Green/Cement Board Overlapping Tub Flange vs Leaving Gap Between Board and Flange

Discussion in 'Shower & Bathtub Forum & Blog' started by DavidSeon, Sep 24, 2012.

  1. DavidSeon

    DavidSeon New Member

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    Location:
    MS Gulf Coast
    It looks like this subject has been beaten to death on the internet but there doesn't seem to be a consensus, so I thought I'd ask you guys which is better. I'm looking at using Swanstone tub walls with a Kohler tub.

    The Kohler tub instructions say to use furring strips to provide a flat surface for the wallboard to overlap the tub flange:

    Kohler Tub Install Pic 1_edited-1.jpg Kohler Tub Install Pic 2_edited-1.jpg


    But the Swanstone wall instructions say to stop short of the tub flange:

    Swanstone Surround Install Pic.jpg

    The second method, leaving a gap, seems the simplest since you wouldn't need a 1/4" thickness transition with the rest of the bathroom walls. Is there any reason to choose one method over the other, or does it not really matter?

    Thanks,
    Dave
    Last edited: Sep 25, 2012
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    First, I wouldn't use any gypsum based product in a shower. Now, there is a company that makes a green waterproof board that can handle wet, but I don't think it's available everywhere. Moisture resistant drywall has a moisture resistant surface...the interior is still gypsum, and will suck moisture up like a sponge and often gives problems.

    If tiling, it depends...you can't cantelever the tile too far...as long as more than half of it is supported on the wall surface, it can be okay to leave it short. You dont' walk on the wall, and it's unlikely that you'd be applying pressure right above the tub edge, so a little cantelever is okay. What they are leaving out on that diagram that most techniques recommend is a vapor barrier on the studs, lapped over the tiling flange.

    What some people do is to cut the studs so when you slide the tub in, the tiling flange is flush with the rest of the stud surface...then, the wall isn't sticking out compared to the rest of the room.
  3. johnfrwhipple

    johnfrwhipple Bathroom Design & Build - North Vancouver, B.C.

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    Leaving a gap between GreenEBoard and tub tile flange

    I like to leave about an 1/8" - 3/16" of a gap between the top of the tub's flange and the GreenEBoard.

    A better backer board is Wonder Board Lite. I stopped using GreenEBoard two years back.


    JW - "When it's perfect. It's Good Enough."

    www.No-Curb.com - my blog site on Barrier Free, No Curb, No Dam and Hobless Showers
    (604) 506-6792 jfrwhipple@gmail.com
    Last edited: Dec 12, 2014
  4. johnfrwhipple

    johnfrwhipple Bathroom Design & Build - North Vancouver, B.C.

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    Sealing the gap between cement board and tub deck

    You should have solid blocking behind this point.


    JW - "When it's perfect. It's Good Enough."

    www.No-Curb.com - my blog site on Barrier Free, No Curb, No Dam and Hobless Showers
    (604) 506-6792 jfrwhipple@gmail.com
    Last edited: Dec 12, 2014
  5. johnfrwhipple

    johnfrwhipple Bathroom Design & Build - North Vancouver, B.C.

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    .
    Last edited: Dec 12, 2014
  6. johnfrwhipple

    johnfrwhipple Bathroom Design & Build - North Vancouver, B.C.

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    The key is using a waterproofing material that laps overtop of the tub's tile flange.

    I have shared many ways of doing this.

    SK 25 Mesh

    Laticrete Hydro Ban Sheet Membrane

    Noble Company NobleSeal TS or CIS all good

    So is Dal Tile's DalSeal TS


    JW - "When it's perfect. It's Good Enough."

    www.No-Curb.com - my blog site on Barrier Free, No Curb, No Dam and Hobless Showers
    (604) 506-6792 jfrwhipple@gmail.com
    Last edited: Dec 12, 2014
  7. nestork

    nestork Janitorial Technician

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    Location:
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    Dave:

    I think the whole idea behind having the tile backer board above the tub lip is something that's been practiced since the 60's, when people used drywall or greenboard as a tile backer for lack of a better alternative. The idea was to avoid the possibility of any water leakage forming a puddle in front of the tub lip and being wicked up into the greenboard.

    Hardie backer board is very much more water resistant, but it's still not a good idea to allow the possibility of that happening. The steel fasteners you use could rust out, and you'd end up with mold growing all over the back side of that board.

