We’ve already talked about supplies needed to build a custom shower and how to build a shower pan with deck mud, and now the moment you’ve all been waiting for… How to install Schluter Kerdi!
Ok, really quickly, for those of you who have no idea what I’m talking about… Schluter Kerdi is the orange fabric looking stuff you see rolled up in the picture above. It’s soft and felt-like but also completely waterproof. It’s made by a German company called Schluter Systems and cool guys like Mike Holmes and Scott McGillivray use their products. You may have heard of Schluter Ditra, it’s also orange, but more of a plasticy honeycomb structure, and is used on floors, under tile. I’ll show you today how to apply Kerdi membrane, it’s kind of like applying wallpaper, then *poof* you’ve got an instantly waterproofed shower!
Remember this was my very first attempt at building a shower or using Schluter Kerdi, but it sure did turn out awesome. My point is that if I can do it, so can you!
How to Install Shluter Kerdi in a Shower
1. Plan out and cut the Kerdi
Do yourself a favor and read through all these steps before you cut a single piece of Kerdi. You want to keep your pieces as big as possible, and you want to plan out your seams.
I was able to do the entire shower with only one roll of Kerdi. At $100 per roll, it’s not something you want to mess around with. I cut pieces for the two side walls first. The width of the wall was smaller than the width of the Kerdi, so there was enough left to fold it over to the other wall. When it comes to Kerdi, there are two options for the seams: (1) Seams can be constructed by overlapping the edges of KERDI by 2″. (2) Alternatively, you can butt pieces of Kerdi against each other and cover the seam with KERDI-BAND. I decided to overlap the Kerdi.
Next I cut the piece for the floor, it over hung the shower curb, which was perfect. It’s best to use one piece of Kerdi to cover the curb, rather than create seams. 
2. Mix unmodified thinset mortar to a fairly fluid consistency
If you watch the Schluter videos, you’ll hear the guy say these words over and over. I mixed 1/3 of the 50 lb bag to start and modified the water accordingly. Instead of a 5-gallon bucket, I use gallon water ice containers left over from a home owners association party. They would have just been tossed in the trash, so I don’t give too much thought to using them once or twice, then throwing them out. Cleaning thinset off your tools is a pain in the butt.
3. Apply thinset
The bummer about working with unmodified thinset is that it dries much faster than modified thinset, so you don’t have as much time to work with it. So don’t screw around. Figure out how much of an area you can complete in about 20 minutes. This is why I mixed only 1/3 of the 50 lb. bag at time. This was my first time doing something like this and I knew it would take me 20 minutes just to get the hang of it.
I started on the left side wall first. This is the Schluter Kerdi hanging out waiting to be installed. 
Wet the wall down with a damp sponge first. Cement board soaks up water like a beast, so be generous. (Remember it’s not necessary to use cement board, you can install Kerdi over drywall. I discussed this in part 1.) Then spread a smooth layer of thinset with the flat side of the trowel. I split the wall into top half and bottom half and wet down and applied thinset to only the top half first. Once the area is covered in thinset, use the 3/16″ notched side of the trowel to add additional thinset, raking trowel marks into it as you go. I don’t have any pictures of this part because I was taking a video of the process, so I suggest watching that.
4. Attach Kerdi to the walls
The goal is to embed the Kerdi into the thinset, so the first thing to do is lay the Kerdi in place, then pat it into the thinset with your hands. Make sure the Kerdi is positioned exactly how you want it. I planned to have 2″ of the Kerdi membrane cover the adjacent wall. This meant I would not have to use Kerdi Band in this corner and I was all for one less step. Of course I needed to apply thinset to this 2″ portion of the adjoining wall to embed the membrane into. Once the Kerdi is in place, use a drywall knife (or flat side of a trowel) to push the Kerdi into the thinset using a “smoothing” motion. It’s similar to applying wallpaper, you start in the middle and push the excess thinset out to the sides. Make sure the corners are nice and crisp.
Now do the same with the lower half of the wall. I had minimal cutting around the shower curb, which was easily handled with a utility knife.
