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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.


Bud Cline
Bud Cline is a native of the St. Louis Missouri area where natives are scarce. After early stints at cooking, ditch digging, truck driving, airline and hotel work, a tile installer bloomed in 1976. Bud has been a flooring professional ever since. Having lived and worked in Palm Springs, Des Moines, Dallas, and St. Louis, he is still 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.

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.

10 Responses to Cement Boards Over a Slab

  1. Cesar Gonzalez says:

    I want to tile over my marble .hallway has marble and living room has carpet. I want to combine tiles so Carpet is coming out and now theres about a 1/4 inch difference between tile over marble (hallway) and livingroom? Hope I didnt butcher that question

  2. Doug Glenn says:

    We want to install a floating egineered wood floor in our concrete slab home which was built in 1996. The kitchen, breakfast room, entry and hearth are covered with a hideous looking ceramic tile that won’t come up without the use of a jackhammer. My wife, who is disabled and can not leave the home for any length of time, nor I want to go through the noise and dust created by breaking up the old tile, or using a leveling compound which means removing all the furniture and being out of the home for several days to cure. The only two options I’ve come up with is gluing the existing laminate floor to the slab, or using 1/2″ concrete board to raise the level of the floor even with the tile. We have tall baseboards and the doors are cut high enough to accept the added 1/2″. Any suggestions will be greatly appreciated.

  3. Dino says:

    I will be installing square cut slate on my outdoor porch. Two different thickness require. 2″ thick coping around the perimeter and 1″ thick tiles for the field area. Can I used self leveling cement to bring up the field area by 1/2″.

    Once the self leveling is curried, i will used portland cement/sand mixture to set the field stone. 1/2″ thick mortar would allow my field stone to be level with coping stone.

    I would assume I must use an exterior self leveling product. Do I have to pre-treat the porch concrete prior to self leveling application?

  4. Effie Liu says:

    I am a diy beginner, trying to replace bath room and kitchen floor ( 170 sq ft). I have removed top vinyl sheets and most thick glue on concrete slab that is on 2nd floor.
    But the concrete floor looks damaged. There are exposed cracks without top cement layer, a hole deep 1/2″ and
    a thin glue like layer on the concrete top.

    Can I prepare this concrete floor for a new ceramic tile by
    1) Repair the cracks and hole, what material is better
    2) Grind the top glue,
    3) Level the floor

    It looks to heavy for me . I am interested in Skim-Coat, but have a question on it:
    Can I miss “Grind and Level” stages if applying skim-coat? The concrete slab itself is in good condition.

    Thanks for your attention and quick reply to my questions and post your experience and knowledge.

    I

  5. James White says:

    Agreed with all your input. Well written comments, you should be an English teacher too. Cheers!!

  6. Bud Cline says:

    Smoothing a concrete surface can be achieved in a couple of ways. Grinding the surface would be the most desirable, just depends on the severity of the damage.

    Another methods would be to skim-coat the surface using a Portland cement based patching compound. This would probably be the least expensive method but would require some efficiency using a cement trowel. A primer would be required prior to the application of the skim-coat product.

  7. Jennifer Bergman says:

    We have a cement stoop that needs to be “corrected” before we can put tiles down on it. It had outdoor carpeting on it for years and when we pulled it up (for the new tiles) there was a lot of glue/mastic left on the surface of the concrete. One contractor recommended just putting down cement board. After reading your article, we have tried to find someone to come out to sand/smooth out the surface, but we can’t seem to find anyone that is willing to do such a small job. Neither my husband nor myself are all that “handy” with tools to do it ourselves. Do you have any suggestions? Thank you.

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