    Regardless, Hardie makes tile backer board for tiling, and I'd follow their instructions when it comes to installing their product. But, if it wuz me, I'd misread those instructions and install the board with a smaller gap (1/8 to 1/4 inch above the tub lip) cuz I don't see the reason to have such a wide gap.

    Or, better yet, phone their 1-800 customer service phone number and ask their customer service reps why you need such a wide gap to keep the bottom of the board dry. If they don't convince you that you actually need that much clearance to prevent the bottom edge of the board from wicking up water, then I'd go with common sense and install it with a smaller gap (1/8 to 1/4 inch, say).

    Of course, you'll scuttle your warranty by not following their installation instructions to the letter, but that warranty will only mean they'll provide you with new board. It don't mean they'll come out to your house and redo your tiliing for you.
    Last edited: Sep 24, 2012
  8. johnfrwhipple

    johnfrwhipple Bathroom Design & Build - North Vancouver, B.C.

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    I can not stand Hardy Board. Or Durock.

    The best backer board to date is made by Custom Building Material and is called WonderBoard Lite.


    JW - "When it's perfect. It's Good Enough."

    www.No-Curb.com - my blog site on Barrier Free, No Curb, No Dam and Hobless Showers
    (604) 506-6792 jfrwhipple@gmail.com
    Last edited: Dec 12, 2014
  9. DavidSeon

    DavidSeon New Member

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    47
    Location:
    MS Gulf Coast
    Wow, you guys have given me a lot of information to study and research, and I really appreciate it. My impression is that the backer material I put behind the surround walls is more important than whether the backers overlap the tub flange or not. So for starters I'd like to ask a couple of questions about that part:

    1) Several of the replies mentioned tile and it sounds like I should forget about gypsum, including green board, altogether and use one of the cement board types. Is that still the case even though I'm installing solid glue-up panels instead of tile?

    2) Is a moisture barrier still necessary with cement board? Poly? Liquid Waterproofing? Both?

    3) For gypsum or cement board the surround instructions specify "To ensure proper adhesion of the Swan Corporation product, the wall surface must be sealed with a water or oil-based primer/sealer." I don't suppose that would be sufficient waterproofing?

    4) With fairly rigid surround panels can I get away with furring strips and 1/4" cement board to keep a 1/2" wall thickness, or would it be better to use 1/2" board, leaving a small gap above the tub flange? (the surround panels will come all the way down the 1 1/4" tub nailing flange anyway).

    I need to do a little homework before I ask questions about the materials mentioned for the later steps, but I do appreciate all the help.

    Dave
  10. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Solid panels, I'd consider 1/4" shims and 1/4" Hardibacker. Hardibacker is (I think) the only one the manufacturer recommends as a wallboard in a shower at 1/4". Again, with solid panels, you can probably forget the vapor barrier, as the panels should be totally waterproof.
  11. nestork

    nestork Janitorial Technician

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    Location:
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    Yeah, I'd agree with that, especially if you're gonna be using plastic panels where there's less chance of water leaking into the wall. Lots people just glue tub surround panels to bare drywall and those installations last a long time. A plastic tub surround is more reliable than ceramic tiling.

    You definitely don't want to go with drywall or greenboard, but there is a gypsum based tile backer board made by the Georgia Pacific company called Dens-Shield that's gypsum based, and as easy to cut and install as drywall, but is very water resistant and suitable for use as a tile backer.

    http://www.gp.com/build/densshield-tilebacker-board

    I used Dens-Shield to replace the water damaged drywall in my sister's finished basement after she had a mini-flood, so that now, she can get up to a foot of water in her basement without having any significant damage to the walls down there.

    The Cadillac of tile backer boards is Hardie Backer Board, and you want it in the 1/2 inch thickness for better rigidity. I've found Hardibacker to be the strongest and hardest tile backer board, but I've also found it to be the hardest to work with and install, and it's also the heaviest of the tile backer boards I've used, and that makes it harder to work with, too.

