Install Schluter KERDI Like a Boss
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This post is not a tutorial on how to install Schluter KERDI from the basics. This expects you to already have a good foundation on how to mix the mud, properly use a KERDI trowel at a 45 degree angle and apply the thin-set in straight lines, etc. That’s the easy stuff.
No.
This is directed at people like me who like to nerd out on getting the prettiest, most bulletproof waterproof install of Schluter KERDI that you will never see again (because it’s covered in tile and you’ll never want to rip it out).
Let’s get started!
For those of you following the blog who are not familiar with Schulter KERDI (skip if you know this too):
There are several reasons my showers are encased in this mysterious orange felt covering you may have noticed in past blog posts like this one.
What’s that orange stuff?
Schluter KERDI is a waterproofing membrane.
It ensures water stays in and does not escape behind the cement board of a bathtub or shower enclosure into the framing. As we have established in a past post, grout is porous. If you end up with mold growth behind the wall, it’s going to be because of some other problem from the other side of the wall, not the shower side.
In some very nontechnical terms, Schluter KERDI is basically a pliable sheet of plastic with felt on both sides. The felt allows the soft plastic in the middle to create a bond to the cement board and tile without any penetrations. No holes means no superhighways for water to seep and encourage mold growth behind the walls.
In technical terms, KERDI is made out of modified polyethylene (PEVA) membrane in between the two sides of non-woven polyproplyene. It meets ANSI A118.10 standards for “Load Bearing, Bonded, Waterproof Membranes for Thin-set Ceramic Tile and Dimension Stone Installation.”
If you don’t want to buy an entire roll it is also sold by the foot in tile stores like Floor & Decor, where this picture was taken.
Schluter KERDI is a great product. However, as with all great products, how great it works is all in the install details.
Installing Schluter KERDI per manufacturer instructions:
Whenever you have a seam there needs to be a 2″ overlap of KERDI material, be it full sheets of KERDI overlapping or butted ends of KERDI covered with KERDI-BAND. KERDI-BAND is a 5″ roll of KERDI, designed specifically to span the gap between two sheets of KERDI.
The Schluter installation guide states it does not matter which way you overlap–the two inches is enough material to ensure water does not seep through.
You attach KERDI to Hardie-Backer or some other kind of cement backer board with Schluter SET, ALL-SET, FAST-SET, or unmodified thin-set mortar (I don’t want to talk about gypsum / drywall here).
TECHNICALLY you can start tiling right away. BUT, let’s be honest, have you ever had an install where you didn’t have to pull a tile off to readjust it?
No one is judging here.
I’m going to put it out there that I do. I’m nit-picky and as amazing (and amazingly slow) I am at setting tile, I’m pulling tile off from time to time for various reasons.
If you pull off tile when the thin-set behind the KERDI is wet, the KERDI might move.
IMHO, after twenty-four hours, allowing the thin-set to dry, you can start installing tile. AKA, take a day off before tiling.
Installing Schluter KERDI like a boss–nerd style!
That whole, “it doesn’t matter which way you overlap KERDI” is one of those things that is comforting, but at the same time the engineer in me says, “No way!” If I’m going to this extra trouble to install this fantastic stuff I’m maximizing it’s ability to waterproof. Why tempt fate (aka let water do it’s thing)?
#1 Minimize the number of edges.
There will be no KERDI-BAND if you overlap the sheets of KERDI. If, instead, you butt joint KERDI and overlap with KERDI-BAND there is an extra seam in there: two sides of the KERDI-BAND versus one side of the overlapping KERDI on top. By overlapping KERDI sheets you are cutting the the number of possible water penetrations in half!
A 50% risk reduction is huge!
Side benefit: you save a bit too on the KERDI-BAND (or really, making your own where needed because you can cut five inch strips off a the huge roll of KERDI).
#2. Install Schluter KERDI with a shingle style layout.
If I install it horizontal, shingle style is the only way to go.
Perhaps too extreme, but I’m OK with it–if I go vertical, I’m going to make sure the KERDI is shingle style going away from the shower head.
#3 Choose which direction (horizontal or vertical) based on the least amount of waste.
Either direction is OK, so deciding based on what is left over is a good way to choose.
#4 Avoid buildup.
When you are installing a curb or a shower bench, these corners can potentially have layer upon layer of KERDI, KERDI-BAND and prefabricated KERDI corners. The buildup is also noticeable if you overlap KERDI at the corners instead of a butt joint sealed with KERDI-BAND or curving through a corner.
Also note, it is not possible to waterproof a corner with a 2″ overlap without a prefabricated corner. You’ll have a pinhole left at a vulnerable point.
When it comes to areas like shower benches, you might have a inside or outside corner where you should trim off an inch or two of one leg to prevent another unnecessary layer of KERDI. Or, in a similar fashion, make sure your KERDI-BAND does not unnecessarily extend over a prefabricated corner.
Notice how the KERDI-BAND is shorter on the bottom of the bench, but it extends over the outside corner on top. In both cases there is still a 2″ overlap due to how the prefabricated waterproofing corners are installed.
