I hang Kerdi on cement board and fibercement board all the time.

No worries.

Mongo

Same here.

Sure, you can do it....just be sure to "spritz" or wet the hardi thoroughly before....it's pretty "thirsty."

Yup! That's the way us "Tile Guys" do it all the time.

Been clankin' away at this computer for a couple of hours now, guys. Gettin' a little delirious. Needed to interject a little "amateur humor."

Signed,
Never Hung Kerdi A Day In Her Life

We just went through a learning curve on the Kerdi and Hardi. Thanks to Mongo's earlier post, we knew we'd have to spritz the Hardi -- but found that even that wasn't quite enough. We ended up sponging the Hardi and wetting and booking the Kerdi (a la wallpaper), and that made the installation go much smoother. We also found that using a heavy rolling pin worked wonders for really setting the Kerdi.

Rolling pin, good, now I can throw away my magnesium float.
LOL
Casey

Casey,

I set the Kerdi with my hands, pat it into the thinset with my hands, then go over it with a 4" taping knife held at a low angle to embed the Kerdi in the thinset.

ps, now that you don't need it anymore you can throw that useless cheap piece of crap magnesium float my way. ;)

Mongo

Well, DH said he had virtually no time to work with the stuff before it dried out, so he switched from my spritz bottle to a sponge. And by 'booking the Kerdi' I mean we put it in the bathtub, sprayed it with the handshower, then booked it (just like wallpaper). It worked well - just like wallpapering.

On the 'rolling pin', it actually wasn't the kitchen kind, but rather a very heavy metal roller DH had from some sort of fabrication table.

Thanks, that clarifies.

Sweeby-
Can you clarify- I undestood that the thinset would tend to dry out from the thirsty hardi or skim over from the air; so how does booking the Kerdi help with the first? And how do you keep too much water from interfering with the adhesion in the second scenario?
Thanks!!

Pepper -

I just a few weeks ago put kerdi on top of hardi-board in my shower and it was PAINFUL! If you do not already have the hardi up, I recommend using real cement board (Hardi is a fiber-cement) or drywall.

When I tried putting up the first sheets, which were only 36" x 39", and even though I had SOAKED the hardi, to the point water was pooling on the floor, the thinset was already drying before I got the Kerdi up, and I couldn't get it to adhere properly no matter how hard I tried.

I posted a plea for help and Mongo and Bill both gave me good advice. I ended up putting up a thin, skim-coat of thinset all over the hardi board, letting it dry, and then putting up the kerdi. I still wet the walls first. I also used a larger sized trowel than the one recommend by Schluter. Also - use a good thinset. I think the first one I used was cheap and that was part of the problem.

When I do another shower I'll use real cement board, not Hardi-board.

True cement board (Durck, Wonderboard) is my first choice as a tile or Kerdi backer. One thing I do like fibercement (Hardie) for is when I need thin strips of cement board. To line niche shelves, for instance.

When screwed to the framing, thin strips of cement board fracture more easily than thin strips of fiber-cement.

Now there's nothing particularly wrong with fiber-cement...it's just not my first choice.

Mongo

The taping knife at a low angle makes an enormous difference, and I'm very grateful for that hint. When I tried to use the edge of my trowel [per schlutter video] I could see I was pulling a lot of the 'felt' off the backer.

It also helped me to work in smaller sheets. More waste, yes, but I think you have to have a LOT of practice to hang those huge sheets like mongo does.

Other mistakes I was making early on:
-over working the thinset. Slap it up comb it out, leave every ridge tall and perfect. A hundred times better to have too much before you comb than too little. Too little, and the kerdi will never show you gray, no matter how you sweat. No bulges either, but definetly no thin spots.
-Not paying attention to the direction of the comb. Your air bubbles will travel along the valleys you've made, and it's way better if you only have to move them 20 inches to the nearest edge rather than all the way from the ceiling to the floor [Again, Mongo can, but he has mad skills].
-Not paying attention to the principle of bedding top down, center out. I barely touch it with my hands before going to the tape knife. You don't want to block off the end of the air chanel and trap half a sheet worth of air under there.
-Not paying attention to the direction of my knife. The fastest method [for me and possibly other dilettants] is to stroke the knife at a diagonal to the comb marks, top down, center out, pushing all the air before you with short, overlapping strokes.
-Thinset needs to be pretty darn wet: Needs to stay put when combed, but should be soft and shiney. Seems like I've been using the full measure of water the bag calls for, plus about 10%.
-50 lbs of thinset at a time is too much for an amatuer. Go with smaller bags or only mix half.

Mind, those last two are at high elevation and low humidity: YMMV, which stands for 'your mud may vary', I think...

I like to use a garden sprayer to keep the thirsty Kerdi wet. I simply sponge off the excess.

This thread is 9 years old and it's the Hardi board that's thirsty not the Kerdi.

I'm thirsty. Creative, C'mon over for a beer!

Anytime Mongo. I imagine we could share some good stories. It may take more than 1 beer:)