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

[DavidSeon]

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:




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



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

[jadnashua]

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.

[johnfrwhipple]

The top diagram is how the walls should be built to TCNA specifications. That said most times builders and home owners do not like the overlap.

When you overlap you are forced to use a thinner backer board or build the wall out thicker.

Personally I have been loving the use of Noble or Kerdi at this weak point.



Here is a picture of an old tub that we tiled recently. I used 1/2" Denshield and applied Kerdi Band to the face of the Denshield and tub flange with Kerdi Fix.

After this dried I waterproofed the entire tub surround with liquid waterproofing.

We have just started using Ardex's product line and have been quite impressed with their 8+9 cementious waterproofing product.

Noble Comapny makes a product called Noble Sealnt 150 and you can have it shipped to your house with 6 square feet of Noble Wall Seal - this would be more than enough to water proof the seam.

JW

JW

[johnfrwhipple]

This tub surround got Noble Seal TS and then Aqua Defence.





JW

[johnfrwhipple]

This tub got Hydro Ban and then a smear of Kerdi Fix on the bottom

[johnfrwhipple]

Some Kerdi Band installed onto a tub deck and then coated with 8+9 Ardex.




In the end the hardest part is the connection to the tub to the backer board. Here Kerdi Fix is king. I love the stuff.

JW

[nestork]

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.

[johnfrwhipple]

This job was boarded with Kerdi Board (builder's choice - not mine) and then we came in and Kerdi Banded it.

If you wanted you could use a 6mil poly under your cement board and have that lap the tub rim. Then tape the seams and your golden by local code (here in Vancouver anyway). The examples I showed are all topical waterproofing measures. Doesn't matter which you pick just as long as you pick one!

JW

[DavidSeon]

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

[jadnashua]

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.

[nestork]

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.

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.

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?

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.

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

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%.

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?

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.

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).

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)

[DavidSeon]

Solid panels, I'd consider 1/4" shims and 1/4" Hardibacker ...

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.

... with solid panels, you can probably forget the vapor barrier, as the panels should be totally waterproof.

... especially if you're gonna be using plastic panels where there's less chance of water leaking into the wall ...

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

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)

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

The tub is a Kohler Archer 60" alcove in acrylic:
http://www.us.kohler.com/us/Archer-6...crollTop%3D342


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


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

[CF]

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

[johnfrwhipple]

Sorry to add to your confusion on the subject.

My hope was to show that any product can be used and it depends on what building materials you have to work with.

Perhaps you should pick a setting material for your tile and then we can show you a way to go about making it water tight.

My pictures show a style of waterproofing a tub surround over and above industry standards. They show a way of doing it that does not shim out backer board. In 20 plus years building showers I have seen I think one tub's backer board shimmed out. It is common in the industry to leave the tubs R.O. (Rough Opening) a 1/4" larger than needed.

If you tell me what product you like I will tell you how I would waterproof it.

jw

[CF]

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.