    If you're installing glued up panels, then you don't need the strength of Hardibacker as much. With ceramic tiles, any flexing of the backer board is likely to crack the grout lines in the tiling, and so that's where you'd be better off opting for 1/2 inch Hardiebacker as your tile backer board. That's not saying that the conventional cement boards like Wonderboard, Durock or a gypsum based tile backers aren't appropriate, it's just that Hardibacker is stronger (for equivalent thicknesses) and so there's less chance of it flexing enough to crack a grout line.

    Yep. None of the cement boards or Hardibacker are impermeable to moisture, so the general gameplan is to put up the tile backer board on your studs, then paint a moisture barrier like "RedGard" (which you can get from Home Depot) over the tile backer, and then tile over that moisture barrier.

    Whenever you do home reno work, it's never a good idea to have TWO moisture barriers in a row. That's because if water gets between them (from an overflowing bathtub upstairs, say) then it'll take forever to dry out. So, if this is an exterior wall that your're doing, you'd kinda have to choose between putting up a polyethylene vapour barrier over your insulation OR applying a moisture barrier over your tile backer. In a case like that, and I was planning to tile, I'd probably paint the moisture barrier over the tile backer to block any moisture from getting into the tile backer OR the insulation.

    If you're sure you want to put up plastic panels, then in my opinion, they'll be their own vapour barrier, and I would just glue them over your backer board. If you're installing plastic panels, I don't see nearly as much of a problem using drywall or greenboard behind them. But, if it wuz me, for the incremental cost, I'd use DensShield behind your plastic panels. It's way more water resistant than greenboard, and is actually overkill if you're putting up plastic panels. But, there's absolutely nothing wrong with overkill and I support overkill 100%.

    Your plastic panels are sufficient waterproofing in themselves.

    Lots of times people will stick plastic panels up directly over bare drywall, and that's what those instructions are telling you not to do. That's because no matter how strong a glue you use, the weakest link in the chain is going to be the paper fibers in the drywall face paper. If you glue something to bare drywall, you can pull it off and the surface of the drywall, that is, the paper fibers at the surface of the face paper will come off with that glue.

    So, what they're wanting you to do is paint the drywall first to consolidate the surface and make it harder to pull the plastic panels off. If it wuz me, I would use an interior alkyd primer that'll dry to a much stronger film than a latex primer. And, if it wuz me and I was he11bent on overkill that day, I'd give it two or more coats, leaving the paint roller and tray wraped in plastic bags in the fridge (good) or freezer (better) between coats. And, put each coat on within 24 hours of the previous coat to ensure excellent coat-to-coat adhesion.

    The latter. Use 1/2 inch board and leave a gap above the tub lip.

    And, in case you want to know why, it's because one 1/2 inch thick board is more than twice as strong as two 1/4 inch thick board sitting one atop the other. That's cuz the formula for the deflection of a beam has the beam's height CUBED in the denominator. So, for example, a 2X4 being used as a floor joist has a height if 3 1/2 inches. 3 1/2 inches cubed 42.9. A 2X8 being used as a floor joist under the same small floor will have the same width, but is 7 1/2 inches tall, 7 1/2 cubed is 422. 1/42.9 is ten times more than 1/422, so given the same span between supports, a 2X4 floor joist will bend 10 times as much as a 2X8 floor joist under the same weight.

    So, 1/2 inch thick board is stronger than two 1/4 inch boards. That's why you can bend a deck of cards in your hands, but not a piece of wood that same size.

    And, that's also the reason why a fir 2X12 can be used as a really ridgid floor joist or a really bouncy diving board. It depends entirely on which dimension is cubed.

    Can you tell us if you've already purchased the tub and/or the plastic tub surround?

    Also, can you tell us what the tub is made of? (ie. acrylic, cast iron, enameled steel)
    Last edited: Sep 25, 2012
  12. DavidSeon

    DavidSeon New Member

    Messages:
    47
    Location:
    MS Gulf Coast
    Ok, I'm leaning toward one of the 1/4" backers with shims, or 1/2" cement boards slightly above or against the upper edge of the tub flange, with a sealer or primer painted over it. The DensShield looks interesting considering my skill level.

    Well it is a 5-piece set so I guess it would still be vulnerable at the vertical seams.

