The importance of a well-prepared substrate cannot be overemphasized.


All finished floor installations must begin with a clean, dry substrate.

There are a host of “standard” procedures on how to prepare substrates to accept finished floor surfaces. In a perfect world, these procedures could be applied to every installation, and subfloor issues would be easy to identify and resolve, and callbacks would be minimal. But, this is not a perfect world.

The importance of a clean floor cannot be overemphasized. Just as paint cannot bond to a dirty wall, flooring products cannot adhere to a poorly prepared substrate. Sometimes it’s as simple as cleaning the surface and allowing it to dry before installing the new floor. Other times, additional steps must be taken to assure that the materials being installed can adhere properly to a substrate.

With changes to the design of cement backer boards for lightweight applications, the advent of floor heating systems and an abundance of new products being introduced to the market, it is imperative to understand product compatibility. For example, a poured gypsum underlayment may be too absorbent and ultimately cause the finished floor to delaminate or crack (refer to TCNA Handbook Method F180-07). In this case, the solution would be to apply a sealer to assure compatibility with the primer or adhesive you will install over it. While these types of products are helpful, they may not be appropriate for every installation. A compatibility test is always recommended.

Other common substrate challenges include working with damaged substrates, installing over existing floors, dealing with uneven or cracked surfaces, properly treating floor joints and protecting against Moisture Vapor Transmission (MVT). The number of products available to address these issues is numerous, but there’s one little known solution.

Best known for protecting finished floors from failure caused by lateral substrate movement, self-adhering elastomeric crack isolation membranes can often resolve a number of substrate issues. In situations where substrates are damaged or a renovation calls for installing a new floor over an existing one, they’re often the most economical route to achieving a suitable surface ready to accept mortars, adhesives and epoxy setting materials.

A good example is the ability to salvage a concrete substrate that is severely cracked yet structurally sound. This is a scenario not uncommon in shopping malls, commercial buildings and private homes throughout the United States. The costs in time and money to remove and replace the slab can be significant.

As a result, the materials preferred for the finished floor may become cost prohibitive, and of course, installation time is delayed. Because crack isolation membranes can often be used to cover a damaged concrete slab at about 25 percent of the cost it would take to replace it, the ability to use the original finished floor product of choice is restored.

Another advantage to using membranes in renovation projects is that many are much thinner than alternate subfloor materials. Usually 1/8-inch or less in height, they add very little to the elevation of the subfloor. A residential kitchen renovation no longer has to mean choosing between a dishwasher that will close and a new ceramic tile surface.

Another substrate issue some crack isolation membranes can resolve is protecting finished surfaces from Moisture Vapor Transmission (MVT), a problem common to new construction and installations in low-lying geographical areas. Some membranes offer certified resistance to MVT.

Crack isolation membranes are also useful for treating floor joints prior to new floor installation. Understanding the various types of subfloor joints is critical to designing and executing proper ceramic tile installation. What are commonly called expansion joints are really four different kinds of joints in sub floors; here is a look at each.

Control joints

Control joints are the most effective method of preventing dry shrinkage cracks. Before a concrete slab cures, control joints are placed anywhere from 20’ to 25’ on center or on column centers (refer to method EJ171-07 in the TCNA Handbook). Without these joints or cuts in the concrete, the cement will develop random dry shrinkage cracks. Control joints permit horizontal movement in the plane of the slab. Because concrete cures from the top down, the concrete tends to curl or lift. With the control joint, however, the cracks develop at these planned points. These breaks in turn, cause the cement to snap at these points.

Cold or Construction joints

These joints occur where concrete work is concluded for the day. Unlike control joints, these breaks go all the way through the slab. Sometimes they are treated with expansion joint material. At other times, they’re simply handled as stopping points.

Isolation joints

These joints separate columns or walls from the slab and permit horizontal and vertical movement. They extend through the full depth of the slab and use a joint filler.

Crack isolation membranes can often be used to expedite finished floor installations by resolving many subfloor problems.

Expansion joints

A true expansion joint, made of metal or plastic, works freely from the subfloor. Because it is designed to expand and contract with both horizontal and vertical movement, this type of joint should never be covered with a flooring product.

Handling control joints begins with their placement by the structural engineer/architect, who has in mind how the floor will move and where the joints should be located to control lateral shrinkage. Shrinkage for each slab at this joint is about 1/8-inch, totaling 1/4-inch for the two slabs. Once their location is set, they are generally formed with saw cuts that are about one-quarter the thickness of the slab. They also can be made with grooved forms, by scoring, or by insertion of molded non-metallic strips into plastic concrete. According to literature published by The World of Concrete Center, “The spacing, in feet, of control joints should equal two to three times the thickness, in inches of the concrete slab.”

While certainly important to the success of the finished flooring, these joints cause both challenges and dismay for the architect or floor designer. The tile industry has long recommended that these joints be brought up through the tile, and that tile be cut to meet all these joints in the slab. If they don’t line up, something has to make a break between the two so that the tile doesn’t crack when the slab shrinks – and a break in the placement of tile on the floor means a break in design.

Products that erase joint problems

Attractively designed tile patterns and durable floor surfaces need not be sacrificed to placement of control joints. Crack isolation membranes can solve these problems. A two-component, sheet-applied membrane can transfer the movement at the joint in the subfloor to the natural pattern of the tile. That way you don’t have to cut the tile to meet the joint. Instead, the movement is transferred to the tile edge closest to the edge of the membrane.

The soft control joint still presents a problem, however, resulting in tile breakage from traffic and wheels. Another product can solve this problem. Preformed expansion joints protect the tile by placing a pre-molded expansion joint made of anodized aluminum and polyurethane sealant between the tile. This raises wheel loads over the tile edge, preventing breakage and damage to the tile.

Now that we’ve reviewed the numerous ways a crack isolation membrane can be useful for substrate preparation, there is one important factor to consider when choosing one specifically for crack isolation. The crack bridging ability and structural movement capability of a membrane are two different features. A product that promises to bridge a 3/8-inch crack doesn’t necessarily have the capability of handling 3/8-inch of lateral substrate movement. A product’s data sheet should explain the elastomeric qualities of the membrane to bridge movement of the subfloor. These are critical factors when selecting the right membrane for the job.

A problem-free floor starts at its foundation. Crack isolation membranes are fast becoming a key to successful substrate preparation with excellent results.