Kerdi membrane question.

Discussion in 'Ceramic & Stone Sales and Installations' started by Mill-End, Jan 16, 2017.

  1. Mill-End

    Mill-End Pro Member

    Hi guys and gals. I've been installing for about 15 years now and can probably count on both hands how many ceramic jobs I've done. I mostly do wood/lvt,lvp/lam. type stuff. Only ceramic tile jobs I've done have been floors on new concrete so pretty cut and dry.

    I've been remodeling my bathroom and I'm finally getting to the custom shower. I built a 5'x5' shower and used the kerdi system. Pan/drain with membrane over greenboard. I tiled the cieling by flat troweling unmodifed mortar to the membrane then raking out mortar as instructed. Went ahead and back buttered the 12x12 porcelain tile for safe measure and slapped them up. Pulled a couple down and definitely got 90-95% coverage.

    Only problem I am having is the mess. Wow, I was covered in mortar. Luckily I covered the flat pebble floor with ramboard or else I'd be cleaning mortar in those cracks for days. 90% of the mess came from the raking.

    My question is, what is the technical reason why I couldn't flat trowel the membrane and rake the mortar on the tile? I've googled it and found many, many people saying don't do it, but most of the reasoning is simply it's not what's recommended.

    I mortared some spare membrane I had on a spare piece of green board. Let it dry over night and tried my method. Put a thin layer on the kerdi and troweled the tile with a 1/4x3/8 trowel. Set the tile and pulled it back off and definitely had just as much coverage as the ceiling. I then scraped the mortar off and did it again and let it sit 24 hours. I tried prying the tile off and destroyed the greenboard in the process.

    So it seems to me it will in fact work even though it's not the recommended practice. Well hell, glueing carpet over existing glue isn't recommended either and people do it all the time with no problems. Am I safe to go ahead and use this method for the walls?
  2. Mike Antonetti

    Mike Antonetti I Support TFP Senior Member

    I would not have used greenboard, I only use hardiboard, especially on ceiling and water areas.

    The flat trowel theory which is absolutely correct helps to lock the setting material into the surface. Absorbent surfaces such as slab should be dampened, otherwise it robs moisture from thinset/adhesives etc.

    This tile ripup we did yesterday they didn't flat trowel on slab, the whole floor pulled up rather easily. Almost 900 ft of tile removed in 8 hours, a very expensive mistake to the homeowner, very lucrative day for us.

    Attached Files:

  3. Mill-End

    Mill-End Pro Member

    Hardiboard with Kerdi membrane over it? I thought the reasoning for Kerdi membrane was so it could totally waterproof drywall type materials.

    As for flat troweling, I completely understand the reasoning for flat troweling the surface first, which I did for the ceiling and floor then troweled out mortar for the tile. My question is what would be so wrong with me flat troweling the membrane then troweling on the back of the tile as long as I'm getting the same coverage?
  4. Mike Antonetti

    Mike Antonetti I Support TFP Senior Member

    I thought I didn't totally get your question. Flat trowel both and comb the tile? That is perfectly fine. I think I've seen an automatic machine that trowels the back of tile on an assembly line type system.

    Drywall has no business in a shower area. It happens, it can last for some years but many failed without Kerdi. Maybe it's approved by Schluter, I'm not fluent in the product, or that method, you used thinset to bond kerdi to the drywall?

    A few tile setters here, JCW?, Jaz?

    I'm not a fan of tiling ceiling, but when I do it ain't coming down due to any fault of mine. My waterproofing choice is Redgard liquid membrane,
  5. Mike Antonetti

    Mike Antonetti I Support TFP Senior Member

    Just two days after I posted, we are removing a tile floor where someone is renovating the bathroom. Here is how not to fasten drywall. The fastener head should not penetrate the paper, only countersunk slightly. This is even done on ceiling drastically reducing strength as the force is downward so the fastener is only holding on maybe 1/4". On the wall downward pressure the fasteners pin has to pull a long distance and still the board won't go anywhere.

