Cement Boards Over a Slab
Using Cement Boards Over Concrete Slab Construction
It seems that ceramic tile installation has an abundance of myths that pop up all too frequently regarding Do-It-Yourselfers on “help forums” and DIY websites. So-called tile experts offer advice and opinions based on, well… I’m not sure what some of the comments are based on or where some people get their (mis)information to tell you the truth. I want to address the subject of installing cement board products over a concrete substrate.
It is imagined by some that products like Durock, Hardibacker and others can be used to fix imperfections in the surface of concrete or to override a previously painted or sealed concrete surface. Installing tile over painted or sealed surfaces is usually not good practice.
In fact, the industry takes a dim view of such things and it has been known for many years that these types of coatings (paints, varnishes, sealers, etc.) are bond breakers of the worst kind when it comes to tile adhesives.
Some would think a simple solution (and path of least resistance) is to cover the offending coating rather than removing it. Where many experienced tile installers congregate on the Net, like here at The Floor Pro Community, this suggestion gives rise to smiles and even in some cases laughter. It is because you cannot effectively or easily install cement board over a concrete surface. This isn’t to say it can’t be done, but what a hassle.
Some cement boards (or “CBUs”- Cementitious Backer Units) can be like rodeo bulldogging when trying to wrestle panels down to the concrete floor. The darned things are usually warped and some lesser known brands are of questionable quality. To install CBUs, they must be attached to the substrate with nails, screws or staples every four to eight inches in both directions. Ceramic and stone tile installations require the backer boards to be installed into a bed of thinset mortar at the same time. Thinset would usually not be tacky enough to hold a board in place by itself, but imperfections in the substrate and in the panel itself could be enough to tweak the board in an undesirable fashion. This would result in warps, lippage and an overall uneven and out-of-plane surface condition.
Over wood structures, installing CBUs isn’t difficult at all, but over a concrete floor several issues arise. The fasteners of choice would have to be screws, obviously, but which screws would you use? Regular CBU screws are made with threads for anchoring into wood and metal. Standard nails and staples are, of course, out of the question. Concrete nails are too difficult to get countersunk without breaking out the concrete. Concrete screws (Tapcons) could be used but they have a tiny head and would pull right through any warped or distorted cement board. The use of washers is out because they can’t be countersunk either. In addition, concrete screws would require pilot holes be drilled to accept the screws. What about the piles of debris that would be extracted from the hole under the CBU panels as it is drilled? Where would that go?
No sir, installing cement boards over concrete just isn’t a very practical idea. In fact, I think it would create more problems than it would solve. The best way to address any inferior concrete surface in preparing it for a tile installation is to use a direct approach.
Concrete floors should be mechanically abraded by some means. Sandblasting, shot blasting, grinding and sanding are all acceptable means of attack. Never use chemical strippers, as they may continue to bleed up through the slab for a long period of time. In the event fillers are required to correct a substrate, always use products that are made of Portland cement, never use gypsum products for a base in a tile installation.
There are also “Self Leveling Compound” (SLC) products available, which in most cases would be the more suitable approach to correcting a concrete surface in preparation for a tile installation. These may take more skill to apply than fastening some backer board panels, but if you follow their installation instructions, they can provide a bullet-proof, flat surface for the installation of your new ceramic or stone tile flooring.
This article was originally published Jan 11, 2013. If you would like to discuss this article, or any of the other articles you have found here at TheFloorPro.com, please join us in the Articles Discussion Forum or post a comment below. If you are not yet a member of The Floor Pro Community, we invite you to register now, it’s free and gives you the opportunity to enjoy all the features of the site.
Bud Cline is a native of the St. Louis Missouri area. After early stints at various service jobs, a tile installer bloomed in 1976. Bud has been a flooring professional ever since. He has been married to his beautiful wife since 1981 and enjoys his three great kids. When he isn’t being with family and friends, installing tile, or spending time at The Floor Pro Community, Bud spends some of his precious spare time lake fishing.