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trophywalleye 06-24-2011 08:21 PM

Cement Board Questions
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Hi All,

I am about to install durock cement board around my bathtub on three walls. I have a question, and look forward to your experience and any other installation suggestions offered.

1) I have researched online, and there seems to be a debate about bathtubs with tiling flanges. I have a Mirolyn Sydney. The debate is whether or not you bring the cement board down to the top of the tiling flange (and tile down to the tub, with a space behind), or to overlap the tile flange with the cement board and have the bottom angled into the tub.

My bathtub installation manual doesn't describe the installation, but shows the cement board on the top of the flange and the tile overlapping down to the tub edge. (Figure 5)

IF I bring it down to the top of the tub flange, do I need horizontal support (or a 2x4 horizontally) between my vertical 16 on center 2 x 4 studs? I don't want somebody in the tub to push against it and it flex and crack the tile or such. I doubt it would do that as it is pretty rigid stuff.

Please see the attached photos.


Bud Cline 06-24-2011 08:34 PM


IF I bring it down to the top of the tub flange, do I need horizontal support (or a 2x4 horizontally) between my vertical 16 on center 2 x 4 studs?
Nope! Not if you use 1/2" board and the stud spacing doesn't exceed 16" o.c.

SuperHans 06-24-2011 10:53 PM

If I'm understanding the manual right, they show the wall material brought down to the top of the flange and then the tile hangs off the wall material by the height of the flange.

I suppose that would work but a viable alternative would be to fur all the walls out 1/4 inch after the tub goes in. That way you can bring your cement board all the way down and you don't have any partially supported tile. If you had the cement board flush with the studs and bend it over the flange, the finished surface may end up looking funny. Good Luck.

Gary in WA 06-24-2011 11:41 PM

I may be seeing it incorrectly but I believe the poly wants to be inside the tub flange to channel any water/moisture through the b.b away from the framing.......


ccarlisle 06-25-2011 10:37 AM


Gary is right in commenting that the poly vb should come down the wall and end inside the tub...the problem becomes a minor one about how to finish off that gap created between the cement board, the tiles and the poly. Some caulk it, some grout it - but there isn't one definitive answer that I know of, if you choose to use that system...add to that the preferred use of total vapour barriers (right down to the floor) in cold climates - and you have a whole variety of options.

That's why many have opted for the Kerdi membrane - or a liquid waterproofing membrane on top of (not behind) the cement board. Just one less thing to worry about.

You may already be passed the stage where Kerdi etc membranes come into the picture, however. So, Gary's observation is the next best thing IMO: over the lip.

trophywalleye 06-25-2011 11:54 PM

Thanks for the comments.

I will not be furring the wall, nor will I overlap the flange with the cb.

If I do not require the horizontal supports, meaning the vertical supports are sufficient for rigidity, I will run the cement board down to the top of the tub flange.

My best plan then, unless specified otherwise, is to run the kerdi all the way down the cement board to the tub, using kerdi-fix sealant to adhere the kerdi to the plastic tub, and non-modified thinset to adhere the kerdi to the cb.

Finally thinset in between the tub flange and the tile.

Any comments?

As for the vapor barrier, I believe I have it right. I'm in a cold climate, so it runs the whole wall (meaning behind the tub). I will not be draping a second piece to hang over the tub flange as dual layers may cause more problems.


RE: Kerdi Shower

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Posted by bill_vincent ( on Fri, Nov 23, 07 at 16:31
Thanks, Sally. :-)
Do you "caulk" the Kerdi to the tub flange?

You basically caulk it to the tub's edge (the flat part between the flange and tub itself).

Does the backer board still go over the flange?

You stop it right at the top of the flange, otherwise it'll kick out the bottom of your wall.

Do you still leave a gap at the bottom of the backer board?

Asked and answered

If so, do you attach the kerdi to the tub or just the backerboard and then caulk the gap?

Caulk the gap with Kerdifix, and make sure it's bonded to the bottom of the Kerdi.

ccarlisle 06-26-2011 06:32 AM

Yeah; there should be two beads of caulk at the tub/tile area: one bead joining the cement board to the top of the flange and one bead sealing the tile to the tub...

Bud Cline 06-26-2011 01:32 PM

No vapor barrier is to be used behind the wall boards when using KERDI on the surface.:)

ccarlisle 06-26-2011 02:50 PM

Ahhhh, 1947 was such a good year...:laughing:

trophywalleye 06-26-2011 09:57 PM

Hi Bud,

Ok now I'm worried... I have my cement board up over the vapor barrier. The one wall with the vapor barrier is an exterior wall, which is per the building code in Ontario. Kerdi is not technically considered a vapor barrier from what I have read.

Should I not use the Kerdi now? I really don't want to take the cb down to remove the vapor barrier... however I really want to use the Kerdi as per the price I paid, not to mention the credibility of water protection. Will it destroy my outer wall by installing it. Will it really trap moisture in between? Currently we do not have air conditioning, but possibly in the future.

trophywalleye 06-26-2011 10:13 PM

And to note, if there is a bit of moisture the cb is 100% mold proof. Not sure how much moisture will form.

Bud Cline 06-26-2011 10:38 PM

I think you are now over-thinking the process.

KERDI is not sold as a vapor barrier per se, but of course it is, it has to be, simply due to its composition.

I think if it were me I would keep going rather than change anything at this point. But that is up to you.:)

ccarlisle 06-27-2011 06:32 AM

Just slash the poly vb you already have wtih a sharp need to take it down.

Schluter's Kerdi membrane is a vapour retarder not a vapour barrier; there's a big difference...Kerdi therfore doesn't let physical water molecules through it but it does let water vapour through it. The whole point is to keep the water molecules from entering the wall cavity - but at the same time, we want the wall+tile assembly to dry to the inside, in northern climates.

trophywalleye 06-27-2011 07:31 AM

Hi Guys,

One last note...

I talked to a Schluter supplier... and since I already have the Kerdi, and feel more comfortable with it... I was told to drill a bunch of 1/4" holes through the cb and vapor barrier to break the seal so it can breath.... I will then thinset the holes and kerdi the wall...

Does this sound like it may work?

Please let me know.


ccarlisle 06-27-2011 07:46 AM

Why cut holes and then thinset them?

Cut the holes - or slash it - like he says, then put up the cement board using appropriate screws, then thinset the Kerdi to that...

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