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 water-stable 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 "water-stable" 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:


  • Plywood, particle board, or solid wood/hardwood - can warp and swell, causing cracks and loose tiles
  • Old tile, linoleum, or vinyl flooring - adhesive may not stick, any issues/cracks will travel up to the new tile
  • Cement slabs - can crack overtime, causing tile to crack. Moisture from the slab can weaken thinset and grout

You can tile over some of these surfaces if you have an appropriate underlayment in between them and the tile.


For example, we had a customer with old vinyl flooring in good condition (no peeling edges, no brittleness or cracks), but there was asbestos underneath and they didn't have the budget for proper abatement. They decided to use Schluter Ditra mat on top of the vinyl, following the manufacturer's guidlines, and tiled on top of the Ditra.

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

Cement Board (and waterproofing for wet areas)

Schluter Kerdi fabric is in stock in Kent, WA

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

Underlayments for Tiled Countertops

1/4" Cement Board + 3/4" Plywood

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 membrane) to the cement board, or consider installing Wedi board instead of cement board.

2" Wedi or Kerdi Board


You can also set 2" thick Wedi or Kerdi board directly on your cabinetry for a solid, lightweight, durable, and waterproof surface that can be cut with a box knife or hacksaw.


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

1/8" Ditra Mat + 3/4" Plywood

1/8" Ditra Mat provides under-tile stability and waterproofing at half the thickness of cement board. 

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