How to Get Walls Flat and Plumb for Tile Installation
- Isaac Ostrom
- Sep 30
- 7 min read
Updated: Oct 1
If you’ve ever tried setting tile on a wall that’s out of whack—bowed, leaning, or full of bellies—you know the pain. You set one tile, it looks good, then the next one wants to kick out. Suddenly your cuts don’t line up, your corners look crooked, and you’re fighting lippage the whole way up.
The truth is: most of the headaches people run into with tile start with wall prep. If your walls aren’t flat and plumb before you set that first piece, you’re setting yourself up for a battle.
In this post, I’m going to walk you through exactly how I prep walls for tile. We’ll use flashing strips, a laser, straight edges, and a good stiff mix of thin set to render walls flat and plumb—even if the wallboard is already up.
This is the method I teach in my two-day Tile Coach training classes, and it’s something I use on real-world jobs all the time.
Key Takeaways (for Homeowners & Pros) [TLDR Version]
Flat walls first. Don’t skip prep. It’s faster to fix walls before tiling than fight them during installation.
Flashing strips work wonders. They’re the thickness of a credit card, peel out clean, and make rendering walls simple.
Use a laser or a 6-ft level. The longer the level, the more accurate.
Thin set consistency matters. Stiff mix for strips, looser mix for filling.
Burn coat is key. That bonding coat locks thin set into the substrate.
Full coverage only. In showers, you need 95% mortar coverage—no dot setting.
Clean as you go. Keep tools, hands, and edges clean to save yourself frustration.
Tools You’ll Need on the Jobsite
Flashing strips (plastic, about 1/32” thick, 8 ft lengths)
Laser level or a 6-ft spirit level
Thinset mortar (ProLite recommended; lightweight and non-sag)
Trowels: flat, margin, ¼" notch, ½" notch
Rubber mallet
Straight edges (L-shaped aluminum set, a level, or a cut MDF board if you’re in a pinch)
6-ft level (4-ft works, but 6-ft is better)
Painter’s tape
Sponge and clean water bucket
Gloves (thin set is alkaline and rough on skin)
Shop vac (for crumbs and debris before screeding)
Step 1: Why Wall Prep is Critical
I start the video showing a sheet membrane already installed on a wall with a nasty belly in it. The wall wasn’t just bowed, it was out of plumb.
If I had tiled over it as-is, here’s what would’ve happened:
Lippage between tiles (edges sticking out)
Inconsistent cuts at the corners
Tiles that wouldn’t line up across the wall
A whole lot of frustration
Flat walls = easier installs. Plumb walls = consistent cuts. Get those two things dialed in and the tile work becomes smooth.
Sometimes you get lucky and you’re framing or boarding yourself. In that case, you can shim studs, plane them, or use cardboard shims behind wallboard to get it close before waterproofing. I go over those methods in other videos.
But most of the time, you walk onto a job where the wallboard is already up. At that point, flashing strips and rendering are your best tools.
Step 2: Burn Coat and First Strip Placement
[00:05:00 – 00:10:00]
First thing I always do is a burn coat. That’s just a thin layer of thin set pushed hard into the substrate with a flat trowel. It keys into the fibers of the membrane or board so the next layer of thin set bonds mechanically.
If you’re mixing thin set stiff (and you should for this part), the burn coat keeps it from shearing off.
Next, I start buttering thin set onto the wall where the first flashing strip will go. In the video, I measure about ⅜" out of plumb at the top compared to the bottom. That tells me how much build-up I’ll need.
Drop the strip into the mortar, line it up with the laser, and lightly beat it in with the mallet. Check it with the level. Adjust by adding or scraping thin set.
Pro tip: Always err on the side of less thin set. You can push strips deeper into the wall, but adding material later is a pain.
Step 3: Setting Additional Strips
[00:15:00 – 00:30:00]
Placement matters. Strips need to be positioned so your straight edge can ride them when screeding. For showers, I usually run one close to the inside corner and one near the curb or edge.
Sometimes one side of the wall needs a ton of build-up (I had one side that was over an inch out). Other spots only need a thin coat. That’s why I switch between a flat trowel for heavy build-up and a notched trowel for lighter spots.
Think of it like old-school mud floating. In California, we’d use redwood furring strips and fat mud. Flashing strips are the modern version—thinner, cleaner, and faster.
Step 4: Let It Cure, Then Fill In
[00:36:00 – 00:52:00]
Usually, I let the strips cure overnight before filling between them. For small jobs like backsplashes, you can use speed set and do it same day.
