Schluter Kerdi Waterproofing: 7-Day Test

Isaac Ostrom
Jan 8
8 min read

Updated: Jan 9

I conducted a 7-day flood test on a Kerdi build to see what happens when you stop guessing and actually stress the system—by filling it and letting it sit for a full week, then tearing it apart and looking underneath.

I’ll say the same thing I said in the video: I’m not a scientist. I’m a tile guy. I do these tests for fun, and I want you to draw your own conclusions.

What I am going to do in this blog is walk you through exactly how I built the model, exactly how I installed the Kerdi (step-by-step), the schedule of the flood test, what we found during tear-out, and what I think installers and homeowners should take away from it.

Schluter Kerdi Waterproofing: 7-Day Shower Wall Test & Pro Installation Tips

Key Takeaways for Homeowners & Pros [TLDR Version]

I built a solid, square model so the substrate wasn’t the excuse. Drywall was screwed off 8 inches on the perimeter and 12 inches in the field.
I used Laticrete 317 unmodified, let it slake for five minutes, remixed it, and kept it a loose consistency so the fleece bonds well. Schluter recommends unmodified thinset to meet their warranty criteria.
I do inside corners first, then band, so I don’t end up with buildup where I don’t need it.
I finished the Kerdi install Saturday around 4:00, let it sit Sunday, then filled it Monday morning for a 7-day soak.
During tear-out, most areas looked dry, but we found moisture at a seam right on the crease where the band folds. It looked like it seeped through that crease.
We also found one wet area where the drywall was actually damp and the wet thinset was against the drywall, meaning moisture got under the band in that spot.
ASTM testing for these membranes only requires 48 hours. This was 7 days, which is beyond what they’re required to pass.
No matter what system you use, the final installation is on us, the installers. That’s the real point.

Tools Required

Here’s what you need to replicate what I did:

Drywall and screws (fastened tight, consistent pattern)
Drill/driver
Mixing bucket and mixing paddle
Margin trowel
Trowel for embedding membrane
Utility knife
Flat knife / putty knife (for checking seams and doing tear-out inspection)
Water source and a way to safely hold water for the flood test

7-Day Flood Test

A normal flood test is usually 24 hours or 48 hours depending on what you’re following. In the video I specifically call out that ASTM testing for these types of membranes only requires a 48-hour test.

So why did I go 7 days?

Because I wanted to see how far water can penetrate over time. This is a stress test. It’s not a perfect representation of shower use, and I say that too: the argument is that a shower is never going to be flooded for seven days straight; it dries out between uses.

So don’t read this as “your shower is going to be full of water for a week.”

Read it as: where are the weak points when water has time to work on the details?

Step 1: How I Built the Test Model

Before I even touched Kerdi, I knew everybody was going to ask the same thing:

“Was the model built right?”

So I show it. Drywall was installed and screwed off solid: 8 inches on the perimeter, 12 inches in the field.

I wanted to remove excuses. I even say straight up this model is solid and I don’t see anything structurally that would compromise tile or waterproofing.

Step 2: Mixing the Thinset Properly

For this test, I used Laticrete 317 unmodified.

Here’s the step-by-step on how I mixed it and how I recommend you mix it:

Mix your thinset thoroughly.
Let it slake for five minutes.
Remix it after slake.
Keep it a loose consistency so the Kerdi bonds well.
Check your bond as you go. I literally peel back and check how well it’s grabbing the fleece.

If you’re a homeowner reading this, this is the part you can’t easily “eyeball” when hiring someone. But a rule of thumb is that the thinset should be mixed to a "milkshake like" consistency. Where it can pour out, but it should still be trowelable and hold a notch shape for a few seconds before spreading out. Having the thinset loose like this extends open time, increases the mortar transfer between the surfaces, and helps the ridges collapse for full mortar coverage between the layers.

Step 3: My Kerdi Install - The Sequence

3A. Inside corners first

I always do corners first before I do my band. The reason is simple: I don’t want too much buildup where I don’t need it.

Step-by-step:

Place your inside corners.
Spread thinset where the corner will land.
Place the corner and embed it.
Hold the corner in place and push down to fully seat it.
Burn it in. I mention in the video that in my other videos I really burn my corners, and I did the same here.

3B. Then install the band

Once corners are locked in, then I move to the band.

Step-by-step:

Spread thinset for the band run.
Lay the band into the mortar.
Embed it tight and consistent.
Watch your seams and overlaps. 2" overlap is required at all seams and corners.
Keep buildup under control. Avoid triple layer overlaps.

And yeah, even on a clean build, stuff happens. I had a little bubble I didn’t like and I fixed it.

3C. Quality check the whole thing

After the Kerdi was installed, I’m looking at corners, overlaps, and coverage. I was checking coverage as I went.

Step-by-step:

Visually inspect all corners.
Visually inspect all seams.
Make sure your overlaps are clean and consistent. 2" minimum at all seams and corners.
Check that you didn’t miss embedding anywhere.
If something looks questionable, fix it now.

Step 4: The Flood Test Schedule

Here’s exactly how I scheduled it:

Saturday around 4:00: Kerdi install was done.
Let it sit all day Sunday.
Monday morning: filled it with water.
Let it soak for seven days.

