren’t able to install a mortar bed, then I would install 5/8″ thick DensShield Tile backer board that has a moisture barrier surface per their directions. Shim out the backer board to make it square with the tub. Then I would install 1/4″ Hardibacker over that. That should bring you out in front of the tub flange. Then install a backer rod foam and install an ASTM C920 sealant over that per the manufacturer’s directions.

  7. Alan says:

    Do you tape and thin set the joint at the cement board to tub flange transition? If not, what if anything do you do for it prior to installing the tile?

    • Donato Pompo says:

      If you have a moisture barrier behind your cementitious backer board such as 15lb roofing felt then it should over lap the flange of the tub so water reaching the membrane can be directed to the drain.

      Normally that tub flange to substrate should be filled with an ASTM C920 sealant that is normally a 100% silicone or polyurethane sealant. A polyethene closed cell foam backer strip should be installed first to recess about 1/4″ to 1/2″ the opening to then apply the sealant over it. This will keep water from getting into the wall cavity.

      After installing the tile it is normally recommended to fill that tub to tile joint with the same sealant.

  8. rick says:

    i have done many duroc cement board installations where the board sits 1/8″ above the bath tub lip(allowance for flexing)…a 6″ water proofing band goes over the lip and behind the board…i use a schluter sealant to fill the gap(or, you can add 6″ water-proofing membrane by filling opening with sealant then adding adhesive with 6″ band over the area..any condensation or water will wick away… when walls are tiled i again use a schluter product to seal around tub(1/16″ gap)…i have never had a failure…there are differant ways to approach the job, from strapping out the tub, angle notching the cement board to fit over the tub….i’ve seen it all…new builders are even notching out the studs to fit the tubs in eliminating strapping so board goes just above the rim of the tub…do the research and don’t rush the job!

    • Donato Pompo says:

      Good advice. If you do what you say properly it can work. Although per TCNA details for bathtub installations show the backer board overlapping the flange of the tub. The vapor barrier that should be behind the cementitious backer board is also suppose to overlap the flange. The idea in theory is that moisture that migrates through the backer board or moisture that condenses on the moisture barrier that then has a bath to the drain in the tub.

      Of course if you caulk that bath either behind the tile or at the tile to tub transition then moisture can’t travel to the drain. There is a debate in the industry on whether that joint should be left open for weeping moisture. Those who say to caulk it with an ASTM C920 sealant so you don’t have a cavity that can develop microbial growth believe that what moisture that is retained in the backer board, the thin-set, and the grout will dry out fairly rapidly and isn’t a problem if everything is installed properly and water can’t build up and flow over the tub flange into the wall cavity.

      We always specify to waterproof the surface of the backer board with a liquid applied mortar and then caulk the tub to wall tile joint. Sometimes we will apply bitumen Jiffy Seal flashing to flash over the tub flange and the stud blocking and apply the moisture barrier paper over the Jiffy Seal for added projection. Although you can’t allow the silicone sealant come into contact with the bitumen flashing as they are not compatible and can cause staining.

    • Donato Pompo says:

      Yes the backer board should overlap the shower pan flange. You might need to shim the backer board out to keep it plumb and square with adjacent walls.

  9. Kathy says:

    I am using Hardiebackerboard and a shower pan. I read not to red guard backer board because the mortar needs to adhere directly to backerboard. I was planning on using epoxy grout. This is what I was going to do. Caulk around shower flange and wall. Then apply house wrap to wall with it overlapping flange, Then hang backerboard with it covering flange .5 inches. Tape and mud seams, red guard seams and screw heads. Hang tile with it going past shower flange to .25 of shower ledge. Caulk ledge, leaving a weep hole in the corner. The question I have, should I redguard all of the backerboard or only the seams?

    • Donato Pompo says:

      Yes you should waterproof the entire backerboard. If it is a cementitous backerboard then it absorbs moisture. You want it to be waterproof. Caulk should be an ASTM C920 sealant e.g. pure silicone. The building moisture barrier paper should be weather lapped so you start at the bottom at the shower pan and work up.

