Kids Bathroom Redo - Kerdi and Hydroban Questions [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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07-03-2019, 11:27 AM

Thank everyone in advance for any and all help. I have done a number of tile projects and have normally found my answers just lurking and searching around. I looked around quite a bit on both of these and didn't find the exact answer. Answers, or direction to the proper thread is appreciated.

I will warn everyone in advance, I am a mechanical engineer by education, so...

Also love looking at all your work to admire and get ideas.

Now, onto my questions:

So, in thinking I was doing right (I guess looking at the Ditra install instructions I just assumed, I know, the Gypsum concrete was concrete backer board) I installed 1/4" Hardie on my new subfloor.

To get everything right, I pulled up the original subfloor, it was 5/8" ply with 1/2" ply glued and stapled then 6" porcelain.

To have a level tile to carpet transition, I sistered all 2x10 solid joists (max unsupported 11.5') with 2x10 shaved to 8.75" high. I then shaved the existing joists to a range of 8.75" to 9" where the floor would be. Also, ran cross bracing, you know just to over do it. Laid 2 layers, 23/32" plywood, bi-directionally.

Then, dun, dun, DUN. I installed 1/4" Hardie on the new floor. It is installed correctly with thinset underneath, and screwed with CBU screws. I now have a really solid floor at perfect height for Ditra, and 3/8" tile to be flush with the carpet. Was really after the additional mechanical relief and waterproofing from the Ditra.

Knowing the underlying support is good, and the Hardie was just to give a Portland substrate to mortar the Ditra, is this still a no-no. My problem now is A) getting the backer up and B) Getting back to level as I glued and screwed the second layer of plywood.

Any thoughts, advice, condolences greatly appreciated.

On the Hydroban. I have done Redguard in the past and was just meh with the process. Looking at Kerdi and Hydroban here, I decided I would use Hydroban.

This is a 5' tub/shower. I have two niches I built. All new drywall and mud throughout. When doing the niches and two vertical corner seams I have seen people add fabric to the thinset, and not, prior to painting on the Hydroban. Have also seen Hydroban direct with no thin set. What do the experts consider best practice?

Finally, the drywall/paint installer (I hate mudding so I had paint and mud done by a pro) took the paint further into the tub alcove than I would like, maybe 4" - 6". Will I have any issues with the Hydroban over the painted surface?

Thank you all so much for any advice on the above items.

Have a great 4th of July!!!

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07-03-2019, 12:17 PM
Boy Mitch you really went way beyond what was necessary. Assuming a ceramic/porcelain tile installation you could have simply replaced the old 5/8" plywood with 23/32" and addded 1/8" Ditra or 5/16" Ditra XL and it would have been at the same height or lower compared what you have there now and there would have been no notching required. Your existing joist structure was more then adequate. The old 5/8" was technically bare minimum adequate assuming you didn't exceed 16" OC joist spacing.

Did you follow the recommended fastener schedule for dual layers of plywood to avoid any voids due to waviness between sheets? Page 17 of the Ditra handbook:

For 3/4" ply it's 6" in the field and 6" on edge. That is a LOT of screws and more than most people know to do. Also NO adhesive between the sheets. I see that you added adhesive which can be a problem because it produces high and low spots which creates voids between the sheets unless you went out of your way to scrape the glue even and create a consistant glue laminate thickness. Most people just squeeze out beads of adhesive from their caulk gun which creates high and low spots between the plywood sheets allowing for flex between the beads.

I don't see much issue attaching the Ditra to the Hardie assuming the Hardie is installed properly. Although, it is not specifically mentioned as an approved substrate in the Ditra handbook listed above. Just make sure you use a modified thinset mortar and adequately sponge down the Hardie to avoid moisture being sucked from the Hardie.

You say you used drywall to sheet the shower? This is not an approved substrate for Hydroban liquid membrane. So that is a no go. You can use Kerdi over drywall but because you used drywall joint compound you must now use a latex primer over the joint compound because it is water soluble and the moisture from the thinset can turn it to mush.

The latex paint on your drywall is no problem for Kerdi (infact it's latex primer that is required over your joint comopund as listed above). I would however lightly sand the finished paint to give it some grip (especially since it's likely gloss or semi-gloss). Neither latex paint nor drywall nor joint compound is an approved substrate for Hydroban liquid membrane in a shower. Hydroban does not require tape on gaps smaller 1/8" or less or corners but many people do anyway for extra security (including myself). Hydroban requires the cement board or hardie backer to installed per the manufacturer instructions which means suitable fiber tape and thinset to bond the sheets together prior to hydroban.

