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Step-by-Step Guide to Build a Waterproof Shower Pan in a Day with Kerdi and Ardex


Alright guys — today’s one of those satisfying job days where everything comes together. In one solid day, we tore out an old shower, got our new substrate up, floated the mud pan, set the drain, built the curb, waterproofed it with Kerdi, and sealed it all with Ardex 8+9. By the end of the day, this shower was ready for a water test.


If you’ve been following me for a while, you know I’m all about systems that save time without cutting corners. This setup — Durock fiberglass mat board, Schluter Kerdi membrane, and Ardex 8+9 — gives me a fully waterproof shower, not just a “water management” system.


Whether you’re a homeowner getting ready to tackle your first mud pan or a tile pro who’s always up for picking up a few tricks, this walkthrough’s gonna take you step-by-step through everything I did on-site — from drain flange to final waterproofing coat.



Key Takeaways for Homeowners & Pros [TL;DR]


  • You can prep, float, waterproof, and finish a full shower pan in a single day

  • Use Rapid Set Mortar for packing around drains to save time

  • Durock’s fiberglass mat board makes an excellent waterproof substrate

  • Combine Kerdi membrane + Ardex 8+9 for a 100% waterproof system

  • Maintain ¼” per foot slope toward the drain — no exceptions

  • Don’t over-pack your deck mud — it weakens the surface

  • Never wrap Kerdi over a curb in one piece (use strips and bands to avoid bubbles)

  • Most important: take pride in the craft — it shows in the details



Tools & Materials I Used


Primary Materials

  • Durock Fiberglass Mat Board – waterproof gypsum-based panel for walls

  • Schluter Kerdi Drain Flange & Membrane System

  • Rapid Set Mortar Mix – for packing and securing the drain

  • Deck Mud / Dry Pack – for the sloped pan

  • Ardex 8+9 – liquid waterproofing membrane

  • HydroBlocks or Kerdi-Fix Sealant – for bonding foam curb sections


Tools

  • Mixing buckets & drill

  • Level (2’ and 4’)

  • Trowels (margin, notched, and finishing)

  • Grinder (for prep)

  • Wood float

  • Utility knife

  • 6” putty knife

  • Measuring tape



Step 1: Prepping the Substrate


We started by tearing out the old shower down to the studs. Once everything was clean, I installed Durock’s fiberglass mat board — not their cement board, but their waterproof gypsum-based version. This stuff is awesome behind shower systems when you’re floating.


With the walls up, I ground down the floor to remove debris and flatten out any humps or leftover thinset. Then I prepped the 2” ABS drain stub-out and checked the height. You want the top of your drain flange to sit about 1–2 inches above the slab — high enough to support the mud, but not so high that it messes up your finished height.


One huge tip: keep that flange perfectly level. If it’s off, your whole pan will drain wrong. West Coast guys like me usually use ABS drains (they’re black), while PVC drains are more common elsewhere (light gray). Just make sure you match your pipe material to your glue — ABS cement for ABS, PVC cement for PVC.



Step 2: Setting the Drain Flange


I used Rapid Set Mortar Mix to lock the drain flange into place. This mortar is great for quick-setting structural applications — it’s rock hard in about 15 minutes, so mix small batches and move fast.


Pro tip: Mix it looser than you think. It’ll tighten up quickly as it slakes. You want to get it packed fully underneath that drain — no voids, no flex.

Once the flange was solid, I checked it with a level. Perfect. Some installers push the flange into wet mud, but I prefer gluing it first and packing underneath — less mess, more control.


If you’re on a raised foundation, keep in mind that your pipes can flex a little. In

that case, use the mud bed itself to stabilize the drain and prevent movement.



Step 3: Building the Shower Curb


For the curb, I used two pieces of 2-inch Kerdi-Board, ripped cleanly on the table saw for straight edges. I glued them together with HydroBlocks sealant (same chemistry as Kerdi-Fix) — that silane-based sealant cures with humidity and stays flexible.


Once the curb sections were bonded, I topped it with ½-inch HardieBacker to give it some rigidity for point loads like shower doors.


I always check my curb for:

  • Level (you don’t want your door to swing open by itself!)

  • Square off the back wall

  • A slight pitch toward the drain — even 1/16” helps keep water from sitting

Once set, I sealed the joints and edges and got ready to float the pan.



Step 4: Floating the Mud Pan


This is where the magic happens. I bonded my deck mud to the slab using a thin slurry coat of modified thinset (Ardex X5). Always apply your deck mud while the slurry is still wet — that’s what creates a true bond.


I laid out my perimeter slope: ¼ inch per foot from the farthest corner to the drain. That’s code, and it’s also what keeps your shower floor from pooling.

Then I filled in the mud bed, starting around the drain and working outward. Here’s where most beginners mess up:

  • Don’t add too much water to your dry pack. It should hold shape when squeezed, not ooze water.

  • Don’t over-pack it. If you hit it too hard, you break the sand bonds and cause flaking the next day.


I tamped the mud firmly but evenly with a wood float, building the slope in stages. My goal was a smooth, consistent pitch that leads directly to the drain from all sides.

