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Durock around Tub

jkarschner's picture

Question: I am doing a new shower tile install in my bathroom. I have the durock slightly above the flange of the tub, but what do you use to cover the gap between the tub flange and durock? It's not a big gap, but in all videos ive seen there is something there, whether it is tape etc. I ask because where the tub flange ends there is still a little bit of tub left over that starts curving down the wall and there is a small hole (about the size of a quarter) where the 2 pieces of durock meet. Is there a special tape I should use, then mud to cover these small gaps and small hole? I know my tile will eventually cover but I want to make sure I do this right.

Adjacent panel of cement (post #214639, reply #1 of 11)

Adjacent panel of cement board, inside and outside corners need to be taped with fiberglass tape and embedded in thinset (just like taping a drywall seam except the tape needs to be compatible with the thinset). I like to use Redguard waterproofing on top of the cement board. Lots of information on all of this over at the  forum

Thanks so much for the reply! (post #214639, reply #2 of 11)

Thanks so much for the reply! I definitely do plan on using redgard. When you say adjacent piece of cement board do you mean to cut out and extra piece and tape and mud it to the other one? I think the small hole is too small for me to do something like this. Could i just tape over the small hole and then use thin set over it to cover it, then redgard it? Also, you are saying to tape along the bottom of the durock and have the tape go onto the tub flange as well? I know to do all connecting pieces with tape, but do you always have to do mud over the tape on the joining pieces? Sorry for the noob questions, first time i've ever done a shower like this.

I'm not sure from your (post #214639, reply #3 of 11)

I'm not sure from your description where exactly your small hole is, most times you can fill gaps with thinset cover with tape and a layer of thinset over the tape. The flange is supposed to be behind the cement board so you are not taping to the flange itself only to adjacent boards. If your cement board was kept high and there is a gap below the board and the flange that's not the best thing. The cement board should be almost tight to the the top of the tub (1/8 to 3/16 above). You could probably trowel in some thinset allow it to dry then run tape partially over the filled thinset and onto the cement board and then thinset over the tape.

What you want is a continuous surface for waterproofing. No gaps 

Got it. Yeah the gap between (post #214639, reply #4 of 11)

Got it. Yeah the gap between the tub flange and the durock is very inimal, just wasnt sure if i should be taping that, but from the sounds of what you're saying, that's a no. Thanks for your help

Hey there,    You (post #214639, reply #5 of 11)

Hey there,

   You definitely don't want a gap between the top of the tub and the bottom of the cement board. The tub has a flange you said, so that flange should be behind the cement board.  It is usual practice to add furring strips to the studs in the tub surround after the tub is installed so that the cement board can land on top of the tub edge, "jumping" the flange.  If you land the board on the tub and screw the board tight without the furring strips then you will bend the board against the flange.  This makes the tile job look bad, but it is a better water detail than holding the board above the flange.  You can't expect sealants to fill the gaps you have, and tile and grout are not waterproof so they cannot be relied on to properly waterproof that area.  My suggestion would be to remove the cement board above the tub and drop it down tight onto the tub, covering the flange, then deal with the small gap between boards by using mesh tape (for cement board) and thin set.  Up high in the feild is a better place for a gap or crack, not an inside corner where vertical meets horizontal.  Think of the tile as purley aesthetic.  A properly prepped tub surround is waterproof before tile in my opinion.  Good luck.  I did a quick drawing of a good detail at this critical spot.  

tub_detail.png51.07 KB

I am assuming the tub was (post #214639, reply #6 of 11)

I am assuming the tub was intalled first and cement board was installed next. I typically back/cut  the bottom of the cement board so that it fits snugly OVER the flange, and maintains its vertical integrity. If you don't do this, the cement board won't lie in a straight plane, and this will impact the appearance of the tile.  In my installations there is a 1/4" or less gap between the tub and the bottom of the tile backer. Mud and tile close the gap leaving only a miniscule gap between tile and tub. The tub flange MUST be behind the tile backer, else you risk leakage.

Mel Fros

vapor barrier (post #214639, reply #7 of 11)


Since you are using Durock (cement board) instead of Denshield,

I assume you have installed a vapor barrier acrosss the face of the studs prior to the Durock installation.

It's required to waterproof the installation, otherwise if the grout fails or, as is typical,

moisure wicks through the grout and the Durock, into the wall cavity, It can cause moisture damage and mold growth

in the wall cavity, including rotting the studs. I would suggest using a 6-mil poly.

Good Luck


Poly is not needed if you use (post #214639, reply #8 of 11)

Poly is not needed if you use Redguard.

red guard (post #214639, reply #9 of 11)



Then you need to "red guard" the whole area of the shower ie. all the Durock surfaces, to achieve water tightness.

Why bother with all that extra work? Just start by using Denshield in the first place and there's no need for Red-guard at all.

Denshield is much easier to work with and it's 100% waterproof, just seal the I.C.'s and butt joints as you install the pieces with

an appropriate sealer and your done, ready to set tile.



Geoff, I would still use (post #214639, reply #10 of 11)


I would still use redguard over the dense shield. I know it's fiberglass but you want a continous waterproof membrane. The denseshield should be taped at the seams, corners and screw holes filled then redguard, then tile. Similar installation details are used in cavity wall construction or EIFS applications where denseguard (same prodcut as far as I am concerned) is used it is covered in waterproofing.




Densguard/Densshield (post #214639, reply #11 of 11)


I'm speaking specifically about Densshield,which is waterproof, Densguard is not.

If you install as designed, it is 100% waterproof, no need to do Redguard. I've been using it for a few years

and it is superior to any other product on the market, in regards to backer-boards.

I took the time to submerge a 3" wide x 12" long piece of Densshield in a container of water, 6" submerged,

6" exposed above the waterline, I left it in this condition for  2 weeks. There was no absorbtion in the lower

section  or wicking of moisture into the upper 6". That was done to prove to a non-believer of Densshield's

integrity, and it worked. Proper sealing of butt joints, i.c.'s, and o.c.'s is a given in any istallation. I have

installations that were done in the mid- 90's, when the product first appeared, that are still dry and tight.



just my 2 cents