To trowel thinset correctly, first use the flat side of the trowel to firmly key mortar into the substrate. Then add more mortar and comb it into straight, parallel ridges with the notched side of the trowel. Set the tile while the mortar is fresh and move the tile perpendicular to the ridges to help collapse them.
The goal is not to create attractive mortar lines. The goal is to create consistent mortar coverage underneath the tile.
One of the most common installation mistakes I see is focusing on the trowel size while overlooking how the mortar is actually applied. You can have the correct trowel and still end up with poor coverage if the mortar is swirled, allowed to skin over or never properly collapsed beneath the tile.
If you are still deciding which notch to use, start with our Complete Tile Trowel Size Guide.
Quick Answer: How Should You Trowel Thinset?
- Prepare the substrate. Make sure it is clean, sound and properly prepared for tile.
- Mix the mortar correctly. Follow the setting material manufacturer's water ratio, mixing time and slake instructions.
- Key the mortar into the surface. Use the flat side of the trowel and firmly press mortar into the substrate.
- Add additional mortar. Apply enough material to create full ridges with the notched side.
- Comb straight, parallel ridges. Do not create swirls or random patterns.
- Keep the trowel angle consistent. This helps maintain a consistent amount of mortar.
- Set the tile while the mortar is fresh. Do not allow the surface to skin over.
- Move the tile perpendicular to the ridges. This helps collapse the mortar into the valleys.
- Lift an early tile. Check actual mortar transfer and coverage.
Mike's Tip: The most important part of troweling mortar is what happens after the tile goes down. If the ridges remain standing underneath the tile, something needs to change.
Why Does Thinset Troweling Technique Matter?
A notched trowel is essentially a mortar metering tool. The notch size controls how much mortar is placed on the substrate, but your technique determines how consistently that mortar is distributed.
Poor troweling can create:
- Large air pockets beneath the tile
- Insufficient mortar coverage
- Unsupported tile edges and corners
- Uneven finished tile height
- Excessive mortar squeeze through
- Visible mortar lines behind translucent glass
Good technique helps the mortar spread into a continuous layer beneath the tile.
Step 1: Prepare the Surface Before Troweling Thinset
Before opening a bag of mortar, make sure the substrate is suitable for the installation.
The surface should be:
- Clean
- Structurally sound
- Free from dust, grease and contaminants
- Properly waterproofed where required
- Sufficiently flat for the tile size
A larger trowel should not be used to compensate for a badly uneven floor or wall.
This becomes especially important with large format porcelain tile because a long, rigid tile can bridge low areas in the substrate.
Step 2: Mix the Thinset Correctly
Always follow the mortar manufacturer's mixing instructions.
Do not add extra water simply because you want the mortar to spread more easily.
Most cement based mortars require a specific amount of water, mixing period, rest or slake period and final remix.
The finished mortar should hold the trowel ridges without being overly stiff or excessively wet.
For more information about choosing setting material, see our guide to white thinset, gray thinset and mastic.
Step 3: Key Thinset Into the Substrate
Before creating mortar ridges, use the flat side of the trowel to firmly work a thin layer of mortar into the substrate.
This step is sometimes called keying in, burning in or force bedding the mortar.
The objective is to establish intimate contact between the mortar and the substrate rather than simply placing loose mortar on the surface.
Hold the flat side of the trowel against the substrate and apply firm pressure as you spread the mortar.
Step 4: Add More Mortar Before Notching
Once the surface has been keyed in, apply additional mortar.
You need enough material for the notches to create full, consistent ridges without exposing the substrate between passes.
If the trowel is scraping the surface clean as you comb, you may not have enough mortar in front of the trowel.
Step 5: Comb Thinset in Straight Parallel Lines
Thinset should be combed into straight, parallel ridges.
Avoid:
- Circular patterns
- Swirls
- Curved ridges
- Crisscross patterns
- Random changes in direction
Straight ridges provide trapped air with a path to escape when the tile is moved across them.
When mortar is swirled, pockets of air can become enclosed between the ridges.
Which Direction Should You Trowel Thinset?
Trowel all mortar ridges in the same direction.
With rectangular tile, I prefer to run the ridges parallel with the shorter dimension of the tile whenever practical.
