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Old 10-21-2018, 03:03 PM   #1
1hawaii50
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Kerdi Shower Install

I am going to be starting my bathroom project within the next few weeks. I am tearing out the acrylic tub/shower enclosure and I am going to be putting in a walk-in shower...32x60 with an offset drain. I'm leaning towards the Durock Shower system, with a corner bench and 2 niches. According to what I've found on their website, you can use either modified or unmodified thinset to install the waterproofing membrane. I will be gutting the bathroom to the studs, and re-framing the shower to make sure that it is square. I will be using Durock Cementboard to enclose the shower. I am also on a concrete slab, there is a square cutout in the slab for the existing tub drain. I know that I have to change the drain over to a 2" drain for the walk in shower. Then I will need to fill in the cutout area with concrete, to fully support the USG shower pan. For my questions:

Would you recommend modified or unmodified thinset to bond the shower floor to the concrete slab?

Modified or unmodified thinset to bond the waterproofing membrane to the cement backerboard?

As far as grout, I would like to use Spectralock, but I can't find it anywhere other than Amazon...what grout is recommended for the shower floor and walls?

I am using a small, octagon, mosaic tile for the shower floors, and 4x12 porcelain tiles for the shower walls.

Any help is appreciated!
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Old 10-21-2018, 03:30 PM   #2
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Welcome to the forum, Joe.


To simplify things, I'd use modified thinset mortar for everything.

How small are the tile for the floor? I don't know what the restrictions are on tile size for the Durock shower tray. It's worth looking into.

As a general rule, we advise people to center the shower drain when possible, and to consider a mud floor rather than the tray, for a number of reasons.
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Old 10-21-2018, 03:31 PM   #3
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Welcome, Joe.

1. Modified. But I would recommend you not use the foam tray at all and instead use deck mud to create your own properly sloped shower floor to perfectly match your shower footprint and drain location.

I would further recommend you move your drain to the center of your shower floor.

2. Modified.

3. Whatever grout you want. Others may make some specific suggestions.

My opinion; worth price charged.
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Old 10-21-2018, 04:36 PM   #4
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I'm on a concrete slab, the only way to center the drain is to cut the foundation. I don't know if the slab is under tension or not, so I wasn't really looking to cut the slab. Are there problems with the presloped pans? I've never done a mud bed, so I figured the pan would be the way to go.
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Old 10-21-2018, 04:58 PM   #5
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Moving the drain location in SOG floors is not a terribly difficult procedure, Joe, and it results is a more more acceptable floor in my estimation.

If you do, in fact, have a post-tensioned slab, you do need to be careful not to cut any of your cable.

The downsides I find in the pre-fabbed foam trays is they're not all sloped the required 1/4" per foot; they require the subfloor to be absolutely flat and level before installation; you must usually cut them down to size to fit your shower footprint, including the drain location, or use mud to fill in around the ones too small for your shower; cutting them results in an unlevel perimeter; they tend to get dents in them from your knees while tiling; they're very expensive.

Other than that, I think they're fine. If I had a dozen identical showers to install in the same building and the footprint and drain locations were all constructed to specifically fit a particular manufacturer's foam tray and had perfectly level and flat subfloors, I'd very likely look into using them. For a one-off, I'd not even consider it.

But that's entirely up to you. Lots of people install lots of those foam trays. Lots of other people make their first mud bed when construction their very first shower. We've seen lots of successful mud shower floors produced by our visitors here. We've also seen lots of them go through the cutting and fitting and floor leveling to use the foam trays, too. Dealer's choice. I favor the mud.

My opinion; worth price charged.
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Old 10-21-2018, 05:09 PM   #6
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Next noob question...how do I know if my slab is under tension or not? I'm not opposed to cutting to center the drain if I don't destroy my slab in the process...lol! My home was built in the mid/late 1950's and our neighborhood has few basements due to the large amounts of sandstone in the area.
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Old 10-21-2018, 05:49 PM   #7
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Here's some ideas. CX may have some others.
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Old 10-21-2018, 07:49 PM   #8
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Joe, if you cannot verify the type of reinforcing in your concrete SOG by visual inspection of the exposed perimeter, the only reliable, non-invasive, way I know of determining the type of construction, presuming your house was built within a code compliance jurisdiction, would be to check the records in said jurisdiction for information on the foundation inspection that would have taken place before the concrete was poured.

If you don't determine that beforehand, I recommend you not even consider saw-cutting anywhere in the concrete and using other means of penetrating the slab with care until you determine what you have. If you encounter steel reinforcing bars, in one location you still can't be 100 percent sure you don't have any post-tensioning cables, but you can be a little more comfortable in an area as small as moving a shower drain.

If you encounter what will probably look like plastic tubing, chip around it carefully. The most common colors with which I'm familiar are blue and orange, but I'm sure they come in other colors, too. The plastic sheath around the cables is actually pretty tough and you're likely to see it before you chisel into it. But you can cut through it and the cable very easily with a concrete saw. DON'T DO THAT. While it may do not much more than put a couple big divots in the side of your slab, it can be a whole lot more dramatic than that.

I like to start such concrete demo with saw cuts, so I'd recommend doing whatever is necessary to verify the type of reinforcement before you begin your demo.

My opinion; worth price charged.
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Old 10-21-2018, 09:08 PM   #9
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I'll put in a call to the building inspector tomorrow...but I'm willing to bet he will have no clue what I'm talking about....a lot of records/permits have gotten "lost" through the years...
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Old 10-22-2018, 07:55 AM   #10
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Hi Joe,

No matter where you locate your drain you'll have to do some chopping into your slab to install a new trap. The trap for tub drains is right against the wall -- the tub overflow is actually the drain, not the drain you see in the bottom of the tub.
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Old 10-22-2018, 08:40 AM   #11
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Another issue with leaving the drain against the wall is with the opposite wall being 5' away. In order to get the recommended 1/4" slope per foot the opposite end of the pan will need to be 1.25" thick (or more) If you carry that 1.25+ around the perimeter you can see the extreme slope at the drain.

Note that USG does offer a custom sized foam pan. You send them the measurements, including exact drain location, they fabricate a foam pan to your measurements. Useful in many ways, including circumstances where one cannot get a drain perfectly centered. As long as the drain is less than 4' from any side wall their custom pan will be only 1" tall at the perimeter.

That's what I went with, their custom pan (about 42"X84). While it is certainly more expensive than a mud pan it was the best option for my situation.
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Old 10-22-2018, 10:32 AM   #12
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I believe the chances of a home built in Cleveland in the 50's having a post tension slab are extremely small. At least based on looking at my much newer slab, it's easy to see from the perimeter.
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Old 10-22-2018, 12:50 PM   #13
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I just spoke with my neighbor...he cut into his slab some years ago to run some drains and he said there was nothing in the slab but concrete. He rented a jackhammer and broke up a bunch of the slab in one bedroom of his house to put in a spare bathroom.
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Old 10-22-2018, 12:51 PM   #14
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I'm thinking I'll get the 4 inch saw from Harbor Freight, add a concrete blade and use that to score the slab...then I'll use my air hammer with a chisel bit and see if that's enough to break up what I need...
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Old 10-30-2018, 12:13 PM   #15
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OK, If I am going to build the shower curb out of brick, on a concrete slab, can I then install the waterproofing membrane directly over the brick or do I need to lathe/mud the brick first? I was leaning towards the USG/Durock membrane, but now I am thinking Kerdi instead...

If I am going Kerdi...drywall, kerdi membrane down to floor, then build the shower floor, embedding the bottom of the drywall/Kerdi into the perimeter of the pan?
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