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Ceramic & Stone Tile Underlayments


by Bud Cline - September 2006

Ceramic & Stone Tile Underlayments



Of the entire inventory of floor covering products on today's market, ceramic and stone tiles probably lend themselves more to do-it-yourself home improvement projects than almost any other flooring product but there are rules and special underlayments required to insure a proper and lasting installation.



Tile in its elementary form is easy to install. It doesn't take long to get the hang of it, however if you research installing ceramic tile you will find that the techniques are many and there are in fact absolute rules that should be followed to insure a suitable return on your investment.


If someone tells you that a tile installation is only as good as its foundation and its preparation, you better believe it. In the case of tile floors the structure must be a suitable one. Not all structures are suitable for a tile installation. Floor deflection is one of the major causes of a floor tile installation failure. The maximum allowable deflection for any ceramic tile floor is what is known as: L/360. This means that your tiled floor span can only deflect (under a 300 pound point load) one three hundred-sixtieth of the length of the span.
In the case of all natural stone tiles the allowable deflection is half of that of ceramic: L/720. All of this technical talk is explained in another area of this website so let's get back to underlayments.

Tile Underlayment Products

CBU's:
» wedi

Sheet Membranes:

Liquid Membranes:
Some of the more common and readily available hard-surface tile underlayments are called "cement board" or "Cementuous Backer Units" or in the trades "CBU's". CBU's come in several forms and generally two nominal thickness sizes and inch.

Half-inch CBU's are always used over wall studs; shower walls, tub surrounds, steam rooms, etc, never use the quarter-inch version on walls or ceilings. Quarter-inch and/or half-inch CBU's however can be used on floors. You should know that CBU's offer absolutely no structural benefit and they are simply a proper "tooth" for the tile-setting materials to bond with. CBU's are not in any way detrimentally affected by moisture or water submersion. Any structural improvements required to meet minimums should be completed before the CBU's go into place.

A traditional CBU is basically a sheet of cement and aggregate between two layers of fiber-mesh. There are other variations that have the same attributes and there are versions that have a waterproof covering on one surface. All CBU's used on the floor are required to be installed in a full bed of thinset. The thinset is used not to bond the CBU to the substrate but instead to insure there are no voids anywhere under the CBU. In addition fasteners are used to install the CBU products. Follow the printed manufacturers instructions available with all CBU products.

In addition to the CBU style underlayments there are also a variety of plastic underlayments that serve a slightly different purpose. These plastic underlayments are not necessarily interchangeable with the above CBU's and usually have a specific purpose. These will be discussed under another topic.


Here are a few (underlayment related) things you wouldn't want to do when installing your new tile.
  • Tiling directly to plywood isn't usually recommended for the diy'er. Sure it can be done and done successfully but there are particular methods and products and unless you are familiar with the variables I wouldn't suggest you do it.
  • Tiling directly to vinyl flooring also is not recommended under most circumstances and this is discussed in another topic.
  • Tiling directly to particleboard or luan plywood is FORBIDDEN, it just doesn't work. In fact, here's what the industry has to say on that subject:
AMERICAN NATIONAL STANDARD SPECIFICATIONS FOR THE INSTALLATION OF CERAMIC TILE (ANSI-1999) ANSI A 108 Page 13
AN-2.4.3 CAUTION: Wood-based panels such as particle board, composite panels (veneer faces bonded to reconstituted wood cores), luan plywood and soft wood plywood all expand and contract with changes in moisture content and are not recommended as backing materials for ceramic tile...


You also wouldn't want to install tile directly to any metal surfaces such as fireplace surrounds, or to most painted surfaces, or to any acrylic or fiberglass surfaces. This isn't to say that there are no means to do this but in most cases there are specific methods and special adhesives required to accomplish a satisfactory installation.

From time to time when diy'ers decide to build their own showers the question of tiling directly to a vinyl shower pan liner arises. The answer here is also NO-WAY, this cannot be done under any circumstances.


Installing your own ceramic and stone tile is fun and can create a genuine feeling of pride when the project is complete. Tile projects can be somewhat costly and the last thing anyone wants to have happen is to have a tile floor installation fail for one reason or another especially when failures can be easily avoided. There are many pitfalls that can be prevented by doing a little research and getting some legitimate advice from experienced tradesmen. Please don't assume that everyone offering advice actually knows of the many things you can and cannot do on the road to a successful tile installation.

A proper base is a must and without one you run the risk of a failed project and wasted dollars. Proper foundations in tile installations can take on many variables and do require some minimal research.
Bud Cline

About The Author
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 since 1976. 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 TheFloorPro.com, Bud spends some of his precious spare time lake fishing.




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» Industry Publications & Information «
WFCA Moisture Vapor Testing Position Statement, from the World Floor Covering Association

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