Maybe some of the Kerdi experts can chime in on how to adequately protect your drywall from the tub to drywall transition area. I'm not sure if you run a bead of Kerdifix and seal Kerdi band to the tub or what. Also the area where the tub meets the drywall along the vertical wall of the tub can be highly problematic as this area sees a ton of moisture and you definitely want your waterproofing to extend a few inches past the tub as seen here:

Ahh okay I did a search and here is the official word:

When I use Hydroban over CBU in a tub/shower I leave a small space between my cement board and the the top of the tub tile flange or tub rim and fill it with Hydroban seal and caulk and then cut my hydroban in and apply it over the caulking with a brush and protect the tub with painters paint. Just make sure you remove the tape before the Hydroban kicks.

Hope that helps...

07-03-2019, 01:32 PM

Thanks for the response. I know the floor was overdone. Early on the wife was thinking about large format Trav. So, out of fear, I went with the strong subfloor, 14" OC sisters, and the cross-bracing. I did actually spread adhesive with a margin and laid a ton of fasteners (between 6 - 7" spacing). Made the Hardie fasteners a trick, but never doubled.

Now you have we confused on the Hydroban though. Their info says acceptable for wallboard install:

Just no continuous submergence?

07-03-2019, 02:02 PM

I know that they list "Gypsum Wallboard (Interior Applications Only)" as a listed substrate. But I don't think they have a shower in mind in that application. Feel free to contact Laticrete technical support and get it directly from the horses mouth but I have never heard of drywall being a suitable substrate for Hydroban liquid in a shower.

In fact in this instructional video you can see they specifically call for an "approved suitable cement backer board":

Now the other side is whether that use is a code compliant installation. In most jurisdictions drywall and even moisture resistant drywall is not a suitable backer board in a shower. Schluter provides a specific certification for approved use over drywall which will satisfy most building departments (but some stubborn ones still may not approve it). I don't believe there is any such certification for Hydroban liquid.

Keep in mind two coats of Hydroban has about twice the vapor permeability of Kerdi.

Will it work? Maybe but I still wouldn't do it. Further waterproofing the edge of drywall where it meets the tub will be especially tricky and difficult IMHO. Let us know what Laticrete tech support says as I'm curious to know their official stance.

In a tub application I would strongly prefer a CBU (at least in the lower section) or a foam board. I don't like the idea that a bead of caulking properly bonding to the tub surface is the only thing protecting the edge of the drywall from turning to moldy mush.

07-03-2019, 02:22 PM
Unless things have changed, only hydroban sheet membrane is certified for use in a shower (well, other than Kerdi) over drywall.

You said you used your ply at cross install? Both sheets must be installed perpendicular to the joists. Hopefully, I read that wrong. The second layer isn’t required for a ceramic install, but a properly installed second layer is with natural stone. There is a big difference in stiffness across the x-y axis.

07-03-2019, 04:49 PM
Well, now I must go feel shame. Thankfullly, the floor is not stone, just porcelain.

Guess I am going to be returning the Hydroban for Ditra. Now I need to think about the drywall at the bottom of the install...

07-16-2019, 08:15 AM
Hey Mitch - I just confirmed with our Tech Services Department, TCNA says that gypsum wallboard is not a suitable substrate for showers.
John's technique of using HYDRO BAN® Adhesive and Sealant at the top of the application is a good system.
HYDRO BAN should not be applied over paint. The membrane may stick but it is not a confident bond.

09-02-2019, 10:24 AM

Thank you for your response. I am going to re set the tub wall areas.

As always, JB forum is a great resource.

09-02-2019, 10:52 AM

I corrected, but kept the Ditra. Pain in the butt, but needed to be done. I now have porcelain 12 x 24 laid over Ditra using Schluter Set. I mixed it at just a hair over 6 Qts per 50 Lbs and followed the 5min/10min/3min mix/slake/mix instructions.

I have read a ton of the threads about using unmodified between porcelain and Ditra, and understand you are relying on hydration to cure the thinset.

Here is where I am at: Laid my tile Saturday evening. Finished the install at about 8:00pm. Here we are about 36 hours later. I know the Schluter instructions say 24 hours before applying grout. I am using Spectralock Pro+ for grout. As I am relying on hydration for full cure of the thinset, installing the "impervious" epoxy grout at this point should not adversely effect the curing of the mortar, correct?

I have used the Spectralock on a number of installs before, but this is my first Ditra installation so I am seeking extra advice to ensure I stay on the right track and thought process.

As always, thank you all in advance for your help and responses! Hope you are having a great Labor Day.