Expert Guide to Fixing Shower Tub Gaps with Wet Shimming & Waterproofing
- Isaac Ostrom
- Oct 20
- 7 min read
What’s up everybody — Isaac here, your Tile Coach. In today’s episode, we’re tackling one of those problems that pops up on almost every bathroom remodel: what to do when your tub doesn’t fit perfectly between the walls.
You set a new acrylic tub, everything looks good, but then you notice a gap between the tub flange and the wall studs. Maybe it’s a quarter inch, maybe it’s five-eighths — either way, you can’t just slap tile over it and hope for the best.
So today I’m walking you through how I fix that — step by step — using a technique called wet shimming. I’ll show you how to build out the wall correctly, waterproof the flange, and prep everything so your tile install looks clean, flat, and lasts for decades.
This is Step 1 in my How to Tile a Tub Shower series — Waterproofing and Prep — and trust me, this is where the whole project’s success starts. You can’t get a good tile job without good prep.
Key Takeaways [TLDR for Homeowners & Pros]
Don’t float the wall with mud around an acrylic tub — it’s risky and unnecessary. Use wet shimming with a waterproof backer board instead.
Always set the backer board flush with the tub flange, not overlapping it.
Use a lightweight waterproof board like Hydro Ban Board (LATICRETE) — it’s durable, easy to cut, and fully compatible with thinset systems.
Wet shim using LATICRETE MultiMax Lite or a similar hybrid mortar for fast setup and strong bond.
When fastening, use 2-inch corrosion-resistant screws spaced 12 inches apart on studs 16 inches O.C.
Seal the tub flange joint with Hydro Ban Adhesive & Sealant and Hydro Ban Band for a watertight connection.
Check your walls with a 4-foot level first — find and fix dips before shimming.
Clean as you go! MultiMax Lite is sticky — wipe off the tub edge before it sets.
Proper prep equals professional results — your tile will sit flatter, your waterproofing will last longer, and you’ll avoid cracked grout or leaks later.
Understanding the Problem: Why Tub Gaps Happen
Most tubs — especially acrylic ones — are designed to fit into a five-foot rough opening. But not every house is built to that exact size. Framing variations, drywall build-out, or even slightly bowed studs can leave you with a gap between the tub and the wall.
Acrylic tubs also have thick mounting flanges that sit against the studs. If that flange pushes the tub forward even a quarter inch, it throws off your wall plane.
In this project, the back wall had about a ⅜-inch gap at the flange and nearly ⅝-inch at the outer edge. That’s way too much for tile to bridge cleanly.
And if you just caulk or float over it, you’ll end up with a bowed wall, lippage in your tile, and a joint that’s bound to leak. So instead, I fix it the right way: by wet shimming a waterproof backer board to bring the wall out perfectly flat and plumb.
Why Not Float the Wall?
Floating is an old-school mud technique that’s great on masonry, but on acrylic tubs? Not so much.
Acrylic is soft — one slip of your trowel or hawk, and you’ve just left a permanent scratch. Plus, mudding takes more time, more cleanup, and adds unnecessary weight.
Wet shimming is faster, lighter, and safer for modern tubs. It gives you the same result — a flat, solid wall — without the risk of damage.
Step 1: Assessing the Wall and Finding High & Low Spots
Before you start throwing thinset at the wall, figure out what you’re working with.
Use a 4-foot level to check for bellies, humps, and dips. In my example, I had what I call an “eighth-belly” — an eighth of an inch dip in the middle of the back wall.
That tells me exactly where I’ll need to add thinset when shimming. Make notes with a pencil right on the studs — mark where to build out and where to stay flush.
This prep work might take 10 minutes, but it saves hours later when you’re trying to get your tile to line up.
Step 2: Choosing the Right Wall Board
I’m using Hydro Ban Board from LATICRETE. It’s lightweight, waterproof, easy to cut, and made for showers and tub surrounds.
If you’ve used Kerdi-Board, GoBoard, or DensShield, this will feel familiar — but Hydro Ban Board has a bit more rigidity, which I really like. It doesn’t flex much and holds screws tight.
I always set the board to land right at the top of the tub flange — not overlapping it. If it hangs over, you’ll end up trapping water behind it. Instead, you’ll seal that small joint between the flange and the board later with waterproofing band and sealant.
Step 3: Wet Shimming with Thinset
Now for the fun part — the wet shim.
Wet shimming just means applying thinset mortar directly to the back of the wall board before fastening it. The mortar fills low spots, corrects wall irregularities, and holds the board perfectly plumb once it cures.