I took my time, followed instructions, mixed thinset properly, used the right trowel, embedded the fleece as well as I could, did the corners, did the band, did everything I could to build it the best I could.

Step 5: How to Run a Flood Test

Let your waterproofing installation cure based on the system instructions and job conditions. Typically 24 hours at 70 degrees.
Plug it properly so you’re not getting a false failure from a bad drain plug.
Fill it with water to the appropriate level.
Leave it alone. Don’t mess with it.
Verify water level at the end of the test window.

My test window was seven days. That’s the point of the whole experiment.

Step 6: The Tear-Out and Inspection

This is where it gets interesting, because a lot of people stop at “it held water.”

I wanted to see what was going on underneath.

Here’s how I recommend doing a destructive test inspection:

Start peeling back and inspecting corners first.
Look for obvious wet areas.
Use a knife to get under seams and check if moisture traveled.
Pay attention to folds, creases, and transitions.
Keep separating layers until you can see the substrate condition underneath.

In the video, we’re checking corners and calling out that certain areas are dry and look good.

Then we start finding moisture spots.

What I Found After 7 Days

I’ll break this down into two categories: what looked great, and what made me pause.

What looked great

We had areas that were dry and looked good. That matters.

That tells me a lot of the installation did exactly what you’d want it to do.

Where we found moisture

We found a few spots where moisture got in, and the one that jumped out was a seam right on the crease where the band folds. It appeared to seep through the small area at the crease.

That’s not me guessing. That’s what we saw when we got under it and checked it.

Then there was the bigger one:

There was a wet area going down to the drywall. The drywall was actually damp, and the thinset that was wet was against the drywall. That means moisture got down under everything, the band and everything.

That’s the part that’s worth paying attention to, because it’s exactly where installers have to be sharp: seams, folds, corners, transitions.

How I Want You to Interpret This

I say it directly in the video, and I’m not changing the message here:

I’m not a scientist.
Don’t take this test as gospel.
Take it for what it’s worth.

Also, in fairness to Schluter (and any manufacturer), ASTM testing only requires 48 hours for these membranes. This test was 7 days.

What This Means for Homeowners

If you’re a homeowner reading this, here’s the practical takeaway:

Your shower’s success has a lot more to do with the installer's detail than the brand or waterproofing system used.
The weak points are almost always seams and corners.
A flood test is still a smart idea, even if you’re not doing 7 days.

And listen, I’m not saying every job needs a week-long stress test. What I am saying is that when you see an installer who takes waterproofing seriously, pays close attention to detail, and trusts their work, that’s the person you want building your shower.

What This Means for Installers

This is the line I want to leave you with because it’s the truest thing in the whole video:

Regardless of what any manufacturer says, the final installation is on us, the installers.

So here’s my installer-to-installer summary:

Build your substrate solid.
Mix your thinset right, and don't skip slake time.
Keep your mortar workable for fleece.
Corners and seams deserve your best effort, not your fastest effort.
Inspect your work as you go.

And if you’re the kind of setter who gets stuck in “my system is the only system,” I say in the video: use what you feel is right. If you feel Schluter, great. If you feel vinyl liner, awesome. If you feel hot mop, cool. They all work when installed properly.

That’s where I’m at with it.

If you want to keep learning with other setters who actually care about this stuff, the Tile Coach Forum is the best place I know to have real conversations without the noise. That’s a natural next step if you’re serious about leveling up.

FAQ Section

Q. Did you follow the instructions during the install?

Yes. I specifically say I took my time, followed the instructions, mixed thinset properly, used the right trowel, embedded the fleece as well as I could, and did the corners and band the best I could.

Q. What thinset did you use for Kerdi?

Laticrete 317 unmodified. I let it slake for five minutes and remixed it, and I kept it loose so the Kerdi bonds well. Schluter recommends unmodified thinset to meet their warranty.

Q. Why do you do corners first instead of band first?

To avoid unnecessary buildup where I don’t need it. I always do corners first before band for that reason.

Q. How long did you wait before flooding it?

I finished Saturday around 4:00, let it sit Sunday, then filled it Monday morning.

Q. What did you find when you tore it apart?

Most areas looked dry, but we found moisture at a seam right on the crease where the band folds, like it seeped through that crease.We also found a wet area that went down to the drywall where the drywall was damp and wet thinset was against it, meaning moisture got under the band in that spot.

Q. Is a 7-day flood test realistic for a real shower?

Not really. The point is stress testing. I also say the argument is a shower won’t be flooded for seven days straight because it dries out between uses.

Q. What does ASTM require for flood testing membranes like this?

ASTM testing for these types of membranes only requires a 48-hour test. This was a 7-day flood test.

Q. What’s your main conclusion?

I’m leaving it the same way I left it in the video: you guys are smart, use your own judgment and common sense, and remember the final installation is on us installers.

Final Thoughts

If you made it this far, here’s what I hope you take from the whole thing:

This wasn’t about trying to make a product look bad. It was about seeing what happens when water has time to work on the details. We saw areas that were dry and looked good, and we saw moisture show up in places that should make every installer slow down and pay attention: seams, folds, and transitions.

And I’m going to repeat the one line that matters most:

Regardless of what any manufacturer says, the final installation is on us, the installers. So use what system you believe in. Just install it like your name is on it, because it is.