When to Back Butter Tile (And When You Can Skip It)
The Bottom Line:
Back buttering tile means spreading a thin, flat layer of thin set mortar on the back of each tile before setting it into the mortar on the floor, and it is required for any tile larger than 12 inches, any tile being installed in a high traffic area, any natural stone tile, and any time you want to avoid hollow spots and cracked tiles down the road.
Quick Summary
You have probably seen a tile installer do this. They spread thin set on the floor with a notched trowel. Then they flip a tile over, skim a thin layer of mortar on the back, and press it into place. That extra step is called back buttering.
Some people say you only need to back butter large tiles. Some say you never need to do it if you use the right trowel. Some say it is a waste of time.
Here is the truth. Back buttering is not optional for most modern tile installations. It is required by the Tile Council of North America (TCNA) for tiles larger than 15 inches on any side. It is also required for natural stone, for tiles being installed outdoors, for floors with heavy traffic, and for any situation where you want full coverage and zero lippage.
Skipping back buttering leads to hollow spots. Hollow spots lead to cracked tiles. Cracked tiles lead to angry customers and costly repairs. That is the long and short of it.
Let me walk you through exactly when you need to back butter, when you can skip it, and how to do it correctly so your tile jobs last for decades.
What Is Back Buttering Exactly?
Let us define the term clearly. Back buttering, sometimes called back skimming, is the process of applying a thin, uniform layer of thin set mortar to the back of a tile immediately before setting it into the mortar bed on the floor or wall.
The key word here is thin. You are not slathering a thick layer. You are skimming the back of the tile with enough mortar to fill the little waffle pockets or ridges on the tile's underside. The goal is to create a flat, consistent surface that will make full contact with the mortar on the substrate.
Here is what back buttering is not. It is not the same as troweling mortar onto the tile with a notched trowel. That would be too much mortar. Back buttering uses the flat side of the trowel, not the notched side. You are spreading a skim coat, not a bed.
Think of it like this. The mortar on the floor is the mattress. The back butter on the tile is the fitted sheet. You need both for a good night sleep.
Why Does Back Buttering Matter?
Without back buttering, the bond between the tile and the mortar is incomplete. Here is what happens.
When you spread notched mortar on the floor, you create ridges. When you press the tile into those ridges, the ridges are supposed to collapse and spread out, filling the space under the tile. But if the back of the tile has any warpage, any dust, any release agent from the manufacturing process, or any deep waffle pattern, the mortar will not make full contact.
The result is a hollow spot. You can hear it when you tap on the tile. Instead of a solid thud, you get a hollow clicking sound. That hollow spot has no support. Over time, foot traffic or heavy furniture can crack the tile right at that hollow spot.
Back buttering solves this problem by giving the tile a flat, sticky surface that bonds completely with the mortar on the floor. The thin layer fills the waffles and creates a uniform contact surface. When you press the tile down, the mortar on the floor collapses into the back butter layer, and you get full coverage.
The industry standard for tile coverage is no less than 80 percent for dry areas and no less than 95 percent for wet areas or exterior installations. Without back buttering, especially on large tiles, you will not hit those numbers.
When Is Back Buttering Required?
Let me give you a clear list. These are the situations where back buttering is not optional.
Situation 1: Tiles Larger Than 12 Inches on Any Side
This is the big one. The larger the tile, the harder it is to get full coverage without back buttering. A 12x24 inch tile has a lot of surface area. The waffle pattern on the back is designed to hold mortar, but those pockets can trap air. Back buttering forces mortar into every pocket and ensures full contact.
The TCNA requires back buttering for tiles with any side longer than 15 inches. Many tile manufacturers require it for tiles over 12 inches. Read the instructions on your tile boxes. If they say back buttering required, do it.
Situation 2: Natural Stone Tiles
Natural stone like marble, travertine, limestone, and slate are more porous and more variable in thickness than ceramic or porcelain. They also tend to have a smooth or mesh backed surface that does not grip mortar well. Back buttering creates a mechanical bond between the mortar and the stone. Without it, stone tiles are much more likely to crack or come loose.