For this, I use LATICRETE MultiMax Lite — a super sticky, lightweight mortar that spreads like whipped cream and sets up beautifully.
Here’s how I do it:
Mix your thinset to a creamy consistency.
Butter the wall or the back of the board — whichever’s easier to reach.
Use a ¼-inch notch trowel for about ⅛-inch build-out, or a ½-inch notch if you need closer to ¼-inch.
Press the board into the wet thinset, check it with a level, and adjust while it’s still workable.
You’ll feel it “grab” almost immediately. Once it’s positioned right, move on to fastening.
Step 4: Fastening the Wall Board
Even though MultiMax Lite will hold the board in place while it sets, you still need mechanical fasteners for warranty and long-term strength.
I use 2-inch LATICRETE screws spaced 12 inches apart on studs 16 inches O.C.
Drive each screw until it just dimples the surface — don’t over-tighten or crush the foam.
If you marked your studs ahead of time (which I always do), this part goes quick. And since Hydro Ban Board has pre-marked grid lines, keeping your screw pattern straight is easy.
Step 5: Waterproofing the Tub Flange Joint
This step is where most people mess up — and it’s the most important part for keeping water out of the framing.
Here’s exactly how I waterproof the joint between the wall board and the tub flange:
Protect the tub flange with blue painter’s tape.
Run a generous bead of Hydro Ban Adhesive & Sealant along the flange.
Spread MultiMax Lite thinset on the wall just above the flange using a 3/16-inch V-notch trowel.
Embed Hydro Ban Band (or similar waterproofing strip) into the thinset, pressing the bottom edge into the sealant bead.
Smooth it tight with a putty knife to remove air bubbles.
Fill any visible gaps with more sealant for extra coverage.
What you end up with is a fully waterproof, flexible seal between your tub and wall board — one that won’t crack when the tub moves or flexes slightly under weight.
Step 6: Waterproofing Seams and Corners
Once the boards are fastened and your flange joint is sealed, move on to corners and seams.
Apply MultiMax Lite with a V-notch trowel, then embed Hydro Ban Band over every joint with a 2-inch overlap. Smooth out the wrinkles and make sure you’ve got full coverage.
This creates a continuous waterproof barrier across the entire wall assembly.
Step 7: Inspect and Clean
Before you move on to tile, do a quick checklist:
Are all screw heads recessed but not overdriven?
Are seams, corners, and the tub flange fully sealed?
Is the wall plumb and flat from corner to corner?
Did you wipe off any stray thinset from the tub edge?
If yes, you’re ready for tile layout and installation in Step 2.
Additional Pro Tips
Trowel Size: Match your trowel notch to how much build-out you need. Remember: half the notch = your finished bed.
Work in Sections: Don’t butter the entire wall at once — thinset skins over quickly. One board at a time keeps things manageable.
True Work Pants: I wear True Work T3 pants on every job. They’re water-resistant and perfect for handling thinset splashes.
Product Choice: Not sponsored — I just like how Hydro Ban Board works with MultiMax Lite. It’s strong, flat, and bonds like crazy.
Check Framing: Sometimes the real problem isn’t the tub — it’s bowed studs. Plane or shim those first if needed.
Frequently Asked Questions (FAQ)
Q1: Can I wet shim directly to studs without drywall?
Yes — that’s actually ideal. As long as your studs are spaced correctly and you use proper fasteners, you can wet shim directly onto them.
Q2: How far should the backer board sit above the tub?
I leave about ⅛ inch above the flange, then bridge that gap with waterproof band and sealant.
Q3: What’s the best thinset for wet shimming?
A lightweight hybrid like LATICRETE MultiMax Lite works great. It sticks, it holds its shape, and it’s easy to trowel.
Q4: Can I screw the board before the mortar dries?
Absolutely. The mortar will hold its bed thickness, and screwing while it’s wet actually helps lock the board in place.
Q5: Do I need washers like with Kerdi-Board?
Nope. Hydro Ban Board’s dense foam core doesn’t require washers — just screw heads flush to the surface.
Wrapping Up
So that’s how you handle tub-to-wall gaps like a pro. Don’t float the wall, don’t rely on caulk — build it out correctly with waterproof foam board and wet shimming.
Once you’ve got your Hydro Ban Board installed, fastened, and sealed, you’ve got a rock-solid, waterproof foundation ready for tile.
It’s faster, cleaner, and more durable than old-school methods — and it gives you that laser-flat wall your tile deserves.
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.
Until then — keep your walls flat, your joints tight, and as always, happy tiling!