Here’s how the filling goes:
Mix thin set looser than you did for strips. Still stiff enough to hold, but easier to spread.
Wet down the cured strips and mortar with a sponge. This promotes a better bond.
Load thin set onto the wall with two flat trowels.
Screed between strips with a straight edge, using side-to-side motion. Don’t just drag straight up or you’ll create voids.
Fill low spots immediately instead of trying to fix them later.
I show in the video how much easier the aluminum straight edges cut compared to a wood baseboard. Cleaner lines, less drag.
Step 5: Strip Removal
[01:02:00]
Once the fill coat cures, peel the flashing strips out. They come out smooth, leaving a line about the thickness of a credit card. No grinding. No patching.
When you notch thin set for tile, those little lines just fill in naturally.
That’s the beauty of this system—it sets you up for a perfectly flat, plumb wall without leaving extra junk behind.
Why Not Just Spot Set?
Some people ask: why not just throw some blobs of thin set on the back of each tile and adjust as you go?
Because it doesn’t meet standards. TCNA requires 95% mortar coverage in wet areas like showers. Dot setting leaves huge voids where water can sit. Those voids are breeding grounds for mold, and they weaken the bond.
Rendering walls with flashing strips means you can trowel a full, consistent bed of thin set behind every tile. No voids, no mold, no failures.
A Word on Thin Set Choice
In the video, I tested a few different mortars. Cheap LHT thin sets were a nightmare. Lightweight mortars like ProLite or Ardex X77 made the process smooth.
Lightweight thin sets stick better, build thicker without sagging, and give you longer working time. ProLite, for example, is non-sag, mixes creamy even when stiff, and has great open time—perfect for this method.
Real-World Tips From the Jobsite
Clean constantly. Every time you get thin set on your hands or tools, stop and wipe. If you don’t, you’ll spread mess everywhere.
Use painter’s tape. Protect edges with tape—it takes 15 seconds and saves 15 minutes of cleanup.
Stir buckets often. Thin set behaves better when stirred. Teach helpers to keep it creamy.
Train the next generation. If you’re a pro, bring in an apprentice. This is how I learned, and it’s rewarding to pass it on.
Set expectations. Tile is hard. Don’t go in thinking you’ll whip out a shower in a weekend. Prep, patience, and practice make the difference.
Frequently Asked Questions (FAQ)
Q. How do I get a wall flat before tiling? Use flashing strips and thin set to render the wall. Install strips vertically, screed thin set between them, and remove strips once cured. This leaves you with a flat, plumb wall ready for tile.
Q. Do I need a laser level for tile wall prep? A laser makes it easier and faster, but you can also use a 6-ft spirit level and pencil lines. The longer the level, the more accurate your strips will be.
Q. Can you tile over a wall that’s not flat? Technically yes, but you’ll fight lippage and uneven cuts. Industry standards call for no more than 1/8" variation in 10 ft. Anything worse should be fixed before tiling.
Q. What thin set is best for rendering walls flat? Lightweight, non-sag mortars like ProLite, Ardex X77, or Laticrete Multimax Lite. They spread easily, allow thicker build-up, and don’t slump.
Q. Do flashing strips stay in the wall? No. They peel out after curing, leaving a gap about the thickness of a credit card. That line gets filled when you trowel thin set for tile.
Q. Is this method good for DIY tile setters? Yes. It looks intimidating, but once you practice loading and screeding thin set, it’s straightforward. It saves you from fighting crooked walls later.
Q. How long does wall rendering take? For a shower: one day to set strips, one day to fill and screed. Backsplashes can be done in one day with speed set.
Q. Why can’t I just spot-bond tiles to fix crooked walls? Because it leaves voids. TCNA requires 95% mortar coverage in wet areas. Spot setting creates gaps where water and mold collect, leading to failure.
Final Thoughts
Getting walls flat and plumb is the foundation of every good tile job. It’s not glamorous, but it’s what separates clean, professional installs from hack work.
Flashing strips make the process faster, cleaner, and repeatable. They pop out clean, they cost next to nothing, and they save you hours of frustration during the actual tile setting.
If you’re a DIYer—take your time, practice with thin set before jumping in, and don’t rush it. If you’re a pro—train your helpers on this method and watch your installs get faster and cleaner.
If you want to go deeper, check out my hands-on training classes at tilecoach.com. We cover everything from wall prep to waterproofing to advanced tile setting techniques.
Until next time: prep right, keep it plumb, and happy tiling.
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