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liquid waterproofing (redgard) over durock cement board - bathroom FLOOR

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liquid waterproofing (redgard) over durock cement board - bathroom FLOOR

Hey guys and gals,

Will be doing a bathroom gut and will be laying durock cement board over plywood sub floor. I don't want to use the Ditra membrane product but rather tiles directly over the cement boards. Do I need to apply a liquid waterproofing layer (redgard) on the cement boards before laying down the tiles with thinset?

I would think that makes sense since ovbiously the cement boards aren't waterproof, but I've seen many sites now showing installation of the tiles over the cement boards without any liquid waterproofing.

For the shower walls, I won't be using Kerdi either, but rather a vapour barrier behind the cement boards, so no liquid waterproofing there. Just not sure about the floors...
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I would use thinset under the cement board to secure it to your subfloor, but screw down the sub floor first. As for the waterproof membrane, normally you incorporate the shower into the floor so they're both one continuous waterproof membrane. Keep in mind that the schluter products do more then waterproof.

I'm curious why you would want to waterproof the floor cement board but not the shower/tub CB? As you've mentioned cement board is not waterproof it's water resistant therefore eventually the shower/tub area CB will get mositure. Even if you've done a great job, eventually moisture will get passed the grout and into the cement board. A good thing about CB is it will eventually dry and most likely won't get damaged but mold will eventually grow between the CB and tile or CB and wall.

I would rething your waterproofing method since the vapour barrier will help protect the studs, but what about where you screw in the CB, you've penetrated the vapour barrier at those points. If anything the shower/tub area will have the potential to have more moisture then the floor......no?
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Why don't you want to use Ditra? It's by far the superior way to go. Very simple to install as well. Besides water-proofing it has the added advantage of keeping your tiles from cracking if the house shifts.
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Maymybonneliveforever wrote:
Apr 10th, 2012 8:17 am
I would use thinset under the cement board to secure it to your subfloor, but screw down the sub floor first. As for the waterproof membrane, normally you incorporate the shower into the floor so they're both one continuous waterproof membrane. Keep in mind that the schluter products do more then waterproof.

I'm curious why you would want to waterproof the floor cement board but not the shower/tub CB? As you've mentioned cement board is not waterproof it's water resistant therefore eventually the shower/tub area CB will get mositure. Even if you've done a great job, eventually moisture will get passed the grout and into the cement board. A good thing about CB is it will eventually dry and most likely won't get damaged but mold will eventually grow between the CB and tile or CB and wall.

I would rething your waterproofing method since the vapour barrier will help protect the studs, but what about where you screw in the CB, you've penetrated the vapour barrier at those points. If anything the shower/tub area will have the potential to have more moisture then the floor......no?
Regarding the continuous waterproofing transition from shower to floor, it's because I didn't mention it's a four piece so there's a tub. I won't be waterproofing the walls since with the vapour barrier I would be creating a barrier pocket with two barriers.

With the floors though, since there's no vapour barrier (poly 6mm) on top of the sub floor like how there would be on the studs, that's why I'm asking whether or not I should put a layer of liquid waterproofing on the cement boards first. I will already be attaching the cement board to the sub floor with motor thinset, so are you saying that's a good enough vapour barrier, so no additional redgard needed on top of the cement boards?
Therion wrote:
Apr 10th, 2012 8:22 am
Why don't you want to use Ditra? It's by far the superior way to go. Very simple to install as well. Besides water-proofing it has the added advantage of keeping your tiles from cracking if the house shifts.
Two factors really: cost and the fact that ditra/kerdi have downsides as well. I'm sure they're good products, but they're still not perfect. Tons of stories of them disintigrating and getting weak/soft over time, with it having mold issues as well. Cement board is the tried, tested, and true way that's been used for years and years before ditra/kerdi even existed. Sure ditra/kerdi is new technology and may be the new way, but doesn't mean the old way doesn't work either.
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gheart008 wrote:
Apr 10th, 2012 11:30 am
Two factors really: cost and the fact that ditra/kerdi have downsides as well. I'm sure they're good products, but they're still not perfect. Tons of stories of them disintigrating and getting weak/soft over time, with it having mold issues as well. Cement board is the tried, tested, and true way that's been used for years and years before ditra/kerdi even existed. Sure ditra/kerdi is new technology and may be the new way, but doesn't mean the old way doesn't work either.

