Kerdi Over Mortar?

[rdtompki]

I'm remodeling our large master bath from the studs up. Plumbing, electrical, insulation, ventilation, sheetrock myself, having someone else do the tiling, but I really like the idea of Kerdi for shower and whirlpool tub surround. Our compromise since the tile installer is a committed "mud" guy is that I'm going to Kerdi over the mortar. This appears to be an approved procedure according to the TCA guide. So, I've got two questions.

1. I assume that the mud can be applied directly over sheetrock. Can't use a vapor barrier as the Kerdi will provide that function. Is sheetrock/mud/thinset/Kerdi/thinset/tile an acceptable approach for a shower? I'll certainly use a Kerdi drain.
2. For plywood substrates, horizontal and vertical, can the mud be applied directly to the plywood followed by Kerdi and tile, or do I need a substrate under the mud for isolation? What substrate would you recommend? I'll be using the Kerdi on the vanity and tub surround (deck and wall)

Will use Ditra on the floor, of course.

Rick

[Mike2]

A real good mud bed can't be beat Rick. Done right it's still the best. Why Kerdi on top of that?

Don't get me wrong, Kerdi is a great product. Have used it and will use it again. But I'm no mud guru either.

[rdtompki]

This is the second shower replacement due to leaks (actually, replacing the entire bathroom now). Our tile guy, whom we really like, does not do Kerdi, but I really like the idea of having the water barrier right under the tile. I realize this is an extra expense and more work for me, but that aside it would seem to be the best of both worlds: it's the engineer in me.

I've read enough to know that a good mud job with proper preslope, liner, vapor barrier will outlast me so I could go either way. I was a bit put off by the hot mop approach, however, of this particular tile installer, so I would want to insist on preslope and CPE liner.

The whirlpool tub surround is another matter. I don't know how much water winds up on the surround, but having the waterproof membrane (Kerdi or Ditra?) immediately under the tile and possibly wraping Kerdiband around the mud/plywood edge seems like a good way to go rather than having the barrier under the mud. Am I overthinking this? Redoing a 250 sq. ft. bathroom is an expensive proposition and I never want to see another tile problem as long as I live (last shower install had not preslope; line directly on slab!).

Rick

[cx]

Sounds like you're mixing methods here, Rick, and not all to your advantage.

If you plan to use a mud pre-slope and PVC or CPE liner, you can't use the Kerdi on the shower floor, which is where you gain the greatest advantage from the Kerdi system.

And building mud walls for a shower is a good way to make everything plumb, square, and flat, but it's pretty gross overkill if you plan to install Kerdi. Don't get me wrong, I'm really a big fan of overkill, but you're just not getting much benefit from this particular brand of overkill and it's gonna cost you substantially more than just using the Kerdi system over properly prepared sheetrock walls and a properly built mud pre-slope.

I'd suggest you chose one or the other. Either shower construction method, properly done, is likely gonna outlast your need for it. The Kerdi does give you the advantage of not having water past the back of the tiles, which is a distinct improvement over the mud. But the mud over a proper moisture barrier has been a proven method for many, many years. The water-soaked pan is it's biggest drawback to my mind, but with proper pre-slope and weep-hole construction, that's not a serious concern in most households.

I think you should chose one method or the other and stick with it.

My opinion; worth price charged.

[rdtompki]

I'm inclined to agree. If I can't persuade my installer to try Kerdi then I'll probably just go with mud. I'm drawing the line, however, at the hot mop and will insist on a CPE liner which, I believe, will have to extend over my wood-framed triangular seat (or we could build a monument seat within the liner I would guess).

I misspoke in the last post in that I realize I can't mix Kerdi and CPE

I assume something like a tub surround doesn't see enough water to worry about moisture passing through the grout and mud. Generally, I've seen tar paper or similar used between the mud and plywood.

[Kirk Grodske]

Go with the Kerdi. it is the best for your application. If your tiler won't work on Kerdi, find another. Big room means probably poor ventilation so Kerdi will help reduce mositure in room by not letting it get to the mud bed.

[rdtompki]

I will have decent ventilation as I'm putting in a 250 cfm remote Panasonic vent fan (if we remember to use it!). It's a bit late to switch installers as we've used quite a bit of this gents time for consultation, etc. I'll try the Kerdi over sheetrock approach again. Better do it soon before I finish the rough-in on my shower valve. It's current set at a depth for mortar.

Rick

[rdtompki]

Given I'm going to full-court press for a Schluter bathroom I'd like to verify my approach to bench and tub surround. The bench will be stoutly framed, no PT wood, 3/4" plywood top; top and sides covred with sheetrock. I understand the sheetrock is necessary over the plywood because Kerdi doesn't have the isolation ability of Ditra.

The tub surround will be ditra over 3/4" plywood very will supported by 2x6 stem walls. Is the single layer of 3/4" plywood enough or should I apply a second layer of plywood.

I've got one transition from tub surround to shower 6 feet away from the shower head. Seems like I should either overlap the Kerdi inside the shower over the edge of the ditra-covered surround or use Keridband to joint the horizontal Ditra and vertical Kerdi. Does this make sense?