After the left wall was complete, I did the wall on the right side, followed by the middle portion of the middle wall. I can’t remember my reasoning exactly, I guess it was because the top section was going to be pieced together with two smaller sheets. But it looks like that middle section was pieced together as well. Use full pieces if your worried about it. I made sure there was a 2″ overlap on every seam, so I know I’m good. I was on a mission to only use one roll of the Schluter Kerdi. Mission accomplished. 
A few action shots. A big thank you to my 7-year-old daughter for these.
All the walls complete. Note the bottom of the middle wall, 2″ of Kerdi is on the shower floor. Again, no Kerdi-Band will be necessary at this seam. Just to be clear, the Kerdi membrane on the center wall stops at the corner, not 2″ from the corner. So there is a 2″ double layer of Kerdi running up both sides of the middle wall. This is a requirement because it is a seam. 
5. Apply thinset and Kerdi to the shower floor
So you will be a pro at this point, but that’s good because the shower floor is the trickiest part because of all the corners (step 6). So there are four walls attached to the shower floor, if you include the inside of the shower curb, which we are so get on board with it. And you now know, wherever two pieces of Kerdi meet (the seam) you need to have a 2″ overlap. Right? So be meticulous in your planning (step 1) and make sure you consider all the seams. Alternatively, you could just cover the seams with Kerdi-Band, which is half the thickness of the Kerdi membrane by the way.
- Back wall – Kerdi from the wall overlaps the floor 2″ and will be under the Kerdi on the floor
- Left wall – Kerdi on the shower floor will overlap and come 2′ up the wall
- Right wall – same as left
- Shower curb – there won’t be a seam because one sheet of Kerdi will cover the floor and the curb
Um, so yeah, maybe this isn’t a project for dumb girls DIY because there’s a lot of thought and visualizing involved. But you’re smart, so don’t worry, you’ve got this.
Now that that’s figured out, apply the thinset and Kerdi the same way as above. (1) Wet the floor, (2) apply a smooth layer of unmodified thinset mortar, (3) spread additional thinset with 3/16″ notched trowel, (4) embed the Kerdi with drywall finishing knife, start in the middle and push thinset to the edges.
Wrapping the Kerdi over the curb was challenging because it was hard to get the thinset to push up and over the curb. It wanted to go back down and under the Kerdi on the floor instead. But it all worked out fine.
6. Cover all corners
Preformed waterproof corners come with either the Kerdi kit or Kerdi-Drain. You need to add these to the corners because there are multiple seams in these locations. There are different preformed corners for inside corners and outside corners, so make sure you have everything all figured out before you apply any thinset. An outside corner is used where the shower curb meets the wall.
7. Apply seals around shower components
Both a pipe seal and mixing valve seal come with either the Kerdi-Drain. They’re like a combination Kerdi Band with a plastic ring that goes around the fixture.
I was only able to use the pipe seal because the other did not fit. But that’s okay, I tested to make sure the cover will protect the opening. 
To apply the valve seal, just add thinset to the Kerdi that is on the wall, slide the pipe seal over the pipe and embed the membrane into the thinset.
8. Let the thinset dry
This is the easiest step! Give it a good day to dry.
9. Perform a water test
They make a handy drain plug just for this purpose. It was really annoying to fill the shower pan with water because we don’t have a sink in the bathroom yet, but hopefully that won’t be the case for you. Fill it a few inches high, then mark the height along the wall. Come back 24 hours later and see if the water is still at your mark. Once I got the drain plug figured out we were good to go! I didn’t lose any water over night. Woohoo!
So that concludes the “building” part of the shower. Mine sat like this all summer while I hung out at the beach with my kiddos. I have since tiled and grouted the floor (with those same annoying hex tiles, God help me!) and I am currently finishing up the wall tile. But I feel like the hard part is behind me because the shower building is what intimidates most people. If you’ve read this far, you seem pretty determined, so I’m confident you can make it happen. Just maybe try not to take as many breaks as I have.
I can’t wait to use my shower, even though this is just the guest bathroom. Now on to tiling!
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Immense thanks. I had no idea where to go with my shower. I got tons of ideas in addition to discovering the wealth of Kerdi products You’re not only extremely adept at making a shower, but also at explaining it with a webpage so well! Props.
Thanks Rodrigo! Good luck 🙂