    Add ceramic tile to ceiling increases pull strength. Where to go for hurricane? Bathroom where rain can get into ceiling, with wind, I guess it would be a perfect storm scenario but it could happen. A few weeks ago around 1:30 am I get an alert on my phone, (amber alert style) saying tornado warning coming thru neighborhood, I wake my wife, tell her and she's back asleep snoring 2 minutes later. But it got me thinking where to go, bathroom was safest as slider doors and windows in other rooms. My bathroom may be getting hurricane rated window as a safe room.

    Attached Files:

  6. Lazarus

    Lazarus Pro Member

    Mike~Only problem with CBU's under Kerdi is the fact that they suck out the moisture in thinset so rapidly that it's a race to get the Kerdi up & embedded in time. Minimum when I HAVE to use it is to use a pump-up sprayer to wet the walls before the thinset goes on.

    Schluter training in 2007 showed that drywall is the preferred substrate, and of the scores of showers I've done, there has never been an issue or failure...and I do follow up. :yesss:

    Done properly, the drywall never gets wet. If it does, it's because of plumbing issues. With CBU's, you'll never know it...until the damage to framing, etc, is massive. With drywall, at least you'll have some advance warning. For this reason, I always put in new mixers and any suspect plumbing lines.
  7. Mike Antonetti

    Mike Antonetti I Support TFP Senior Member

    Yes, they are some thirsty bastards! I can't get around the drywall thought, Don't even like Denshield, I did hear the word drywall passing thru an Ardex 8+9 demonstration.

    Drywall is not very rigid as well. My buddy texts me about a rental that tile in tub is falling off, I'm booked, don't have time for that bs. So greenboard and mastic! If it were Kerdi done properly then yes it should never see water. I suppose if that were considered do my bathroom and a tilesetter wanted to use, I guess I would relent. I'd prefer they switch to an adhesive versus thinset, not a fan of that method either. Hell I'm a fan of nothing! (This is my mood after a long 4pm day, two sips of coffee!)
  8. Lazarus

    Lazarus Pro Member

    Well, Greenboard hasn't been "code" for 2 or 3 decades and, like you, guess I'm set in my ways 'cause I'll never use mastic in a wet area....seen too many fail, and yes, I've seen some last, as well...but I don't like the odds.

    Interestingly, ceement board has more flex than drywall. Ever put CBU in a ceiling with 16" joist spacing? Seen a couple where they sag between joists. Never with 1/2" drywall though. (go figure)
  9. Mike Antonetti

    Mike Antonetti I Support TFP Senior Member

    Yes I don't like CBU, only Fibreboard, Hardiboard, not sure what I may have said above, Durock crackles, I dislike it, won't use that one either.

    Fiberboard I'm not happy about delaminating issues but not much I can do.

    No mastic for me as well, but behind Kerdi just like adhesive for Ditra by Ardex AF 207?

    I do like grinding Mastic on floor, easier than portland thinset.
  10. Mike Antonetti

    Mike Antonetti I Support TFP Senior Member

    I spotted this from the road, it's a topic of coverage, outdoors it's supposed to be 95%. I saw the shadow under tile from 50 ft away. They just grouted yesterday as I was eyeing them down. Let alone the edge of tile will be weak. I don't get it, I always focused on the edges of tile, especially when it's in a walk area like to carpet/hardwood etc.

    Attached Files:

  11. Kman

    Kman Tile Expert I Support TFP Senior Member Published

    Although this thread's a little old, I'll give my opinion on the OP's question: There's absolutely nothing wrong with skimming the substrate and combing thinset on the back of the tile. If the substrate is flat, the combed thinset will flatten out anyway. As long as you have wet, sticky thinset on both surfaces, the bond is good.

    The real advantage is that you don't get thinset all over your clothes, the floor, and your new hat. :yesss:

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