Natural stone also often comes with a resin backing or a mesh backing. That backing needs to be fully embedded in mortar. Back buttering ensures that happens.
Situation 3: Large Format Tiles (Over 24 Inches)
When you get into tiles that are 24x24 inches, 24x48 inches, or even larger slabs, back buttering is absolutely mandatory. These tiles are heavy. They need maximum support. A hollow spot under a large format tile is almost guaranteed to crack under foot traffic. Professional installers back butter every single large format tile without exception.
Situation 4: High Traffic Commercial Floors
Retail stores, restaurants, office lobbies, and hospitals see thousands of footsteps a day. The tile needs to be bonded as strongly as possible. Back buttering increases the bond strength significantly. It is a small extra step that prevents huge problems down the road.
Situation 5: Exterior Tile Installations
Outdoor tile faces temperature swings, moisture, freeze thaw cycles, and direct sunlight. The bond needs to be bulletproof. Back buttering is required for any exterior tile installation. The TCNA standards specify back buttering for exterior applications to ensure no voids where water can collect and freeze.
Situation 6: Wet Areas (Showers, Pools, Steam Rooms)
In a shower or a steam room, water will eventually find any void behind the tile. Once water gets into a hollow spot, it can lead to mold, mildew, and deterioration of the thin set. Back buttering eliminates those voids and keeps moisture out.
Situation 7: When Using a Deep Waffle Back Tile
Some tiles, especially large porcelain tiles, have a deep, pronounced waffle pattern on the back. Those deep pockets are designed to hold mortar, but they also trap air. Back buttering fills the pockets completely and pushes out the air. Without back buttering, you will have hollow spots even if you think you pressed the tile down firmly.
Situation 8: When Installing Over a Crack Isolation Membrane or Decoupling Membrane
Membranes like those used to prevent cracks from transferring to the tile have a textured surface. That texture is great for bonding, but it can also create uneven contact if the tile back is not flat. Back buttering ensures the tile bonds fully to the membrane.
Situation 9: When Using a Fast Setting Thin Set
Fast setting thin set hardens quickly. You have less time to press and wiggle the tile into place. Back buttering gives you a head start by ensuring the tile already has mortar on it. The bond forms faster and more completely.
Situation 10: Any Time You Want a Job That Lasts
Even if your tile does not technically require back buttering, doing it anyway is never a bad idea. It adds maybe 10 to 15 seconds per tile. It uses a small amount of extra mortar. And it gives you peace of mind that every tile has full coverage. Professional tilers who guarantee their work back butter everything.
When Can You Skip Back Buttering?
There are a few situations where back buttering is not necessary. But read carefully. These are exceptions, not the rule.
Mosaic Tiles on Sheets
Small mosaic tiles that come attached to a mesh sheet are usually small enough that the notched mortar on the floor provides adequate coverage. The tiles are tiny, so the risk of hollow spots is low. Back buttering a whole sheet of mosaics is impractical and usually unnecessary.
Tiles Smaller Than 6x6 Inches
Small tiles have less surface area and less warpage. The notched mortar from the floor usually covers the entire back of the tile when pressed. That said, if the tile has a deep waffle pattern or if you are installing in a wet area, back buttering still helps. But it is not strictly required.
Glue Down Vinyl or LVT
Vinyl plank and vinyl tile are not set in thin set. They are glued down with specialized adhesives. Back buttering is not a thing for vinyl. This article is about ceramic and porcelain tile.
When the Manufacturer Explicitly Says Not Required
Some modern porcelain tiles are manufactured with a flat back that is designed to bond perfectly without back buttering. Very few. Read the box. If it says "back buttering not required" in writing, you can trust it. Otherwise, do not assume.
The Consequences of Skipping Back Buttering
Let me be blunt. When you skip back buttering on tiles that need it, you are gambling with your reputation.
Here is what can happen.
