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Unread 05-28-2014, 07:44 AM   #1
jim_c
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Why use Durock in a Kerdi (Schluter) shower?

Hey, don't mind me, I'm just trying to learn.
Perhaps there is something I don't understand here.
Why would anyone use Durock board as a substrate and then apply kerdi membrane in a Schluter shower?
Isn't the main benefit of cement board that it can withstand moisture?
It can get wet and still maintain its integrity.
Other than that, it's a more difficult material to work with.
But with a kerdi shower, the substrate will never get wet.
In fact Schluter recommends drywall or their own Kerdi board.
I know they approve Durock, but it's not "required".
So it seems to me that using cement board and then waterproofing it is almost contradictory. (?)

I am not doing a project right now, this is just an academic question to me.
But I am afraid I'm missing something here. (?)
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Unread 05-28-2014, 08:13 AM   #2
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Here's a link to a recent post that shows one of the downfalls of using sheetrock behind the waterproofing. It works if nothing goes wrong.

Cement board is sort of like cement. Cement can get wet and absorb water but it's not going to fall apart by doing so. Kerdi is putting a waterproof layer over the cement board so that it won't absorb any water. It's not contradictory at all- they work in harmony. Not eveyone wants their cement board absorbing water.
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Unread 05-28-2014, 09:31 AM   #3
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I weighed the options and went with cement board over drywall. My reasoning was that I have plumbing behind the shower walls, and plumbing above the shower walls...and a roof that could leak...So although I don't expect the substrate to get wet from water leaking past the kerdi, I can imagine it could get exposed to water from another source. My concern is that there could be a leak that deteriorates the drywall, necessitating replacing the tile...with the cement board I don't have to worry about that.

It would have been much easier to use drywall, that's for sure.
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Unread 05-28-2014, 11:38 AM   #4
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Some folks just feel better doing it that way, Jim.

The restriction on using gypsum drywall in wet areas is far removed from what was initially intended, which was to restrict tiling directly to gypsum drywall in wet areas. The use of drywall in wet areas where a surface applied direct bonded waterproofing membrane is applied is perfectly acceptable to all membrane manufacturers I've ever questioned. The problem is how the building codes are now worded, making it very difficult and expensive to get the necessary waiver, in the form of an ICC ER, to use your product that way. Schluter is the only manufacturer I'm aware of who has gone through that process.

I question the logic of the argument that there might be plumbing behind the wall that might leak. There's plumbing behind lots of walls and some ceilings in a residential construction, including all kitchens, bathrooms, laundry rooms, and odd places in other rooms where copper has to be tied together above the SOG. Would we also recommend using CBU in the entire house as a replacement for Gypsum drywall? I think not.

You wanna use CBU behind your waterproofing membrane, please feel free to do so. But if you're using Kerdi, please also feel free to use Gypsum drywall as your backing material, but please read and follow all the manufacturer's instructions.

My opinion; worth price charged.
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Unread 05-28-2014, 12:32 PM   #5
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In one sense, I understand Steve's argument about the plumbing, roof leak, etc.

While those things can happen in other walls that have sheetrock on them, a tile shower wall would be many times more difficult to repair, cost much more money, and compromise the waterproof layer. To say nothing of the headache of finding out the tile is discontinued.

Having said that, I use drywall in my kerdi showers.
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Unread 05-28-2014, 02:56 PM   #6
wilander
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jim - i like your method of using dens shield with kerdi over the seams that you used in your last reno.
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Unread 05-28-2014, 03:14 PM   #7
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Some people like tile on walls outside of the bathroom to include hallways, kitchens, etc. If you had a leak, tearing out kitchen cabinets and other things can get costly, too. So, like many things in life, you set your risk threshold, and mitigate those risks. In the 30 years I've lived where I am now, I have never had a pipe leak. Replaced a few valve cartridges, but when they leak, they either leak into the shower or the sink, but the guts in the walls themselves don't leak. Now, you certainly need good workmanship when you put things together, but assuming you achieve that, a leak down the line is not much of any risk.

Use whichever approved method you wish, and use good workmanship, and rely on insurance to cover the small risk, should something happen...that's what it's for in the first place.
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Unread 05-28-2014, 08:57 PM   #8
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Thanks

Thank you for the replies to my question.
I realize there are pros and cons to every method, costs and benefits.
I like to analyze the technical principles and the real reasons behind the ways things are done.*
In this internet age I see a lot of advice from well intentioned folks who really do not make logical sense when you look closely. Not very often on this board - which is much appreciated.
Maybe some others will chime in as well. (?)
Again, merci.

