Tile Underlayments & Wallboards

What is an underlayment? What is the purpose of an underlayment?

Underlayments are the support materials that lay underneath your tile on a floor. They sit between your subfloor (usually plywood) and your tile. They provide a waterstable base, make the floor more rigid (less chance of cracked tiles), and are better suited for tile adhesion.


A well-set tile on a sturdy underlayment should never crack, and will last for decades.

What is the difference between "waterstable" and "waterproof"?

Some products, such as cement board, are water stable. This means that they will not warp or fall apart if exposed to or submerged in water, much in the way that plywood or drywall would. But they are not waterproof: water will still work its way through cement board and get to plywood subfloors or wood support beams, and eventually cause mold growth.


Waterproof materials are non-porous and will not allow water or vapor through. These include:

Underlayments for Tile Floors

Cement Board

Cement Backer Board

Sturdy and waterstable, cement boards such as HardiBacker, Wonderboard, and JetBacker are among the least expensive of tile underlayment options.


We recommend Fiberock as an underlayment. This fiber-reinforced gypsum board is silica-free, as easy to cut as drywall, as strong as any other cement board and less expensive than other cementboards.

Schluter Ditra

Schluter Ditra

Lightweight and waterproof, Schluter Ditra is a waffle-patterned underlayment that doubles as an uncoupling membrane. Perfect for absorbing movement and blocking moisture, Ditra is a great option for basement floors, concrete substrates, bathrooms, laundry rooms, and outside spaces.


Ditra requires special thinset and is more expensive than cementboard, but is the perfect product for special situations where you need a thinner, more lightweight, waterproof product.


Visit our Ditra page for more information. Ditra is also available as an in-floor heating mat (DitraHeat) and an insulating sound barrier (DitraHeat Duo).

Wallboard Options for Showers, Tiled Wainscoting, & Backsplashes


If you are tiling a wall in a dry area (such as a kitchen backsplash or a wainscoting), you can tile directly onto painted and/or primed drywall. If the drywall is painted with a high-gloss paint, we recommend lightly buffing it with sandpaper so that the thinset can bond to it.


Make sure the surface is clean and free of dust or grease before installing your tile.


Drywall is not recommended for walls in wet areas such as showers as it does not hold up to moisture. If you already have drywall around your shower/tub and don't want to tear it down, use Schluter Kerdi to waterproof the wall wherever water might hit it.

Extruded Polystyrene Foam Waterproof Building Panels

Wedi Building Panels

If you're tiling a shower or bath tub surround, you need waterproofing. 


Wedi, HydroBlok, and Kerdi Board panels are constructed from  high-density, closed cell, lightweight, waterproof polystyrene foam. Available in the same sizes as standard cement board, they are just as durability but more lightweight and a lot easier to cut and install.


Simply affix the panels to your wall studs with the non-ferrous screws and washers, seal all punctures and seams with polyurethane caulking, and you have a watertight shower surround that won't harbor mold or mildew.


Visit the Wedi page, HydroBlok page, and Kerdi page to learn more about these shower systems.


Check out this TileLetter article: Multifunctional Foam Building Panels Offer Advantages to the Tile Setter

Cementboard (and waterproofing for wet areas)

Schluter Kerdi fabric is in stock in Kent, WA

The only time you need to use cementboard for a vertical installation is when your installation will be continuously exposed to water (primarily showers or bath tub surrounds). If you use cementboard in these areas, you'll need to waterproof them. We recommend the Schluter Kerdi system for waterproofing wallboards.

Underlayments for Tiled Countertops

Cement Board

As with tiled floors, you'll want a cement board bonded to the top of plywood for your new countertops. The water-stable nature of the cement boards insures that, in the event of leaky sinks or spilled liquids, your countertop will not warp or swell.


For extra protection, add a waterproof barrier (such as Kerdi) to the cement board, or consider installing Wedi board instead of cement board.


Check out this TileLetter article: Multifunctional Foam Building Panels Offer Advantages to the Tile Setter

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