Hollow spots. You tap on the tile and hear a hollow sound. That is not just annoying. That is a weak spot. The tile has no support underneath.
Cracked tiles. A hollow tile has no support in the middle. Step on it with a heel or drop a heavy object, and the tile cracks. Now you have to replace it. The new tile will not match the dye lot of the old one.
Lippage. Without back buttering, tiles can sink unevenly into the mortar. One edge sits higher than the next. That is lippage. It looks bad and it is a tripping hazard.
Tentling. In extreme cases, hollow spots can lead to tenting. The tile lifts up at the edges and forms a tent shape. This happens when the thin set shrinks unevenly. Back buttering prevents the uneven shrinkage.
Failed warranty. Most tile manufacturers require back buttering for their warranty to be valid. If you skip it and the tile fails, you have no recourse. You eat the cost.
Callback nightmares. You finish the job. You get paid. Six months later, the phone rings. The customer says a tile cracked. You go back. You see the hollow spot. You know you skipped back buttering. Now you are replacing a tile for free, and the customer is telling their friends not to hire you.
Skipping back buttering to save a few minutes per job is false economy. The time you save is nothing compared to the time you lose on callbacks.
How to Back Butter Correctly (Step by Step)
If you are going to do it, do it right. Here is the proper technique.
Step 1: Mix your thin set correctly. Follow the bag instructions. Use the right amount of water. Mix for the full time. Let it slake. Mix again. If the thin set is too stiff, it will not spread evenly. If it is too runny, it will drip off the tile.
Step 2: Spread mortar on the substrate. Use the correct trowel notch size for your tile. Comb the mortar in straight lines. Hold the trowel at a consistent angle. Do not spread more than you can cover in 15 to 20 minutes.
Step 3: Pick up a tile. Check the back for dust, dirt, or release agent. If it is dusty, wipe it with a damp sponge and let it dry. Some tiles come with a white powdery release agent from the manufacturing process. That powder prevents bonding. Wipe it off.
Step 4: Apply the back butter. Hold the tile in one hand or place it on a flat surface. Use the flat side of your trowel (not the notched side) to spread a thin, even layer of thin set across the entire back of the tile. You want a skim coat, not a thick layer. The waffle pockets should be filled, but the surface should be flat. Think of buttering a piece of toast. Not globby. Just a thin, even coating.
Step 5: Check for coverage. The entire back of the tile should have a thin layer of mortar. No bare spots. No dry waffles. If you missed a spot, add a little more.
Step 6: Set the tile immediately. Do not let the back butter dry out. Once the skim coat skins over, it will not bond properly. Place the tile into the mortar on the floor. Press down firmly. Wiggle the tile slightly to collapse the ridges. Use a rubber mallet and a beating block to tap it flat.
Step 7: Check for lippage. Use a straight edge or a level to make sure the tile is flush with its neighbors. Adjust as needed.
Step 8: Repeat. Every tile gets the same treatment. No shortcuts.
How Much Mortar Does Back Buttering Use?
This is a common question. Back buttering uses about 10 to 15 percent more thin set than setting tiles without back buttering. That sounds like a lot, but in real dollars, it is pennies per square foot.
A bag of thin set costs $15 to $30 and covers 80 to 120 square feet depending on trowel size. Adding back buttering might use an extra half bag per 100 square feet. That is $7 to $15. On a 500 square foot job, that is $35 to $75 extra. Compare that to the cost of replacing a single cracked tile, which can be $200 to $500 in labor and materials. The extra mortar is cheap insurance.
Tools You Need for Back Buttering
Keep these in your truck.
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Margin trowel (for scooping mortar out of the bucket)
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Flat trowel or skim trowel (for spreading the back butter)
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Notched trowel (for the floor mortar)
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Mixing paddle and drill
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Buckets for mixing
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Rubber mallet and beating block
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Sponges and water bucket for cleanup
Common Back Buttering Mistakes
Even contractors who back butter sometimes do it wrong. Here is what to avoid.