For example, underneath a 12 x 24 tile, running the mortar ridges across the 12 inch direction gives trapped air a shorter path to travel than running them the full 24 inches.
The tile should then be moved perpendicular to those ridges as it is embedded.
What Angle Should You Hold a Notched Trowel?
A notched trowel should generally be held at a consistent angle that allows the full notch profile to form.
A common working angle is approximately 45 degrees to the substrate, but consistency matters more than trying to hit an exact number every time.
If you hold the trowel too flat, the notches deposit less mortar.
If you hold it more upright, the ridges become taller and more mortar is deposited.
This means two installers can use the exact same trowel and still place different amounts of mortar depending on their trowel angle.
Why Does Trowel Angle Affect Mortar Coverage?
The notch dimensions printed on a trowel do not automatically determine the final amount of mortar beneath the tile.
The angle at which you hold the trowel changes the height of the mortar ridges.
A consistent trowel angle helps create a consistent mortar bed.
If the angle changes dramatically from one section to another, the amount of mortar under neighboring tiles can change as well.
Should Thinset Ridges Be the Same Height?
Yes. The ridges should be as consistent as reasonably possible.
If one section has tall full ridges and the next has shallow partial ridges, the amount of mortar beneath the tile will vary.
Keep enough mortar in front of the trowel and maintain a consistent angle and pressure.
How Far Can You Spread Thinset Before Setting Tile?
Only spread as much mortar as you can cover with tile before the mortar begins to skin over.
There is no universal square footage because working time changes with:
- Mortar formulation
- Temperature
- Humidity
- Air movement
- Substrate absorption
- Installer speed
Working in smaller sections is usually safer, particularly for DIY installation.
What Does It Mean When Thinset Skins Over?
Skinning occurs when the exposed surface of the mortar begins to dry before the tile is installed.
The mortar underneath may still be workable, but the surface can lose the ability to properly transfer to the tile.
This is dangerous because the mortar can look perfectly fine from above.
A simple check is to lightly touch the surface. If mortar does not transfer to your fingertip, do not assume it is still suitable for setting tile.
Follow the mortar manufacturer's instructions for dealing with material that has exceeded its open time.
Should You Twist Tile Into Thinset?
A small movement during setting can help embed tile, but the goal is to move the tile across the mortar ridges rather than simply rotate it in place.
Movement perpendicular to the ridges helps collapse the peaks into the valleys and allows trapped air to escape.
Large format tile usually requires deliberate back and forth movement rather than a small decorative twist.
How Far Should You Move Tile Across the Ridges?
You do not need to slide the tile a great distance.
The objective is enough movement to collapse the ridges underneath the tile.
The amount required depends on the notch profile, mortar consistency, tile size and tile back.
Instead of trying to measure a specific movement distance, lift a test tile and inspect what happened.
What Should Collapsed Thinset Ridges Look Like?
When you lift a properly embedded tile, you should see mortar distributed across the back rather than distinct untouched ridges separated by empty valleys.
The mortar should have transferred to the tile and spread between the original trowel lines.
Look especially closely at:
- The center of the tile
- The perimeter
- The corners
- Areas around ribs or depressions on the tile back
How Do You Check Thinset Coverage?
Lift one of your first freshly set tiles and inspect the back.
For typical dry interior installations, aim for at least 80 percent mortar contact.
For showers, other wet areas and exterior installations, aim for at least 95 percent mortar contact. Natural stone should also achieve at least 95 percent.
You want good distribution of that mortar without significant continuous voids.
What Should You Do If Thinset Ridges Are Not Collapsing?
If you lift the tile and see pronounced ridges with large empty valleys between them, stop and determine why.
Possible causes include:
- The tile was not moved sufficiently across the ridges
- The mortar was too stiff
- The mortar had started to skin over
- The trowel notch was too small
- The tile back needed additional mortar contact
- The substrate was not sufficiently flat
Do not continue installing tile until you are getting adequate coverage.
Should You Back Butter Tile Before Setting It?
Back buttering can improve mortar contact, particularly with large format porcelain, natural stone and tiles with pronounced patterns on their backs.
Use the flat side of the trowel to firmly key a thin layer of mortar into the tile back.
The objective is not to create another thick layer. You are filling surface texture and improving contact with the mortar on the substrate.
For the full technique, read our Back Buttering Tile Guide.