      • Jason Bredemeier says:

        I am using Goboard with metal tub. Should I overlap the metal lip? Every video I see has it go over top of the lip. I have always overlapped the lip to keep water from getting back behind the tub.

        • Donato Pompo says:

          Yes you should overlap the tub flange lip. You might have to shim the board out to get it to overlap and the board remains plumb to the wall.

          • Vic says:

            Hi, I’m knew at this but i need some advice, I’m remodeling a shower and just installed a shower pan 60×30, how far down do I need the cement board to down on the pan flange and do I need to put a moisture barrier behind the cement board?

  10. Donato Pompo says:

    Yes the cementitious backer board is suppose to overlap the pan flange. You may need to shim it out to keep the wall plumb and square. Cementitious backer boards are suppose to have a moisture barrier behind it that will flash over the pan flange. Start at the bottom of the wall and weather lap each additional row of moisture barrier paper.

  11. Diane says:

    I am using wedi board, with swan acrylic bath surround. I am told you notch out the Wedi board to go over flange, Yet I was told by swan company with the acrylic surround to leave a gap??? Even reading on your site here with questions on this I still did not understand 🙂 Where you say ” So the backer board and the vapor barrier behind it should overlap the flange leaving no more than a 1/4″ gap, but no less than a 1/8″ gap, above the bathtub ledge. ” Are you meaning then to notch out the wedi board, to fit over our enamel steel tubs flange, and YET have a 1/4 gap where it sits at bottom of flange not lip, the bottom ledge of flange ( tub edge itself) leave that gap and fill in? Do you suggest using the wedi tape over the flange and wall, I don’t believe that is suggested for this, but is it a good extra step?

    • Donato Pompo says:

      Wedi board is a foam board and the installation is somewhat different from a cementitious board. You need to follow Wedi’s installation guidelines. They may say you don’t need a vapor barrier behind the foam board because the board itself may act as a vapor barrier as long as you seal the board joints.

      A flange on a bathtub is a vertical leg that extends up from the edge of the tub that is against the walls. Industry standards state that the backer board should overlap that 1″ flange leaving about a 1/4″ gap between the horizontal edge of the tub and the end of the board. That gap should be caulked with an appropriate ASTM C920 sealant over a backer rod.

      Since the walls have to be plumb, cement backer boards should be shimmed out to be within plane of the flange. Perhaps with a foam board you can notice the back of the board to fit over the flange. Again you need to verify that the Wedi data sheet says you can do that.

      Some people do apply a waterproof peel and stick membrane flashing the vapor barrier behind the cementitious backer board to the tub flange to help ensure water can’t get behind the tub into the wall cavity. Or you can caulk that joint between the back of the board and the flange.

  12. Diane says:

    Also Swan ( swanstone ) has its own silicone ( probably just like any silicone) but we will be using Wedi sealant for seams of Wedi board and if we leave the 1/4 gap we will also put the wedi product at that gap at bottom of tub flange where the notched wedi board Sits to.? And After we put up the acrylic walls, then can we reseal the bottom edge of wall again with the swanstone silicone wall area that sits At tub?

  13. art says:

    Here is a curveball…to the point w/ bullet points:
    *have older tub with 1/4″ curved lip/flange
    *previous install is glued on 5 piece tub surround on greenboard-total mess.
    *going to the studs……
    Can I put a H Depot tub surround that screws into the studs on my tub? If so, how is it waterproofed behind the acrylic walls and between the surround bottom and my tub? Manufacturer and H Depot show nothing…only how to screw to the walls. I am thinking to stay clear altogether

    • Donato Pompo says:

      I’m not familiar with the design of the acrylic wall surround. The key is you need to have a watertight connection between the wall surround and the top of your shower flange. The wall should lap over the tub flange and it should be caulked in so water get get int. It is assumed that the wall panels interlock and are water tight in some way.

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