Mistake 1: Using the notched side of the trowel. Back buttering is a skim coat. Use the flat side. If you use the notched side, you are creating ridges on the tile. Those ridges will not collapse properly and you will have hollow spots. Flat side only.
Mistake 2: Applying too much mortar. A thick layer of back butter squeezes out when you press the tile. It gets into the grout joints. It makes a mess. It also wastes mortar. Thin is good. A skim coat is all you need.
Mistake 3: Letting the back butter dry before setting. Thin set has a limited open time. If you butter a dozen tiles and then start setting them one by one, the later ones will have a dry skin on the back. That skin prevents bonding. Butter one tile, set one tile. Work in small batches.
Mistake 4: Skipping the dust wipe. New tiles often have a layer of manufacturing dust or release agent on the back. That dust prevents the thin set from bonding. Wipe each tile with a damp sponge before back buttering. Let it dry or set it while damp. The mortar will bond better.
Mistake 5: Back buttering but using the wrong trowel on the floor. Back buttering is not a substitute for correct floor mortar coverage. You still need the right size notched trowel for your tile. Back buttering complements floor mortar. It does not replace it.
Mistake 6: Not back buttering natural stone. Natural stone is not optional. Do it. Every piece.
Back Buttering for Walls vs Floors
The same principles apply to walls, but there are a few differences.
On walls, gravity works against you. The tile wants to slide down. Back buttering gives the tile extra grip. It also helps fill any voids on the back of the tile that could trap air.
For wall tiles larger than 12 inches, back buttering is required. For subway tile or small wall tiles, you can often skip it if the wall is flat and the mortar is applied correctly. But for showers, always back butter. Water will find any weakness.
Frequently Asked Questions
Does back buttering prevent lippage?
It helps. Back buttering fills the waffles on the tile back, which allows the tile to sit more evenly in the mortar. But lippage is caused by many factors including subfloor flatness, tile warpage, and installer technique. Back buttering is one tool in the toolbox, not the only one.
Can I back butter with the same thin set I use on the floor?
Yes. Use the same thin set. Do not mix a separate batch. Just scoop from your bucket. Keep the bucket covered so the mortar does not dry out.
Do I need to back butter if I use a leveling system?
Yes. A leveling system pulls tiles flush with each other. It does not fix hollow spots. Back buttering ensures full coverage. Use both.
How thick should the back butter layer be?
About 1/16 to 1/8 inch thick. Just enough to fill the waffle pattern and create a flat surface. You should not see the tile color through the mortar. But you should not have a thick glob either.
Can I back butter too much?
Yes. Too much mortar squeezes out and makes a mess. It can also cause the tile to sit too high. Thin is better.
Do I need to back butter mosaic sheets?
No. Mosaic tiles on mesh are small enough that the floor mortar usually covers them fully. Back buttering a whole sheet is messy and unnecessary.
What about glass tile?
Glass tile requires special thin set and special techniques. Back buttering glass tile is different because the back is usually coated with a bonding agent. Follow the manufacturer's instructions for glass tile specifically.
Does back buttering work on uneven subfloors?
No. Back buttering does not fix a subfloor that is not flat. You still need to flatten the subfloor before tiling. Back buttering fills tile warpage, not floor warpage.
The Bottom Line from The Tile Shoppe
Here is my honest advice. Back butter every tile larger than 12 inches. Back butter every natural stone tile. Back butter every tile in a shower or wet area. Back butter every exterior tile. Back butter any time you want the job to last.
The extra time is minimal. The extra mortar cost is trivial. The peace of mind is priceless.
At The Tile Shoppe, we sell the thin set, the trowels, and the tiles. But more importantly, we teach our customers how to install them correctly. Back buttering is one of those techniques that separates amateur work from professional work.
If you are a DIYer, take the extra minute per tile. Your floor will thank you.
If you are a contractor, back buttering is not a suggestion. It is a standard of care. Do it on every job. Your reputation depends on it.
Stop skipping it. Start back buttering.