* For example, I see a lot of advice on car maintenance and repairs - on other sites - which is an area where I have some expertise. About 50% of the advice in this area is flat out wrong, aka BS. Off topic, I know.
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Unread 05-28-2014, 09:11 PM   #9
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As has been said...the major requirement for a surface applied membrane like Kerdi, is a stable panel. Drywall, gypsum board, is quite stable, accepts thinset readily, and makes a good bond with it. The membrane, properly applied is waterproof. Think of your roof and then the ceilings, or the windows and doors verses the walls next to them. Any of them can leak if the waterproofing layer or device fails. At some point, you determine the risk of that happening, and the impact, and decide if say maybe you want to use plaster verses drywall, or maybe like many homes in Europe, where the main structure is primarily stone, brick, or cement, and water, while always an annoyance, isn't destructive, or as quickly or as much...the structure will still be there.

If you install Kerdi properly, the risk is small. A second benefit over cbu, is the sheets are bigger, easily to cut, fewer joints, and the screws are easier and cheaper, too. You can do the whole room with the same materials without the potential transition.

Now, is it worth the cost difference and ease of install? Only you can determine your level of risk that's acceptable...drywall in a Kerdi shower is pretty low on my risk list.
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Unread 05-29-2014, 12:39 AM   #10
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Quote:
Originally Posted by CX
There's plumbing behind lots of walls and some ceilings in a residential construction, including all kitchens, bathrooms, laundry rooms, and odd places in other rooms where copper has to be tied together above the SOG. Would we also recommend using CBU in the entire house as a replacement for Gypsum drywall? I think not.
I don't have kerdi and expensive ass tile and too-many-labor-hours-to-count in the other places where my drywall might get wet from a plumbing leak. Also a damaged ceiling doesn't prevent me from taking a shower the same way a damaged shower does....now, whether or not the fear is a realistic one, maybe not...but if I had to redo my shower for want of a water stable substrate, I'd be spittin' mad. So I paid a few extra $ for hardiebacker. Seems sturdier too...definitely don't think my curb would have done too well with drywall on it.

I'm probably a bit biased as just a month before I started my project a leaking toilet tank took out a good sized section of my ceiling not but a few feet from my my shower now stands.
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Unread 05-29-2014, 07:59 AM   #11
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Again, thanks for the input.
I am not from the facebook generation but I want to improve my understanding in this area, which is why I asked a general question.
Steve, you may be interested to know this. I had a plumbing leak due to a broken valve last year. Fortunately not a major disaster just a small disaster.
The insurance adjuster told me that they have calls every week for flooded bathrooms because the plastic nut at the toilet end of the water supply line cracks and leaks. This is partly a design issue (the cheap way some supply lines are made) and partly a "lack of understanding issue" because folks overtorque the plastic nuts. Polyethylene, the most common plastic, is very prone to stress cracking in many different applications.
Have a great day at work guy and girls!
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Unread 05-29-2014, 11:04 AM   #12
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Jim,

I have had leaks at the plastic nut before, but they made themselves known and I caught them. The leak that caused the damage was from the tank bolts -- the rubber washers inside the tank that seal the bolt hole had deteriorated and started leaking. Could have been related to my kids monkeying around with the toilet, though - I wouldn't be surprised if they were practicing their gymnastics by jumping off the top of the toilet.

In any case, it made me paranoid - and that's probably all it is. Jim and others make a good case that it's just a classic cost/risk decision and the risk is pretty low. I'll still do my next shower with cement board, but I'm just a "recreational user" of Kerdi - I might switch to drywall if I made a habit out of building showers.

Happy shower building!
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Unread 05-29-2014, 11:32 AM   #13
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I use drywall on my kerdi installations. The last bathtub surround I did, right after putting up the Kerdi sheet, I pulled the blanked off nipple on the shower head fitting off. (drop ear elbow). The mixing valve was accidentally left on and out rushed several gallons of water before I could find the valve handle and shut the water off.

My drywall was now completely soaked and so I had to rip everything on that side out.
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Unread 05-29-2014, 11:36 AM   #14
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That's a pretty easy fix, Joseph. I'd recommend you simply not do that.
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Unread 05-29-2014, 12:18 PM   #15
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Most plumbing behind the walls is pretty rugged, solid copper, or maybe cpvc, or pex. If they are installed properly, they last many decades without issues. The bigger issue is when first installing things. Things like that plastic nut on the toilet supply line are not allowed behind the walls for a reason.

The makers of sheet membranes have tested their material with numerous solid backer boards...choose one on that list, and the shower itself will be reliable. OF the choices, pick the one that makes you feel best.

To me, if I install the stuff correctly, I have no issues. You may not feel the same way, and are free to choose whatever one of the tested and approved backers you prefer. Just install it with one of the approved materials.
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