The various brands of cement board, such as HardieBacker, Durock, DenShield, and Wonderboard have become a standard tile backer material for nearly all ceramic, porcelain, and stone tiles for floor, wall, and countertop applications. These convenient panels of cementitious board provide an instant flat, hard surface that bonds with the thin-set adhesives or mortars used to install tile.
Typically, a layer of cement board is screwed to a plywood or OSB subfloor, to to the wall studs for wall installations. While most tile manufacturers do say that their products can be applied directly to plywood, the cementitious surface of cement board provides a better base. And when you are tiling wet locations, such as bathrooms, cement board is not just a good idea—it is necessary.
Cement Board on Concrete?
Normally, installing cement board is not regarded as necessary when you are laying tile onto a concrete slab since this subfloor is already cementitious—adding cement board would be redundant. However, there are three cases where installers are sometimes tempted to install a layer of cement board before applying tile:
- The existing concrete does not provide an adequate, solid base for tiling.
- The concrete has been painted over, and paint is not an acceptable surface for thin-set adhesive or mortar.
- The concrete needs to be raised higher than you can comfortably float a mortar bed.
Would adding an underlayment of cement board help with these problems?
James Hardie Industries, makers of HardieBacker®, and USG, makers of Durock®, indicate that their respective cement backer boards should not be installed over concrete.
- James Hardie: HardieBacker specifications specifically exclude concrete as a base for installation.
- USG: Durock does not expressly exclude concrete, but the material is specified only for minimum 5/8-inch exterior-grade plywood or OSB. One source reports that USG will not officially validate the Durock-to-concrete attachment simply because they have not tested it. The lack of testing may be simply because so few customers express a need for applying Durock to concrete.
The View of Tile Professionals
But the manufacturer prohibitions or omissions are warranty issues. The questions remain: Can you effectively pair two cementitious products—cement board and a concrete slab?
There is no problem with the two materials being compatible. The issue, as Bud Cline of The Floor Pro says, is more about how to attach the cement board to the concrete. A powder-actuated nailer is out of the question since nail depth would be impossible to regulate. Concrete screws, Cline says, have heads that are too small to hold down the cement board.
His recommendation: Work with the concrete surface so that it is strong enough and porous enough to accept tile mortar. Portland cement-based fillers can take care holes and cracks. Painted concrete can be sandblasted, sanded, or ground down to bring up a nice, porous surface.
Most tile professionals, including John Bridge, concur: Attaching cement board to concrete is not an acceptable way to surface the concrete prior to tile installation. Thin-set alone will not help the cement board stick to the concrete slab. Screws are the only logical way to do this, but it would entail an extremely tedious and time-consuming process of drilling pilot holes before sinking the screws. Additionally, you would be fighting against the thin-set bed under the cement board when drilling the holes and driving the screws.
Technically, cement board can be laid over a concrete slab as the base for a tile installation. But doing so is a very laborious, time-consuming process that is likely more trouble than it is worth. A better solution is to prepare and resurface the concrete slab so that it can accept thin-set adhesive or a mortar base onto which to lay tile.