Although we’ve retired the use of cement board underlayment (or CBU's - cementitious backer unit) long ago, while working in the field we see many tile guys still using this antiquated method to provide a bondable surface for their tile installation.
It still works. After all these years and many better products that do the same thing much better, guys still punish themselves using cement board. The dust, the grit, the weight…Forget about it! Ok, they use it, whatever…but do it correctly!
As a home owner or do it yourselfer…BE SURE YOUR CEMENT BOARD IS EMBEDDED IN A LAYER OF THIN SET WHEN USING IT ON A FLOORING APPLICATION!
It is shocking how many installers skip this step because it’s basic, beginner level competence. Let me tell you why it’s so important.
We put thin set under the cement board to support, or bed, the installation. It helps the installation to float on the structure and the thin set makes it so the CBU isn't bonded to the plywood. You need to make sure there is enough thin set to fill the voids in the sub floor. The thin set will allow the CBU to conform to the plywood and negates weak unsupported areas.
If you skip this step you will have air space underneath the tile installation. The air space will allow for vertical and lateral movement in the tile and the one thing you never want with tile is MOVEMENT! I can see the basic, builder grade tile installer saying “Just throw some more nails in it, It’ll be fine!” Wrong. This tile installer is breaking a cardinal rule of tile installation. The tile floor with cement board that was not bedded in thin set will creek and crack leaving cracked tile, missing grout, etc. A costly, failed floor.
People will skip this step to save a dollar and save some time. You don’t have to mix up thin set or spread it, so this cut corner makes the installation ‘easier’. This cut corner is common on builder grade jobs because it’s very rare for the installer to be held accountable. This cut corner causes the future homeowner a ton of problems, and gives us job security.
Remember, if you’re going to use cement board….embed it in thin set and if you’re smart and want to improve…Switch to Ditra by Schluter Systems or a comparable product!
The video below shows an installer with a taste for cement board properly installing it.