Tile Underlayments: What Should I Use Under My Tiles?
What is an Underlayment?
Tile may be strong, but it needs a solid, supportive base. An underlayment is the solid stabilizing layer directly below the tile and the tile adhesive (usually a thinset mortar). Using the wrong one can spell disaster for your tile installation.
Basic Underlayment Types
Cement Board
Cement pressed into a board shape with stabilizing mesh on the surface. Water stable, meaning water won’t cause it to warp or swell, but not waterproof. Water can wick through your cement board to whatever is behind it, causing water damage, mold, or rot. You’ll need to add a waterproof layer if you’re using cement board in a wet area.
PROS
- Relatively inexpensive
CONS
- Heavy
- Brittle
- Not waterproof
- Tricky to cut; you’ll need tools such as an angle grinder with a diamond wheel to grind/cut the board, which is a very dusty process
- Use breathing protection when cutting cement board, as it can release carcinogenic silica dust into the air.
Tile Lines no longer stocks cement board as we’ve found better underlayment options.
Coated Polystyrene
Extruded Polystyrene (EPS) is a high-density, closed cell foam. For tile, it is sold with a concrete and mesh coating. Options include Wedi board or HydroBlok board. PROS
- Lightweight
- Easy to work with
- Insulating
- 100% waterproof.
- Cuts cleanly; anything that cuts wood will cut polystyrene board. You can even use a box knife or a hacksaw.
CONS
- It is a more expensive option, so only use it if you need a waterproofer or a lightweight underlayment option.
Great for shower walls, bathtub surrounds, or places where weight is an issue, such as motor homes and boats.
Flexible Tile Mats
A plastic mat with a raised pattern and a synthetic fiber backing (for better adhesion to the subfloor). Options include Schluter Ditra Mat and DCI mat. PROS
- Lightweight
- Easy to work with
- Cuts cleanly; just use scissors or a box knife.
- Easy transport: carrying 300 square feet of underlayment has never been easier.
- 100% waterproof
- Thin: regular Ditra and DCI mat are only 1/8” thick
- Crack Isolation Membrane: absorbs a certain amount of movement and flex. Ideal for tiling over newer concrete slabs (which can shrink and crack)
- Optional integrated in-floor heating. Instead of adding a heat layer on top of your underlayment (creating a floor that is too tall) you can
- Optional insulation: Ditra Heat Duo has a thick flannel-like layer underneath to prevent heat loss.
CONS
- More expensive per square foot than cement board, but for the price you’re getting so many more features.
These tile mats are an excellent underlayment option for tile floors everywhere.
Drywall
Gypsum pressed between two sheets of thick paper. It’s probably in your house already. Do you want to tile a pre-existing painted wall? As long as it’s clean (no dust, grease, or grime) and not in a wet area (shower or tub surround), go for it! Stable, painted drywall is just fine for tiled backsplashes or feature walls. PROS
You don’t have to remove your existing painted walls if you want to add tile. CONS
If you’re starting with exposed wall studs, cement board and polystyrene boards are stronger and specifically designed for tile. Use them instead.
DO NOT USE:
Wood.Do not tile directly to wood. This includes plywood, pressboard, chip board, hardwood, and shiplap. Wood can contract and expand with temperature changes, warp and swell with humidity changes, and ultimately moves too much for tile. Your tiles are likely to pop off or crack. Tile does not like wood.
Raw drywall.If your drywall doesn’t at least have a paint primer on it, the moisture in the tile adhesive can dissolve the paper coating, and the weight can cause it to tear.
Stucco or plaster.You can’t get a good enough bond, and you run into moisture issues.
So what underlayment should I use under my tile?
First question: where are you installing the tile? Floor, wall, countertop, or outside?
Tiling a Floor
Ah, floors, where tile’s durability is put to the test. Let’s start from the base up: what is your subfloor?
Wood: Plywood, OSB, Shiplap, or tongue-in-groove
1/4” cement board is your basic, cheapest floor underlayment. Spread thinset with a 1/4” notch trowel, lay your 1/4” thick cement board so that the seams do not line up with your wood subfloor panels, and then screw them down with galvanized cement board screws.
If you want added stability, waterproofing, or need to save time or have the shortest floor possible, a tile mat such as Ditra or DCI mat is your best option. Spread thinset with a 1/4” notch trowel, roll out the mat, embedding the fabric underside into the thinset. Add a strip of waterproof membrane (such as Schluter Kerdi Band) to your seams to waterproof the floor.
Concrete
Wait, can’t I just set my tile directly on the concrete slab?
We don’t recommend that for a few reasons:
- Concrete slabs can wick water from outside in. That moisture can creep up into your thinset and tile, causing structural issues or promoting bacterial growth. A waterproofing layer is recommended.
