Crack isolation/suppression membranes basically serve as shock absorbers. When installed correctly, they transfer stress from the crack to the nearest soft joint. Unless they have the ability to move, they are basically useless. When it comes to crack isolation and dealing with control joint relocation, you must be very selective in choosing the appropriate product. Due to many different product variations and their ability to support tile, not all membranes are suitable for all applications.
As an example, most popular liquid membrane systems actually caution against their use for control joint relocation, yet they are consistently used for that application. Additionally, most membranes require a three-tile width when they are approved for that purpose, not the 12” strip commonly found with either liquids or sheets.
Sheet products offer more predictable performance when it comes to crack isolation for several reasons, not the least of which is factory-controlled mil thickness. Error on the part of the installer is much less likely when using a factory-made rather than field-made product. By their nature, sheets have greater ability to withstand more aggressive movement than the typical liquid product. This does not mean that liquid products should never be used. It just means to do your research before using a product for the job.
When using liquids make sure to check your application thickness with a film thickness gauge. Most require around a 30 mil (.030 or 1/32”) thickness or greater. Any less, and it is frankly just colored concrete incapable of absorbing and transferring movement.
Most membrane adhesives are moisture-sensitive and should not be used over a slab with high relative humidity or moisture vapor emission. The “tile setters’ 24 hours,” (typically the next morning) can be a very bad idea when using membranes. With the low water absorption of both the tile and membrane, the water in the setting material isn’t going anywhere very fast. Low temperatures of either the air or the surface may make it even slower.
Also, never allow construction traffic over a membrane. They are hard enough to bond to without being soiled. This can and will affect the longevity of the installation.
The importance of control joints. All concrete slabs require control joints, because all concrete shrinks as it cures. Control joints control where the cracking will occur. Concrete, like all building material, will move with changes in moisture and temperature.
A residential slab with no movement joints begs for the use of a membrane because it is going to crack somewhere. If the slab is already cracked and there is no vertical displacement (in other words, one side is not higher than the other) a membrane will aid in keeping the tile from cracking.
If you are tiling direct to a suspended slab, also known as slab on deck, that installation is also an excellent candidate for a membrane. All above-grade slabs move by design, so these types of installations require some type of deformable installation system to allow for a reasonable amount of distortion.
Other considerations. With few exceptions every tile job ends up with an argument about the need for movement accommodation (aka expansion) joints. Unfortunately these joints are necessary evils and there is no way around the fact that all building materials move, and move at different rates.
It is pointless to install a membrane that allows movement and not provide for that anticipated movement. If an installation is to have the ability to deform, that stress must be transferred someplace. Crack isolation membranes do exactly that, storing energy and transferring it to a different place (the movement accommodation joint) rather than through the tile surface.
Please note that there are few if any guarantees by a manufacturer that a membrane will stop tile cracks from occurring. This is spelled out in the American National Standard Specification for Crack Isolation Membranes for Thin-set Ceramic Tile and Dimension Stone Installation (ANSI A118.12-05) which states:“It should be noted that while crack isolation membranes are intended to minimize the potential for crack propagation from the substrate through the finished tile or stone installation, they may not always be 100% effective in preventing all defects in the finished tile.”
Therefore, most manufacturer warranties are for membrane product replacement. There are very few exceptions to this statement and those that make that exception have numerous caveats attached to any type of material and labor warranty. You can be quite sure if a problem does occur and a claim is filed, the installation products and processes will be gone over with a fine-tooth comb.
About the Author
David M. Gobis, a third-generation tile setter, is an independent Technical Consultant. He has been in the trade for over 37 years and owned a successful contracting business for many years prior to his current position. Mr. Gobis is an Author of over 100 trade related articles and a frequent speaker at industry events. He is member of the Construction Specification Institute, International Code Council, American Concrete Institute, National Tile Contractors Technical Committee, voting member of The American National Standards for Ceramic Tile Installation and Setting Materials (ANSI A108/118), American Society for Testing of Materials (ASTM) C-21 Ceramic Whitewares, and Tile Council of America Installation Handbook committees. You can reach Dave via email, firstname.lastname@example.org