I'm kind of curious as to how mold grows on the ditra if there's no organic substances for the mold to consume. Unless you mean mildew or something else.

I also have a hard time seeing ditra disintegrating or getting weak: it's a pretty darn thick piece of plastic, and it's flexible. I would assume that the problems you're alluding to is due to a bad install. In my house, the bathroom, done by a contractor, used cement boards, etc. The grout lines already started to crack. The kitchen floor, which I had built up similarly to the bathroom floor, but used ditra, hasn't cracked yet. The subfloor beneath the tiles is pretty much the same. The slight exception being that I probably secured the plywood to the joists a bit better than the contractor did.

Yes, ditra is more expensive, but I didn't find it prohibitively more expensive than cement board. E.g. it cost me ~$2/sq ft for ditra. At HD: http://www.homedepot.ca/product/durock- ... -ft/911230 that's $36/32 sq ft. So the price for ditra is about double per sq ft over CB. In the total cost of the kitchen reno, it cost me an extra $200 over the cost of the CB. Which, given the rework that's required now in the bathroom, is well worth it.

If you want to continue to use CB, or, heck, use diamond lathe and make a jersey mud bed (which was used before CB), you just need to factor in the cost of rework. Given my recent experience, there's no way I would go back to how it was done before.


A couple of questions: 1) if you use waterproofing on the CB, how will you prevent water from going through the gaps between the boards? and 2) If you use waterproofing on the CB, will the thinset stick? I guess if you used a modified thinset, it'll stick, right?
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There's no doubt in my mind Schluter's Ditra is the superior product. But I do understand your concerns with the cost, it is expensive.

I wouldn't bother waterproofing your subfloor. Remember, Ditra is an uncoupling membrane before it's a waterproofing method. The waterproofing aspect is a bonus in my opinion. Where Ditra shines is it's ability to prevent subfloor movements from transferring to the tiles & grout. This prevents cracking as mentioned above.

As for installing cement board on your floor first. It really has to be done well. Thinset and screws. Any movement in that subfloor is going to have a direct impact on the tiles & grout. If not done properly, you will no doubt see cracks in the tile where the cement board butts up underneath.

Your method for the shower walls will work. Tiles & grout are not waterproof as you know, and neither is cement board. As the tiles & grout leech moisture, it will transfer to the substrate, in your case cement board. When the cement board gets wet, the moisture barrier will prevent the water from reaching the studs. Cement board contains no organic material, so it stands no chance of growing mold.

My honest opinion? I'd do cement board and Kerdi in the shower. Once again, Kerdi is far superior to moisture barrier behind CB, in my opinion. But if you do Kerdi, do not do a moisture barrier behind the CB, or you'll end up with a moisture sandwich.

This leads me to my next point, what material to you plan to install the tiles with in the shower?
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PhuFighter wrote:
Apr 10th, 2012 11:56 am
I'm kind of curious as to how mold grows on the ditra if there's no organic substances for the mold to consume. Unless you mean mildew or something else.

I also have a hard time seeing ditra disintegrating or getting weak: it's a pretty darn thick piece of plastic, and it's flexible. I would assume that the problems you're alluding to is due to a bad install. In my house, the bathroom, done by a contractor, used cement boards, etc. The grout lines already started to crack. The kitchen floor, which I had built up similarly to the bathroom floor, but used ditra, hasn't cracked yet. The subfloor beneath the tiles is pretty much the same. The slight exception being that I probably secured the plywood to the joists a bit better than the contractor did.

Yes, ditra is more expensive, but I didn't find it prohibitively more expensive than cement board. E.g. it cost me ~$2/sq ft for ditra. At HD: http://www.homedepot.ca/product/durock- ... -ft/911230 that's $36/32 sq ft. So the price for ditra is about double per sq ft over CB. In the total cost of the kitchen reno, it cost me an extra $200 over the cost of the CB. Which, given the rework that's required now in the bathroom, is well worth it.

If you want to continue to use CB, or, heck, use diamond lathe and make a jersey mud bed (which was used before CB), you just need to factor in the cost of rework. Given my recent experience, there's no way I would go back to how it was done before.