- Concrete takes a long time to cure, much longer than you’d think. A slab that was poured less than 15 years ago could still be curing, and could potentially shrink and crack. If it does, those cracks will transfer up to the tile, causing the tile and grout to crack too.
A mat underlayment is best for concrete as it prevents cracks and moisture from getting to the tile. Spread thinset with a 1/4” notch trowel, roll out the mat, embedding the fabric underside into the thinset. Add a strip of waterproof membrane (such as Schluter Kerdi Band) to your seams to waterproof the floor.
If you want heated floors, get the Ditra Heat Duo mat. The extra insulating layer will allow you to get the most heat out of your system.
Old Flooring: old tile, linoleum, etc.
It is always better to remove the old layers and start fresh. Old problems such as cracks or creaks or leaks or unevenness will transfer to whatever you put above it. Covering up a structural problem won’t make it go away; it will come back to haunt you.
Tiling on top of old flooring will add height to your floor. Your doors may be too close to the floor now, to the point where they can’t close or open anymore. You may create uneveness between different rooms; for example your bathroom floor may now be 1/2” taller than your hallway floor, and you have to install a little ramp to transition the two floors.
There are a few legitimate reasons you may not want to remove the old flooring: if the flooring is old enough, it may contain asbestos, and abatement can be cost prohibitive.
If the flooring is in good condition, no damage or uneveness, you can tile on top of it with a mat underlayment. Rough up the surface of the old floor with a large grit sandpaper, then clean up the dust. Spread thinset with a 1/4” notch trowel, roll out the mat, embedding the fabric underside into the thinset. Add a strip of waterproof membrane (such as Schluter Kerdi Band) to your seams to waterproof the floor.
Tiling a Wall
Where is this wall?
Kitchen Backsplash
If you already have painted drywall up, make sure it’s clean and free of any grease or oil and you can tile directly to that.
Some people prefer to add a little bit of waterproofing to the wall behind their sink; you could replace that section of drywall with a piece of 1/2” polystyrene board, or add a waterproof barrier such as Schluter Kerdi to that area.
Bathroom Dry Area, such as a wainscoting, feature wall, or backsplash
If you already have painted drywall up, make sure it’s clean and dust free and you can tile to that. If you’d like to add a waterproof layer behind the tile, just to be safe (such as under the tile wainscoting around the toilet in the kids’ bathroom), we recommend a layer of Schluter Kerdi over the drywall.
If your wall studs are bare, you can install either 1/2” cement board or 1/2” polystyrene board using the appropriate board screws and washers.
Bathroom Wet Area, such as a shower or tub surround
Nothing beats a polystyrene waterproofing system when it comes to building a shower or tub surround.
Use the 1/2” board along with the recommended screws, washers, and sealant to build a 100% waterproof space.
Fireplace Surround
If your firebox is properly built and sealed, the heat will go straight out into the room. The wall around the fireplace should not be getting hot.
That said, for safety’s sake, especially with a wood burning fireplace, 1/2” cement board is ideal for under your fireplace surround tile. You could also use 1/2” polystyrene board, which is non-flammable (it will melt at a high enough temperature, but your fireplace should never be that hot).
Certain drywall products have extra additives to make them more fire resistant.
Tiling a Countertop
Can I tile over an existing countertop?
If you have slab or tile countertops, it’s best to remove them and get down to bare cabinets. The extra weight, plus the structural and installation prep requirements, will be too much.
If you have laminate countertops, then you’re in luck! This is the easiest countertop to tile over. Rough up the surface of the laminate with a large grit sandpaper, then clean up the dust. Spread thinset with a 1/4” notch trowel, roll out your tile mat, embedding the fabric underside into the thinset.
Starting fresh with bare cabinets.
We recommend a 2” polystyrene board. Wedi, HydroBlok, and Schluter all make a 2” thick waterproof board that can be used for countertops. Easy to cut and install, with added water protection.
We recommend a 2” polystyrene board. Wedi, HydroBlok, and Schluter all make a 2” thick waterproof board that can be used for countertops. Easy to cut and install, with added water protection.
Both cement board and tile mats will work, but they require some sort of subtop, such as a 3/4” layer of plywood. This creates extra work, weight, and mess.
Tiling Outside
Outside installations can be tricky, as you have to consider temperature, humidity, slope, drainage, and exposure when picking materials.
For slab patios or wood decks, make sure there’s a slope of 1/4” per foot away from the house. Use a tile mat and waterproof sealant for the seams.
For exterior countertops, we recommend 2” polystyrene board. About the Author: Kathryn Helbling has been a Tile Lines designer and product expert for 10 years.
