Without some degree of surface preparation, it’s highly unlikely that a concrete slab or wood-framed subfloor will be flat enough to meet the requirements for large-format tile. A quick check with a straightedge often shows that a small area or perhaps the entire surface dips and undulates outside of industry standards. The resulting lippage, an unsightly variation in height between tile edges, shows up very quickly with today’s large-format tiles. Because of the difficulty in laying 24-, 36-, 48-inch and up to 10-foot tiles without lippage, contractors regularly use leveling clip systems to hold tile edges flatly together while mortar cures. If the substrate isn’t flat before you start, the installation requires more labor and there’s a greater chance that the completed installation will be rejected.

Some tile contractors attempt to level tiles — even very large tiles — using adhesive mortars. Forensic consultants and manufacturers’ reps don’t see a week go by where an installer hasn’t used the “spot bond” or “5-glob Bob” method as a shortcut to failure. This practice is actually becoming a serious industry problem as many untrained and low bid contractors are entering the trade and not accounting for proper surface preparation. Unfortunately, once they start the project, the cost and burden of “leveling tiles” falls to them instead of the building owners.

Our industry requirements for substrate flatness prior to tile installations are very clear. Any substrate with a tile format that has any side longer than 15 inches requires flatness to vary no more than 1/8-inch over 10 feet and less than 1/16-inch in any 24-inch span. Most concrete slabs are finished within reasonable Floor Flatness (FF) & Floor Levelness (FL) specifications as measured by ASTM E1155, the Standard Test Method for Determining FF Floor Flatness and FL Floor Levelness Numbers.  However, the industry recognizes that results vary when the time comes to install tile. Depending on curing conditions, concrete shrinkage and curling will affect flatness. Additionally, Division 3 measurements do not include slab perimeters, column block outs or variations between joints in the slab. When measured with a 10-foot straightedge months after the concrete was placed per Division 9 requirements, the actual floor flatness is typically out of tolerance. A joint position paper, CSI Division 3 Concrete Specifications versus Division 9 Finishes Floor Flatness Tolerances, recognizes this and makes the provision for bidding on necessary surface preparation prior to tile installation.

Substrates can be flattened using a cement patch in small areas or by placing a cement mortar bed meeting ANSI A108 requirements for recessed and pitched areas, but in most cases, more extensive “flattening” is required over large areas. The fastest and easiest surface preparation method is to use a self-leveling mortar or self-leveling underlayment (SLU). A SLU is defined as a “hydraulic-cement” and a “flowable mortar,” as it’s designed to be poured in a consistency between water and a thick orange juice with heavy pulp.

The name “self-leveling” is somewhat of a misnomer. Even though the SLU can lay flat and seeks its own level, pours must be joined together and it needs to be moved into place using a gauged rake, especially in thin applications. While still flowing, the applicator uses a spiked roller or a smoothing tool to break the surface tension to allow the mortar to relax and flow to a flat state. Most projects using SLUs are just for providing a smooth surface, only filling areas that are far out of tolerance. When specifications require a truly level floor, pins are set throughout the floor as gauges to pour to. Special equipment is also used to screed leveling across the floor, similar to placing concrete to create level and very flat floors.

For decades, recognized industry test methods — specifically for these hydraulic, flowable mortars — did not exist. In 2011, ASTM International released the document, ASTM C1708 Standard Test Methods for Self-leveling Mortars Containing Hydraulic Cements. The test methods measure major characteristics such as compressive strengths, flow healing and set time.  ANSI standards for self-leveling products and methods of installation specifically for the tile industry are under review.

To be successful using an SLU, certain steps are required. Actual jobsite conditions will dictate what is appropriate.

First of all, confirm that the substrate is sound, stable and ready to accept bonding materials.  This means a slab must be strong, not weak from laitance. It has to be free of construction contaminants such as ground-in dirt, gypsum dust from sheetrock and paint overspray. If not, shot blasting to a concrete surface profile ICRI CSP #3 would be best. The concrete should be absorptive. Consider ASTM F3191 Standard Practice for Field Determination of Substrate Water Absorption (Porosity) for Substrates to Receive Resilient Flooring for qualifying the slab.

Certain types of natural stone, manufactured agglomerates and installation materials have moisture sensitivities and therefore limitations when exposed to higher moisture vapor emissions. If this is the case, consider using a moisture mitigation membrane meeting ASTM F3010 Standard Practice for Two-Component Resin Based Membrane-Forming Moisture Mitigation Systems for Use Under Resilient Floor Coverings before placing the SLU.

Slab cracks should also be evaluated. If the crack is from shrinkage, typically hairline to ≤1/8-inch-wide (≤3mm-wide) and dormant, they can be filled with a cement patch. Larger cracks, especially those that are differential (out of plane), should be evaluated by an engineer. They may require epoxy injection or other extensive repairs. Non-moving joints in the slab can also be patched, but any joints designed for movement must be left open.

Application of a SLU requires appropriate surface and ambient temperatures. Environmental conditions such as humidity, wind and sunlight — as well as product and water temperatures — are also factors to consider. Product data sheets will set parameters for these requirements to help prevent problems in regards to workability, fast set, poor flow, weak surfaces and cracking.

The Tile Council of North America estimates self-leveling underlayment at 2 3/4-pounds per square foot at 1/4-inch thickness. When thicker-than-expected lifts are required, an engineer will have to determine that the substrate can handle the added weight of a leveler. If the added weight is a concern, consider a lightweight SLU.

Wood-framed structures with plywood or OSB panels will require attaching wire lath or an equivalent plastic lath for most leveling products. Special fiber-reinforced levelers are also available that eliminate the need for lath.

Most projects can be primed with an acrylic latex primer, as new technology products can bond to porous and non-porous surfaces, including existing tile, epoxy coatings and steel. But based on the service usage of the project and the concrete profile, an epoxy primer with a sand broadcast to rejection might be required. Primers are designed to be applied in thin films so more is not better. They also may require dilution to penetrate into a porous slab. Almost every substrate requires a primer; exceptions include some radiant heat and hydronic floor heat mats.

Self-leveling products are engineered cements. Their chemistry is much more complex than mortars used to set tiles. Consider that an SLU is formulated to expand to offset the loss of water and shrinkage that will occur to help prevent cracks, curling and failure. Of course, this expansion has to be controlled as well as the shrinkage. To accomplish this, strict water ratios with only slight adjustment for climatic conditions have to be maintained. Proper mixing tools, mixing times and techniques are mandatory to blend the chemicals with the cements and sands. These are products where you actually have to read the instructions.

The technology for self-leveling, flowable, hydraulic cement underlayments continues to progress. Some are designed to work better for deep pours, others for very thin applications. Compressive and flexural strengths differ and fiber reinforcement saves time, labor and expense. The use of SLUs is here to stay and will grow as buildings become lighter, substrates become thinner and projects require extremely fast turnaround. If you’re not familiar with using levelers, contact your local manufacturer’s representative to get started.