A primer is applied to the plywood subfloor prior to the installation of the SLC. Primers may also be required for featheredge compounds.

Galvanized expanded metal mesh nailed to the plywood.

Tiles need a flat surface to provide support and eliminate lippage. The current industry standard requires floors, walls, and countertops to be flat to within 1/4-inch in 10 feet. Suitable for tiles less than eight or ten inches, but for larger tiles, this minimum standard may not produce acceptable results. Small tiles are able to conform to minor variations in the setting bed surface, but with larger tiles, two problems occur when installing over less-than-perfect setting beds. Over shallows, it is possible for substantial areas of the backs of large tiles to be totally out of contact with the adhesive. Such tiles tend to sound hollow, are incapable of supporting anticipated weight loads, and are prone to cracking. Bumps and ridges, on the other hand, can also have a pronounced affect on large tiles when they lift tiles up and out of the adhesive bed, and when they cause excessive lippage. The first task for an installer is to survey the floor with a long straightedge to make sure the conditions of the floor match the needs of the tile.

Electrical resistance mat is unrolled over the galvanized mesh.

As consumers trend towards larger and larger tiles, and as contractors install fewer tiles using wet-set mortar bed installation methods, the need to prepare setting beds that are flatter and smoother than allowed by industry tolerances has become vital to successful large-size ceramic or stone tile installations. On many remodeling and all new construction projects, a good specification and reasonable oversight should assure that the structure underlying the tile areas will meet the tile industry standards when covered by backer boards or other dimensionally uniform setting bed materials. But when a sub-floor surface is out of tolerance and the framing cannot be corrected, featheredge and self-leveling compounds, made specifically fore use with ceramic tile, can be used to correct the surface.

Generally, featheredge compounds are used to fill in low spots occupying a small percentage of the overall floor area. They may also be used to fill in large depressions. Featheredge compounds are site-mixed, may require primers or reinforcement, and are installed using trowels for spreading and a straightedge for screeding off the excess. The patching compounds you may use for preparing areas to receive sheet goods or wood strip flooring may not be adequate for tile. For best results, single-source the featheredge compound, tile adhesive, and tile grout from a single manufacturer.

The first of two batches of SLC is poured over the floor.

Self-leveling compounds (SLCs) have been used for many years under a variety of floor finishes. Some SLCs made specifically for use with ceramic or stone tiles can dramatically speed an installation by eliminating the tedious process of fastening tile backer boards in a bed of thinset mortar. Some gypsum based materials that are used beneath carpeting and other finishes may not be suitable for use with ceramic or stone tiles. As with featheredge compounds, it is a good idea to single-source the SLC, the tile adhesive, and the tile grout from the same manufacturer. SLCs are highly reactive materials that require precise measuring of wet and dry ingredients (not guessing with a shovel), and careful mixing practices. To further ensure quality and eliminate problems, make certain that lot numbers on all sacks of SLC you intend to use are identical. This is always good practice when ordering thinset mortar, grout, and other installation materials.

A trowel is used to roughly distribute the SLC mix.

If only a small area of the floor needs correction, I use an economical featheredge compound, but when the whole floor needs to be flat, smooth, and level, I recommend an SLC, and if waterproofing is a project requirement, I prefer to install the SLC prior to application of the waterproofing membrane. SLCs are not generic! Each has its own system materials and requirements for mixing and application so make certain you follow all directions and use accurate measuring containers.

The photos used for this article came from a bathroom floor covered with 1 1/8-inch exterior plywood over 16-inch centers. Also included in the installation is an electrical resistance tile warming system. The main reason for using an SLC for this project was to facilitate the installation of rather small stone tiles over the radiant heat mat. I followed the manufacturer's instructions regarding floor prep, mixing, installation, and curing. For this 40 square foot floor, preparation of the floor, and mixing and applying the SLC took about two hours-ten minutes of which was spent mixing, pouring, and distributing the mix. The installation began with a survey of the floor to check for fastener heads that might interfere with the heating mat, vacuuming away the dust and debris, and wiping the surface with a clean, barely damp sponge. Next, I coated the plywood subfloor with a companion latex primer (Photo 1), allowed the primer to dry to the touch, and then installed sheets of galvanized expanded metal mesh to help reinforce the SLC (Photo 2). The mesh covers the entire floor, and is held in place with hot-dipped galvanized roofing nails.

After an overnight rest to cure and harden, the SLC is ready to tile.

Once the galvanized mesh is flat against the floor, I unrolled the tile warming mat (Photo 3), secured it with staples, and made the appropriate electrical connections. Working with an SLC is not too difficult as long as you plan ahead because you only have about ten minutes between the time the SLC is mixed and when is begins to set up. Mixing should be done as close to the jobsite as possible to avoid wasting time. I want to have all tools and materials ready, with idiot buckets on hand to ensure that proportioning of the wet and dry ingredients is identical from one batch to the next. Mixing is done with a low-speed mixing paddle made specifically for use with SLCs.

In spite of the name, all SLCs require some human assistance to ensure that the bulk of the material is evenly and quickly distributed. Photo 4 illustrates the consistency of the SLC used on this project, while Photo 5 shows a trowel being used to distribute the wet mix. According to my calculations, this project needed two full sacks of SLC to cover the mesh and heat mat. As soon as I had dumped the first full batch and distributed it over one-half the floor surface, I dumped and distributed the second batch and allowed it to dry and harden overnight. The following day, the SLC covered floor was ready to tile (Photo 6).