Today's installation systems allow the incorporation of ceramic (porcelain) and stone tile on virtually any surface. However, historic analysis of the development of these contemporary systems reflects a dramatic shift in accepted wisdom regarding the physical dynamics of the floor assembly that comprises this ideal and popular surface covering.
Self-leveling and Trowel-able Underlayment
One of the oldest methods in the book would be simply building a bed of mortar above the substrate. This is not a recommended method due to the concrete, like other construction materials, expands and contracts with moisture and temperature changes. These volume changes may produce cracks in hardened concrete unless they are properly controlled. Provision for volume changes at predetermined locations that prevent a concentration of crack-producing stress forces. Such provisions are termed control relief joints. The years of trial and error brought in a new method using self-leveling pour. This product is mixed with water to liquid consistency and poured on, then spread with large trowels or rollers. Because it is more liquid rather than a muddy texture, this helps level low areas and evens out the surface of the floor.
Some manufacturers have proprietary products that allow a tile installation (when their products are used in accordance with recommended methods) over control joints, so long as out-of-plane deflection does not occur. Even small shrinkage cracks in concrete can be dimensionally active; continued curing of the slab can cause these cracks to expand or propagate. This type of cracking can be easily avoided in the tile layer by installing the tile over a crack isolation membrane. With this installation, a crack isolation membrane is bonded to the concrete, and tile is bonded (with mortar) to the surface of the membrane. Using a membrane is highly recommended by ITS because of its useful properties in waterproofing, crack isolation, and sound reduction.
Why is Underlayment important?
Even though these preventative measures can reduce (and often eliminate) cracking within the concrete, surface cracking can still occur. Whenever tile is directly bonded to a concrete substrate, cracks occurring in the concrete can cause cracks to transfer into the tile layer - this is often called "reflective cracking." Underlayments do a great job thanks to manufacturers committed to innovative products that make the finished tile installation look the way it should – flat and lippage-free.