I have an issue with water leaking from the side of the tub..I removed the tile to see the issue and I found that the old construction left a gap (around half inche) between the first mold defense board and the drywall(really image and one I drew to explain)

1.Is there a reason for this gap? 2.If I closed this gap would it make an issue (I closed it by adding a mold defense board piece to extend the dry wall to the end of the tub)

Please help.

The water was going from the corner because of a bad tile, through this gab behind the tile to the other corner of the tub and leaking to the floor.enter image description hereenter image description here

  • 1
    The gap would not be the cause of water getting behind the tile. It is normal for there to be a gap below the backer board and the tub. The tile should and does extend down close to the tub and then that gap is sealed with caulk. You need to determine how the water is getting into the space behind the tile. Not sure if your "mold defense board" is a PROPER waterproof barrier.
    – Alaska Man
    Commented May 19, 2020 at 21:38
  • provide a picture from the right side looking at the tile. For so much water to be leaking, either the tile/grount is letting water though or the chalking between the tile and tub is not sealing. I believe your "mold defense board" should be or is a backer board. Commented May 19, 2020 at 22:03
  • Thanks for the reply..so it ok to seal the gap completely (i just cut a dry wall board and cement it there from side to side then I add the mold defense board on top of it down to the edge of the tub.the water mostly from the grout and sealing especially from the tile in the corner as it was loose..the water to reach to the corner taking around 30 min to reach there..it takes both of my two daughter s to shower for the water to start leaking
    – Chad
    Commented May 19, 2020 at 23:00
  • can you take a picture from further away?
    – DMoore
    Commented May 20, 2020 at 0:11
  • That mold defense board should be the equivalent of a cement or backer board to use in a wet location behind the tile. But as you surmised, the leak is water seeping through the loose tile joints/grout and the caulking on the tub rim. Commented May 20, 2020 at 19:02

1 Answer 1


What is a mold defense board? I have done hundreds of tub surrounds / showers and have never heard of this it looks like someone may have used water resistant Sheetrock and attached the tile to that. A gap at the bottom is normal and should be filled with calking. If there is no gap any moisture that sits on the ledge is sucked up buy the Sheetrock So no you should not extend the tile backing. You should seal your grout sealers don’t normally last more than a few years, make sure the gap from the tub to the tile is well sealed this should stop water from getting behind the tile unless there is a plumbing leak. Yes even copper may develop small pin holes and leak I have seen this a few times over the years with well water that had a heavy mineral count. Where galvanized and copper were mixed without a dielectric union, and where metal was in contact with the copper. Plumbers strapping that was galvanized being a big issue plumbers tape or strapping for copper pipe needs to be copper coated. So seal the grout, and the tub /tile gap and if you still have a leak it is probably in the plumbing.

  • Thanks a lot for all the replies and help.The mold defense board is the Green board (water resistant board). I noticed that whole area is moistered even the first layer (the dry wall) and I need now to remove al the boards and apply new one. I live in north Atlanta, is it ok to apply one level of green board (screwed to the stud) and apply Redgard (water proofing membrane) instead of using cement backer board. It will be much easier for me as DIY.please advise.thank you
    – Chad
    Commented May 22, 2020 at 15:11
  • Chad that is a separate question you should close this out by accepting an answer or posting your own answer and ask the new question a new question will get more attention and more opinions.
    – Ed Beal
    Commented May 22, 2020 at 15:22
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    @Chad I agree that this is fodder for a whole new question (as Ed said), but working with cement board isn't difficult. It can actually be easier because the sheets are smaller. The tools are 99% the same as they are for drywall.
    – FreeMan
    Commented May 22, 2020 at 15:40
  • I agree with backer board 100% just like Sheetrock but heavier.
    – Ed Beal
    Commented May 22, 2020 at 15:43
  • Thanks for the advise and replies. I already posted a new question.
    – Chad
    Commented May 22, 2020 at 16:09

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