Do You Comb Thinset on the Back of the Tile?
For typical back buttering, I recommend keying a thin coat of mortar into the tile back with the flat side of the trowel rather than creating another heavy set of ridges.
The primary mortar ridges are normally combed on the substrate.
This keeps the mortar bed easier to control while still improving transfer to the tile back.
How Should You Trowel Thinset for 12x24 Tile?
For a 12x24 porcelain tile, begin with an appropriate large format trowel, commonly a 1/4 x 1/2 inch U notch, Euro notch or another notch recommended by the mortar manufacturer.
Key mortar into the substrate, then comb straight ridges across the shorter dimension of the tile.
Back butter the tile when needed and move the tile perpendicular to the ridges.
Then lift an early tile and verify coverage.
How Should You Trowel Thinset for Mosaic Tile?
Mosaic tile requires the same basic principle but generally uses a smaller notch.
Key mortar into the substrate and create consistent ridges without depositing so much mortar that it fills the grout joints.
Mesh mounted mosaics can often be embedded evenly using a clean grout float.
For the complete setting process, see our mesh mounted mosaic installation guide.
How Should You Trowel Thinset for Shower Tile?
Technique becomes even more important in a shower because wet area installations require high mortar coverage.
Key the mortar into the waterproofed substrate, use the appropriate notch for the tile, create straight parallel ridges and verify at least 95 percent mortar contact.
For large porcelain shower wall tile, see our porcelain shower wall installation guide.
Can You Trowel Thinset in Circles?
You should avoid swirling or combing thinset in circles.
Circular ridges can trap pockets of air underneath the tile when the ridges collapse from multiple directions.
Straight parallel ridges provide a more direct route for air to escape.
Should You Trowel Thinset on the Tile or the Floor?
For most installations, comb the primary mortar layer on the prepared substrate.
Large tile may also be back buttered with a thin keyed coat of mortar on the tile back.
The important part is producing adequate mortar transfer rather than simply placing mortar on both surfaces.
Can You Apply Too Much Thinset?
Yes.
An oversized notch or excessive mortar can create:
- Mortar squeeze through
- Difficulty maintaining tile height
- Uneven installation
- Excess cleanup
- An unnecessarily thick mortar bed
Use enough mortar to achieve required coverage while staying within the mortar manufacturer's allowable thickness.
Can You Apply Too Little Thinset?
Yes.
If there is not enough mortar, portions of the tile may never make contact with the setting material.
This can leave unsupported areas beneath the tile even though the finished surface appears flat.
Your coverage check will tell you whether more mortar or a different notch is necessary.
Common Thinset Troweling Mistakes
Skipping the Flat Side of the Trowel
Do not immediately begin combing mortar onto a dry substrate. Key mortar into the surface first.
Swirling the Mortar
Use straight parallel ridges instead.
Changing Trowel Angle Constantly
A changing angle changes the amount of mortar deposited.
Spreading Too Large an Area
The mortar may skin over before the tile reaches it.
Only Pressing Tile Straight Down
Move the tile perpendicular to the ridges to help collapse them.
Never Checking Coverage
This is one of the easiest mistakes to avoid. Lift an early tile and look underneath it.
Using a Larger Trowel to Fix an Uneven Surface
Correct significant substrate problems before setting tile.
Ignoring Mortar Squeeze Through
Excess mortar in grout joints may indicate that the notch is unnecessarily large or that too much pressure is being applied.
Mike Belk's Rule for Troweling Thinset
I keep the process simple.
Key it in. Comb it straight. Set the tile while the mortar is fresh. Move the tile across the ridges. Then pull one back up and look.
Do not assume that neat looking trowel ridges mean you have a good installation.
What matters is what those ridges do after the tile is embedded.
Bottom Line: Good Troweling Is About Coverage, Not Perfect Ridges
Key mortar into the substrate, comb straight parallel ridges, maintain a consistent trowel angle, set the tile before the mortar skins over and move the tile perpendicular to the ridges.
Then do the most important part: lift an early tile and inspect the mortar transfer.
If the ridges have collapsed and you are achieving the required coverage, your technique is working.
If not, correct it before continuing.
For help choosing the right notch before you start, visit the BELK Tile Complete Tile Trowel Size Guide.