The bottom is one of the two long legs of the prefabricated inside corner:
Schluter KERDI-KERECK-F preformed 90° waterproofing inside corner.
…and the top is along the inside corner of a prefabricated outside corner:
Schluter KERDI-KERECK-F preformed 90° waterproofing outside corner.
This is a textbook install of the prefabricated KERDI corners along with the KERDI-BAND for minimal overlapping, creating minimal buildup (admittedly, if I say so myself, since this was the work of my team at a Schluter Systems Innovation Workshop).
If you forget, laying tile will be more difficult with all of these layers and the necessary thin-set in between.
Note: The prefabricated corners and KERDI-BAND are only 4 mil thick. Compare this with the 8 mil thickness of a KERDI sheet. The attempt to minimize corner and transition buildup is there, since the banding products are all half the thickness of the field waterproofing membrane (KERDI).
Just in case you wanted to make KERDI-BAND DIY-style from KERDI, just remember you will have double the thickness. Take this outside corner as an example:
Overlapping preformed 90°outside corners to waterproof a shower curb.
If you attempt to create the same thing with KERDI the thickness will be double–quite significant (and it isn’t possible to create an outside corner without a pinhole, which also is clearly not abiding by the 2″ overlap rule).
There is a second kind of buildup: definitely avoid the buildup due to thin-set you forgot to squeeze out from behind the KERDI when it was wet. Don’t get in this situation: Beer Saved This Bathtub Tile Surround Install
#5 Consider where your seams overlap.
I figure if the cement board meets in the corner, and the KERDI meets in the corner, and then the tile meets in the corner–what is the one place where water is most likely to infiltrate?
The corner.
So I intentionally made the KERDI turn the corner. The seams are offset.
Some real life examples on installing Schluter KERDI:
Not only did I want to overlap the KERDI and not end on an inside corner as we installed the walls in the first bathtub surround…
And yes, I had a missed opportunity on overlapping the bottom-left piece there on the tub edge. It should have been installed below the long span.
…but in the walk-in shower I also did the floor first with at least two inches going up the wall (aka shingle style). This way the KERDI on the walls would overlap on top of the floor KERDI keeping all seams overlapping towards the drain (still using the Schluter corners). Unfortunately, the picture below is before I did the last corner pieces and the last Schluter gaskets around the plumbing. That’s the way it goes with pre-blog picture taking…
Likewise, if you look closely at the bench seat below you’ll discover it is covered in a continuous sheet of KERDI. KERDI covers the back, seat, and front of the bench, which overlaps the next piece.
Look! There’s the Schluter gasket ready to go on the countertop.
The downsides of these methods.
You won’t save on time. In fact, you’ll most likely need an assistant when hanging the KERDI in huge spans. This is especially true when trying to turn the corner. These methods took a little longer and required the help of my husband for us to cram it in the corners.
Buildup in the corners, mentioned above, is another potential issue if your overlap is not considered along with the other necessary waterproofing layers (such as the prefabricated corners).
I will admit, those corners you see by the shower head (photos below) had some buildup because I did not trim the overlap to minimize the buildup when I put the inside corners in last (not pictured). What I should have done was cut back the overlap coming up from the floor, leaving a 2″ square on both sides of the corner. Next time…
But what about that post on KERDI-BOARD?
In all honesty, I’ll be using KERDI-BOARD as often as possible as I went over in this post: Build a Better Bathroom Faster with KERDI-BOARD! I am still a proponent for KERDI-BOARD over KERDI, but there are times and places for KERDI. For instance, in a bathroom where you want the baseboard to have extra waterproofing–DITRA won’t make the curve from the floor to the wall (although you would probably use KERDI-BAND in this instance, but if you had extra KERDI, use that instead of going out and buying KERDI-BAND).
KERDI wins in terms of not having to buy KERDI-BAND. You can’t get around not buying KERDI-BAND with KERDI-BOARD because, obviously, you are sealing the gap between two boards.
Second, if you against all shower materials ending at the corner, KERDI is your answer because it curves. KERDI-BOARD has to terminate at the corner–the corner is exactly where the boards meet.
More on why you want to use Schluter KERDI:
Durability and longevity
Because of waterproofing, the shower will last a lot longer than if no waterproofing or improperly applied waterproofing were used.
Warranty
If the product is defective it is probably going to show up sooner than later. Just in case, if you follow all of the installation instructions they offer a limited lifetime warranty if you use their thin-set and register the project within 90 days of installation. If you forget, keep your receipts for proof of purchase and they will warranty your shower with a 10 year guarantee. Check out all the warranty details here.
Personally, I’m on the 10 year warranty program as the thin-set system was introduced in 2017. I’ll make sure to update this post if I have to call it in. I have one year left a bathtub enclosure and another four years on a walk-in shower, but I feel like both will be going strong long after I’m gone. That, or someone will decide my style does not match theirs–I love to be a fly on the wall when they try to demo the shower out (although I also might cry) and they discover how hard it will be to take apart.
Thoughtful installation and attention to detail is appreciated here! Have you installed Schluter KERDI before? Share any of your own installation tips below.
