It makes sense to apply cement board to plywood. Plywood is an accepted surface for tile, but a cement product is better. When you are tiling wet locations, cement board is not just a good idea, it is necessary.
So what about installing cement board on a concrete floor?
There are three cases in which you might want to do this:
- 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 thinset.
- 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: HardieBacker specifications specifically exclude concrete as a base for installation. Durock does not expressly exclude concrete, but is specified only for minimum 5/8" exterior-grade plywood or OSB.
- USG: One source reports that USG (Durock) will not officially validate the Durock-to-concrete attachment simply because USG has not tested it. So few customers need to attach Durock to concrete that it is not cost-effective for USG to test for this.
What Do Tile Professionals Say?
But those are warranty issues. The questions remains: can you effectively pair two cementitious products?
There is no problem with the two materials matching up. 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. 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 that 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. Screws would be the only way to do this, yet this would become an extremely tedious and time-wasting 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 screws.
Technically, it can be done. But doing so may entail more work than if you were to repair and resurface the concrete base.