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 thinset adhesives or mortars used to install tile.
In most installations, a layer of cement board is screwed directly to a plywood or OSB subfloor, or more frequently, to the wall studs for wall installations. While most tile manufacturers do indicate that their products can be applied directly to plywood, cement board often provides a better base.
But what about when you already have a hard and solid masonry base in the form of concrete—it is necessary to have an intervening layer of cement board?
Cement Board Not Necessary 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 a cement board layer would be redundant and even detrimental to the overall project. It's best to fix the concrete and then apply an uncoupling membrane.
There are three infrequent cases where installers are sometimes tempted to install a layer of cement board before applying tile:
- Poor Concrete: If the existing concrete does not provide an adequate, solid base for tiling, some tilers may add cement board. Usually, though, it's best to repair the concrete instead of covering it up with cement board.
- Inadequate Surface: Paint is not an acceptable surface for thinset adhesive or mortar. So, if a concrete floor has been painted, some tilers may add cement board to cover up the paint. Etching or sanding down the concrete are often better ways to prepare the surface than applying cement board.
- Height Issues: Some tile installations may require the extra 1/4- to 1/2-inch that a layer of cement board and thinset can provide.
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.
Tile Professionals' Recommendations
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. As Bud Cline of The Floor Pro notes, the issue 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 of 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, agree: Attaching cement board to concrete is not an acceptable way to surface the concrete prior to tile installation. Thinset 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 thinset bed under the cement board when drilling the holes and driving the screws.
What to Do Instead
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 first prepare and resurface the concrete slab. Next, an uncoupling membrane such as Schluter Ditra brand should be installed on the concrete. These mats are now standard for preventing tile installed on concrete from cracking over time. Minor changes in the concrete are not transmitted to the tile and thinset above. Major changes in the concrete are mitigated, reducing the likelihood of cracking.