Once my perimeter was perfect, I screeded across using straight edges and levels, then refined the surface with my wood float and finally a steel finishing trowel.


That last pass compresses the top layer — it interlocks the sand grains and gives you that perfectly smooth, baby-skin finish.



Step 5: Applying the Kerdi Membrane


Now, this part’s for those of you who’ve done a few showers — if you’re brand new, let your mud bed dry overnight. I’m comfortable applying the Kerdi same-day because I know my mix and timing.


I mixed my thinset loose — pancake batter consistency — and troweled it carefully without lifting the trowel (that can pull up the mud). Always move from wet thinset to wet thinset to avoid breaking suction.


I vacuumed the holes in the drain flange so the thinset could really key in. Then I laid my pre-cut Kerdi sheet over the pan, smoothing it from the center outward with my hands first, then a 6” knife to push out any air bubbles.

When working over a soft mud bed, gentle pressure is key — you don’t want to dent the slope.


Once the floor sheet was down, I moved on to the curb.



Step 6: Wrapping the Curb and Seams


A big mistake I see all the time: people try to wrap one sheet of Kerdi up and over the curb in one go. Don’t do that. It’ll bubble, crease, and drive you crazy.

Instead, I use pre-folded pieces for each side, plus a separate pre-folded top piece for the curb. Kerdi’s printed grid lines make cutting and squaring easy.

Next, I installed Kerdi inside and outside corners, then Kerdi Band across all seams. Always pre-crease your bands, set them just inside the corners (not overlapping too far), and press them in with two trowels if you have to — one to hold, one to smooth.


You want thin, even coverage — no thick layers of thinset. Excess build-up can cause wicking, and Kerdi performs best with the least amount of mortar under it that still achieves full coverage.



Step 7: Sealing with Ardex 8+9


Now we’re onto the extra credit. This step takes your install from “good” to bulletproof.


I brushed on a full coat of Ardex 8+9, starting about 10 inches up the wall, covering the curb, corners, seams, and the entire pan.

One thing I always do: extend the Ardex into the PVC or ABS part of the drain to encapsulate the joint. That seals any potential moisture migration point and locks the system together.


Kerdi alone is technically “water management” — Schluter even updated their terminology — but once you hit it with Ardex 8+9, it’s a fully waterproof system.

I also use this chance to fill any tiny voids or creases in corners where thinset might not have fully bonded. Ardex seals those right up.


After about 3–4 hours, this pan was ready for a water test. In one day: we installed the board, floated the mud, built the curb, waterproofed everything, and finished with a second membrane coat. That’s professional-grade work, start to finish.



Pro Tips & Common Mistakes to Avoid


  • Don’t rush your slope math. Check it twice. ¼” per foot minimum.

  • Set your drain flange first. Don’t fight with glue and mud at the same time.

  • Mix Rapid Set loose. It thickens fast.

  • Never over-pack deck mud. Light, even tamping is stronger in the end.

  • Pre-fold all Kerdi pieces. Makes life way easier and prevents bubbles.

  • Use 8+9 as insurance. It’s worth every penny for true waterproofing.

  • Keep your tools clean. Mud buildup will drag and ruin your surface finish.



FAQ: Shower Pan & Waterproofing


Q: Can I use PVC drains instead of ABS? 

A: Sure, just make sure your cement matches — PVC glue for PVC, ABS glue for ABS. Don’t mix them.


Q: How long should I wait before waterproofing my mud pan? 

A: If you’re new, let it dry overnight. Experienced installers can apply Kerdi same-day with a fresh float.


Q: Can I wrap Kerdi over a curb in one sheet? 

A: Nope. Always do it in pre-cut, pre-folded pieces to avoid air bubbles and weak spots.


Q: What’s the slope requirement for a shower pan? 

A: ¼” per foot minimum toward the drain. Don’t go flatter or you’ll end up with puddles.


Q: Why add Ardex 8+9 if Kerdi is already waterproof? 

A: It’s extra insurance. Kerdi uses thinset seams (not 100% waterproof), and Ardex seals those weak points.


Q: How soon can I water test after Ardex 8+9? 

A: Usually after 3–4 hours of drying time — that’s the beauty of it.



Final Thoughts: The Rewards of Quality Tile Work


I gotta say — after a day like this, I walk out of the job site with a pep in my step. There’s just something satisfying about turning an empty, torn-out shower into a finished, waterproof system in one day.


A lot of people think tile work’s all about cutting pretty pieces of porcelain, but honestly, the real craftsmanship happens underneath the tile — the prep, the slope, the waterproofing. That’s what separates a pro shower from a weekend DIY that fails in two years.


If you’re out there feeling stuck in a job that doesn’t fulfill you — think about getting into the trades. Tile work gives you something tangible, something you can be proud of at the end of the day. Doesn’t matter if you’re 20 or 50 — find a good mentor, learn the right way, and you’ll build a skill that’ll last a lifetime.


If you found this helpful, check out the Tile Coach Forum for pro tips, tool talk, and jobsite problem-solving. You can also schedule 1-on-1 tile coaching sessions if you’re diving into your own remodel.


I love doing this stuff, and I love being your Tile Coach. Let’s keep learning, building, and pushing the craft forward.

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