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ADIAGA 07-27-2010 04:05 PM

Using Schluter-KERDI wall system over plaster walls.
Does anyone have any advice for or against using Kerdi over old --but in good shape!-- plaster/metal-lath walls in a tub-shower area? The old unit was a fiberglass 3 piece that did an excellent job of keeping the plaster dry over the past 40+ years.
I removed it and have patched some rough places in the plaster. (Someone swings a hammer badly...I won't say who.) :)
I really do not want to bust up the old plaster/metal-lath combo to install cement board.
I've seen a few videos online about using the Kerdi wall membrane and I'd rather spend 100 bucks there than swing a sledgehammer, possibly scratch my porcelain tub, and have to shim the dickens out of the cement board!
I'm already planning to use Schluter Ditra on the old tongue-and-groove hardwood floor.
I'm a J.O.A.T.M.O.N....Jack Of All Trades, Master Of None....but I've done a lot of remodeling for some friends....never installed tile before. My virgin project, I suppose. Wish me luck...or the winning Lotto numbers so I can hire this done! LOL :thumbsup:

JazMan 07-29-2010 04:16 PM


There is no problem applying Kerdi Membrane over your plaster walls. Just be sure they're in good shape. clean, dry no lose paint etc. Use unmodified thin set to install the membrane and to set the tiles. I suppose you know the rest, if not ask.

However, Ditra does not go directly over hardwood, or any planked floors. If it's hardwood over a subfloor, remove the hardwood and add plywood underlayment as necessary.

I'm thinking your subfloor might be planks too. Let us know how the subfloor system is built starting with the joists and working up.


ADIAGA 07-30-2010 06:50 AM

Thanks for the fast reply. Ok, now I'm stumped. I can use Ditra over OSB board with spacing of joists up to 19" o.c. yet I'm not supposed to install it over 50 y.o hardwood flooring installed over 1"x6" diagonal subfloor on 12" o.c. joists? Seems odd to me. The previous small-diamond-on-webbacking ceramic tile was installed directly over the hardwood over 30 years ago without any cracking. I believe I'm going to go out on a limb here and go ahead with the Ditra over hardwood. Try something new.
10 years ago, I was the first person I know of in this area to use the then-new fast-install 'quick connect' water valves on copper pipe even though everyone said it shouldn't be done but no one had a good reason why not. Now, most everyone uses them or Shark-bite valves! Heck, I probably was the inspiration for the shark-bites! LOL

JazMan 07-30-2010 11:08 PM

I have no idea about those water pipe connector thingies, and wonder what that has to do with Ditra over planks. :huh: I don't know why you're stumped, you asked a question, and I gave you the right answer. :whistling2:

Your comparison of installing Ditra, (or any membrane or CBU), over OSB also has nothing in common with old hardwood floors. Plank flooring is not a stable substrate for tile. It shrinks and expands too much, as I'm sure you've noticed. It expands with higher humidity and shrinks when the heating season comes. Tile may stick to it, but the movement causes the grout to develop hairline cracks and worse.

There is no adhesive or mortar that can be used over hardwood flooring with confidence, regardless of what you're trying to install.....well, except carpeting.


ADIAGA 07-31-2010 09:14 AM


Originally Posted by JazMan (Post 478052)
I have no idea about those water pipe connector thingies, and wonder what that has to do with Ditra over planks. :huh: I don't know why you're stumped, you asked a question, and I gave you the right answer. :whistling2:

The "water connector thingies" was in reference to doing some thinking outside the box. :)
I'm not stumped, so much as puzzled that OSB board on 19" center-to-center spans a far greater area than my hardwood nailed 1x6 subflooring nailed to 16" c-to-c joists and I bet flexes more! :wink:
Also, because I believe that tongue-and-groove hardwood that has shrunk about as much as it's possible to shrink over the last 60+ years will not expand nor contract anymore than OSB board would under similar circumstances. Yes, there are more 'cracks' between the boards, but hardwood flooring that has been properly installed (and this was installed back when craftsmen were proud of their work, unlike today) has been sufficiently nailed to the joists, so 'expansion' would be minimal, otherwise the hardwood would have buckled.
If Ditra is worth the cost, I have confidence that it will 'handle' any expansion/contraction. Otherwise, I can save myself some major money and buy that fancy tub/shower faucet with the savings!:thumbsup:
Thanks for your input, but I'm always willing to try something new.
Check back in 5 years and I may post some pics.
Time, as they say, will tell.

JazMan 07-31-2010 01:49 PM

Yes I know you were trying to reference an example of working out of the box that actually worked, but that doesn't mean improvising in this matter is the way to go, but be my guest........

You keep comparing OSB stiffness to hardwood, but that is not the point. The point is lateral movement, not vertical movement. (deflection). The max deflection for ceramic and porcelain tile is L360 regardless of what the substrate is. Every individual plank is moving as it likes, laterally. That is the concern. One other reason to remove the hardwood is to not build the floor up too much. That's another 3/4" of height.

Wood never stops shrinking and expanding, regardless of its age. Haven't you noticed wider and narrowing gaps, sticky doors and drawers, squeaks?

As far as bonding to hardwood, please tell me which thin set mortar you're planning to use.

ciao e buona fortuna.....:thumbsup:

Bud Cline 08-02-2010 08:56 PM


You asked a question and you got a response from a skilled and experienced flooring person specializing in ceramic tile installations.

I can tell you the response you received from JAZ comes from industry-wide recommendations supported by the Tile Council of North America.

You can think "outside the box" all you want, you can experiment all you want, you can install your products any way you choose. What you are getting here are answers from an industry that knows what does and doesn't work. You do what you want to.

I can also tell you that if you think 60 year old slat wood floors don't expand and contract seasonally, you are simply mis-informed sir/madam. All wood has the abilty to expand and contract throughout its lifetime. Has nothing to do (what-so-ever) with an installers pride in workmanship or whether or not the product is "sufficiently nailed to the joists".:)

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