    Yes, they are here, although both are unopened and returnable locally at no additional charge.:eek:

    The tub is a Kohler Archer 60" alcove in acrylic:
    http://www.us.kohler.com/us/Archer-60-x-32-alcove-bath-with-Comfort-Depth-design,-integral-apron-and-left-hand-drain/productDetail/Archer/425546.htm?brandId=429364&skuId=394481&categoryId=429297&hash=id%3Dfilters%26startIndex%3D20%26scrollTop%3D342
    Back Bath  Kohler Archer® Bathtub_edited-2.jpg

    The surround is a Swantile TI-5 in Veritek (whatever plastic flavor that is):
    http://www.theswancorp.com/index.php?prod=189
    Swanstone Surround Install Pic2_edited-2.jpg

    I would have liked to try tiling it but I'm afraid I'd make a mess of that. Thanks again for all the advice.
    Dave

    Attached Files:

  13. CF

    CF New Member

    Messages:
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    Location:
    ga
    Hopefully, this is an extension of this thread and not a hijack. My project is also a tub/shower surround but I plan to tile it with 3x6 subway tile. Whether the backer board should be shimmed is also one of my issues. The NTCA Handbook 2012, specification B419-12 (p. 149) for cement backer board on a bathtub/shower wall shows a shim as John stated. It also requires a vapor retarder membrane behind the bb OR a waterproofing membrane on the front. Interestingly, Durock's installation instructions for a tub/shower surround does not include a shim. And for water protection, it states that if vapor or waterproofing is required, then buy Durock tile membrane. I was hoping for a bit more clarity. Is the shim for asthestics or is there a functional reason?

    My tub/shower will be used as an everyday shower. Originally, I planned on using the tried and true approach of 15# roofing felt behind the Durock. But after reading about waterproofing the backer board to prevent water getting into it in the first place, that has a lot of appeal. I have read a lot of comments about waterproofing membranes on this site by John Whipple and Jim DeBruycker and also on the JB site. Now I'm thoroughly confused and overwhelmed with the choices. The JB site is partial to Kerdi and since I have no experience with any system, I have no idea. John Whipple's pictures of tubs in this thread with a mix of waterproofing products from Kerdi, Noble, Laticrete, Ardex and Mapei (did I miss one) is exactly why I'm confused. I understand the application of the products but I don't have the experience to combine products from different vendors. I would prefer more of a one stop approach if possible. Can you recommend a waterproofing system for the average or hopefully above average DIYer?

    If I do use a waterproofing system, would using cement backer board be a belt and suspenders approach? If I do make a mistake and the waterproofing leaks, will the cement backer board help or hurt? The cost difference between cement bb and drywall is very small compared to the total project cost. There could be an additional cost if using a liquid waterproofing system over cement bb. Ardex states that one gallon of Ardex 8/9 covers 70 sq. ft. over cement vs. 100 sq. ft. over drywall.
    Thanks
    Last edited: Oct 15, 2012
  14. johnfrwhipple

    johnfrwhipple Bathroom Design & Build - North Vancouver, B.C.

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    The key is picking a system.

    A waterproof system that is topical, that laps the face of the tub and the surface of the cement board or one that goes behind the backer board and you skip the topical.

    Both are Ok

    Both are approved.

    But I never use the old style of poly or builder paper - I just don't like it. Aprroved or not.


    JW - "When it's perfect. It's Good Enough."

    www.No-Curb.com - my blog site on Barrier Free, No Curb, No Dam and Hobless Showers
    (604) 506-6792 jfrwhipple@gmail.com
    Last edited: Dec 12, 2014
  15. CF

    CF New Member

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    Location:
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    I'm confused but not frustrated. You have a lot of good information here and I just need to sort it out. For example the picture of the tub with Noble TS and Aqua Defense surprised me. I would have thought the Noble TS (installed per Noble's spec's) is a waterproofing membrane by itself. I wouldn't have thought to add the Aqua Defense on top of it. Also the Aqua Defense I'm aware of is made by Mapei. I'm not questioning your work, but I don't have the knowledge to do that.

    What I'm looking to do is build a tub/shower surround that will give me years of worry free use. As for the backer board choice, I don't really care. I'm doing this for my home and the difference in cost of Durock vs DensShield vs Hardiebacker vs drywall will not have a big impact on the budget. Also, the time required to cut and fit the backer board is also not important. Being the homeowner gives me those options. What is important is that it not leak for the next twenty years (at least).