A couple of questions: 1) if you use waterproofing on the CB, how will you prevent water from going through the gaps between the boards? and 2) If you use waterproofing on the CB, will the thinset stick? I guess if you used a modified thinset, it'll stick, right?

Ya I hear you. Doesn't look like the cost for the floors at least will be too much more since the sqft of the bathroom isn't that large.

In terms of the mold/mildew issues, I found a few threads on it, this being one of them (pic on page two of thread):

http://www.diychatroom.com/f84/hardibac ... ard-95711/

Don't get me wrong, I'm sure just using durock cement board has it's share of issues, but I'm sure the kerdi/ditra products have their own set of issues as well. Both can have bad installations.

I was seriously considering kerdi for the shower walls, but that's where a lot of the cost would be. People say if you're using kerdi in the shower, then you can just use regular drywall behind that and no plastic vapour barrier, but I'm just paranoid about that (the using regular drywall part). What if the kerdi does fail? Then you're back to the drywall rot issue. That's why I figure you would probably want kerdi on top of cement board, but that's why I figured it would be quite costly to be paying for both cement board plus kerdi.

Ya if using redgard, you have to use a modified thinset or else you run into adhesive issues. For filling the gaps between boards, same as how you would seal the seams between kerdi sheets, with fiber mesh tape and thinset.
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gheart008 wrote:
Apr 10th, 2012 1:11 pm
Ya I hear you. Doesn't look like the cost for the floors at least will be too much more since the sqft of the bathroom isn't that large.

In terms of the mold/mildew issues, I found a few threads on it, this being one of them (pic on page two of thread):

http://www.diychatroom.com/f84/hardibac ... ard-95711/

Hmm.. that pic is related to hardieboard. and it looks like it was used on a floor. You can see where the screws/nails protruded through the board around the drain - a path for water to go through. And then the water reaches the wood subfloor. Not only is that a bad install, that has nothing to do with Kerdi or Ditra :p
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Having done CB and Protegga(like Ditra) for tiled floors, I would never do CB again. Its heavy and so much more difficult to maneuver.

However, if you still decide to do CB - thinset and then nail it down with hot dipped galvanized roofing nails. Start from the center and work your way out, in a 4"-6" grid pattern. Some of the CB products already have the nail spots marked. Like the guys at johnbridge say: The thinset holds it up, the nails hold it down.

Personally, I don't care about waterproofing the floor, because small spills can be wiped up immediately. Large spills are going to flow into your heat vent or out the door into your other rooms at which point the subfloor getting wet is the least of my concerns.

But if you are going to use Redguard, factor that and the CB+nail costs into your calculations and compare it to Ditra or similar.
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Sorry but I quickly skimmed through a couple threads and only read maybe 30% of it but will make a couple comments.


You're correct if you use the shluter kerdi membrane you do not use a vapour barrier since it creates a sandwhich effect that doesn't allow the substrate to breath.

When using kerdi, you don't need CB, or the screws, schluter recomends standard drywall, therefore when all the costs are added up, there isn't much of a difference between standard cement board substrate and standard drywall/kerdi membrane.

Even though you have a tub you use a 4-5" strip of kerdi band along the edge of the ditra to the edge of the tub just below the tile and it creates a continuous waterproof membrane. When you go from the shower curb you contuously run the kerdi right onto the ditra and that's a continous waterproof membrane. I wouldn't even use strips of kerdi to cover the drywall joints I would just overlap them by at least 2", that will save you money.

The DIY forum you linked to is in my opionion not a good forum to ask advice about bathroom install. The John Bridge Forum in my opinion is the best forum to get great advice on a bathroom remodel.

As mentioned before ditra or ditra xl isn't only for waterproofing the floor it helps greatly with expansion. The only concern you should have re the floor and waterproofing is at the tub area in case water spashed onto the floor but again, I never installed ditra xl on my floor for waterproofing.

A vapour barrier is no substitute for a waterproofing membrane since it goes behind the substrate not in front of it. Would you take a shower with only a cement board wall? If not then take a look at the video below.

http://www.youtube.com/watch?v=okGFj-4q2d4
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