    Originally I planned to use 15# roofing felt and cement backer board. But replacing the felt behind the backer board with a waterproofing membrane on the front seems like a better approach in theory. I have to depend on you to tell me if in practice it really is better. And if it is better, then what are good systems to use? There are those who prefer the Kerdi membrane. (I wasn't considering Kerdiboard.) Is Kerdi head and shoulders above the rest? If so, I'll use it. But are there others that are just as good? As you say, you do things above industry standards and that's what I'm wanting as well.

    Also, after the tub/shower surround, the next project is a walk-in shower. I would like to use the same waterproofing system on it.
  16. johnfrwhipple

    johnfrwhipple Bathroom Design & Build - North Vancouver, B.C.

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    I would never use Kerdi Board. I can not stand that stuff.

    I might use it for crafts. Maybe to kneel on. Certainly not to build a shower with.




    JW - "When it's perfect. It's Good Enough."


    www.No-Curb.com - my blog site on Barrier Free, No Curb, No Dam and Hobless Showers
    (604) 506-6792 jfrwhipple@gmail.com
    Last edited: Dec 12, 2014
  17. CF

    CF New Member

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    Location:
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    Now I'm adding to the confusion. I am NOT interested in using Kerdiboard or Wedi at this time.

    Is there a way to compare the durability of the waterproofing systems from Kerdi, Nobel, Laticrete, Mapei and Ardex? For example, how do the sheet waterproofing membranes from Kerdi and Noble compare to the liquid applied systems from Laticrete, Mapei and Ardex?

    As you say, the hardest part is the connection of the tub to the backer board. The Kerdi installation instructions show Kerdi-Band bridging the backer board to the tub. Thin set was troweled on to the backerboard and a bead of Kerdi-Fix on the tub flange. Then the Kerdi-Band was pressed into both of them. Your first picture in post #3 looks exactly like that. Your picture in post #5 shows another example with Hydro Ban liquid coating on the backer board and Kerdi-Fix at the tub flange. In this case did you fill the gap between the backer board and tub using Kerdi-Fix only? There doesn't appear to be a sheet membrane between the tub and backer board. I'm assuming the Kerdi-Fix is the dark grey material in the picture.

    Thanks for your help. This is very informative.
  18. johnfrwhipple

    johnfrwhipple Bathroom Design & Build - North Vancouver, B.C.

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    Hard to tell the durability.

    I have stopped using Kerdi all together. Stop as well using Liquid waterproofing.

    My goto materials today are made from Noble Company, Ardex or Laticrete.



    JW - "When it's perfect. It's Good Enough."

    www.No-Curb.com - my blog site on Barrier Free, No Curb, No Dam and Hobless Showers
    (604) 506-6792 jfrwhipple@gmail.com
    Last edited: Dec 12, 2014
  19. CF

    CF New Member

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    Location:
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    Thanks again for your help. The confusion is much improved. For the sheet membranes, Noble appears to be the easiest for the DIYer to buy. The have an online store at noblecompany.com. Home Depot can get Schluter-Kerdi but I don't think they stock it.

    Is there a durability advantage to using cement backer board vs. fiber cement (Hardie) or gypsum backer board (DensShield or drywall)? The cost difference is not that significant. If somehow the membrane failed would the cement backerboard help or hurt or make no difference?

    For the Noble sheet membranes, do you prefer the Noble TS or the Noble Wall Seal for wall applications? The Wall Seal is 0.025" thick and Kerdi is 0.008".

    Is there still a debate as to whether Kerdi should be installed with modified thin-set vs non-modified thinset? Their instructions still call for non-modified thinset.

    Is there a problem with the membrane seams telegraphing through to the tile? Does that affect whether you install the sheet membrane vertically or horizontally (shingle style)?
    Last edited: Oct 18, 2012
  20. johnfrwhipple

    johnfrwhipple Bathroom Design & Build - North Vancouver, B.C.

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    You can not compare Kerdi to NobleSeal TS.

    The Noble Product is hands down the better product.

    I do not use Kerdi at all anymore.



    JW - "When it's perfect. It's Good Enough."

    www.No-Curb.com - my blog site on Barrier Free, No Curb, No Dam and Hobless Showers
    (604) 506-6792 jfrwhipple@gmail.com
    Last edited: Dec 12, 2014
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