Fast-track construction, poorly specified projects, careless workmanship, and lack of appropriate inspections can result in floors that are not flat and level and which must be corrected. One way to repair such conditions is to use a self-leveling underlayment (SLU).

To provide space for a movement joint, apply self-adhering foam tape around the perimeter of the floor.

A combination of tar paper and caulk keeps SLU off of the closet flange.

If you have never used an SLU before, I recommend you purchase a sack or two of the material you will use, construct a simple frame of 2x4s, line it with plastic film, then mix and pour a batch to see how it behaves and learn what you need to do to ensure that when you are on-site with a real installation, there will be no surprises. A practice run is advisable for anyone who has not used an SLU before, and it is also advisable for the seasoned pro who is using an unfamiliar material. The cost of a sack or two of material is peanuts compared to the cost of ripping out an installation that didn’t go well.

The first step to achieving good results is to choose a material that meets the strength requirements of the installation. Compressive strengths vary from light residential to heavy industrial, and not all brands of SLU are suitable for use with ceramic or stone tiles: some are only recommended for non-tile floor coverings. Only self-leveling compounds made specifically for use with ceramic or stone tiles should be used, and manufacturer’s instructions must be followed.

Preparing for application of most SLUs is quite similar although each brand will have its own specifics. The first step is to ensure the existing floor surface is clean and free of grease, oil, and dust.

The next step is to install ¼-inch thick resilient foam tape to the perimeter of the floor. This creates space for a movement joint – a requirement for every brand of SLU. Since self-leveling materials are made to flow easily, the third step is to fill all floor seams, fastener holes, and other openings with caulk or sealant to prevent draining.

Over wooden subfloors, some brands of SLU require or recommend the use of reinforcing mesh. Reinforcing mesh is usually not required when installing an SLU over concrete. Another step that may or may not be required is to apply a primer to the flooring surface to ensure the SLU bonds to the substrate. Primer can be installed with a brush, roller, or spray. Primer must not puddle and there may be a waiting period before the SLU can be poured.

A 20-gallon party bucket makes a convenient mixing container that can hold three sacks of SLU mix.

Once surface preparation is complete – because SLU materials have such a short pot life and open time - a critical step is to gather all the tools required for mixing and application and stage them where they will be needed: in the few moments it takes to run out to the truck to retrieve a tool, a batch of freshly mixed SLU may already be setting up. When only a very small amount of SLU is required, it may be possible to mix one sack in a 5-gallon bucket. For larger batches, I use a 20-gallon party bucket for mixing, and stage it in the room where the SLU is needed. For large floors, a 30-, 40-, or 55-gallon drum, plus two mixers, may be used. For multi-room applications, it’s wise to contract with a local installer with pumping equipment and specify an SLU made for tile - one that meets the strength requirements of the installation. Once the surface is prepped and the materials and tools staged, the bulk of the work needed to complete the installation of the SLU is finished. Before committing to mixing, however, it’s a good idea to check once more to ensure that all tools are ready to go.

The first step in mixing is to pour a measured amount of water into the mixing container. The amount of water will be determined by the individual brand and by the number of sacks that will be mixed. All the water needed for a batch should be in the mixing container before any powder is added. SLUs should only be mixed with power equipment and a paddle made specifically for this type of material otherwise, excess air may be entrained and weaken the mix. Some brands may require a minimum mixing time, and all brands should be mixed until there are no lumps. To minimize lumps, have a helper slowly pour the powder into the container as you run the mixer. There is no slaking period with SLUs – as soon as mixing is finished, the material should be poured without delay. If possible, don’t just pour the mix in one spot, but rather distribute it around the floor. Spike shoes are available so an installer can walk on the floor without stepping directly into the mix, but I prefer to place 12x12x2-inch squares of wood as stepping stones on larger floors.

To ensure that an SLU mix levels out, it must be distributed evenly across the floor with a trowel.

As soon as the mix is poured, scrape the remains from the container and use a trowel, wood float or a rake to distribute the SLU more evenly. This is the key to good results: SLU materials WILL NOT level themselves without help from the installer. It is important that the entire floor be wet with mix so the fluid SLU will level itself. You do not need to be too exact, but after rough troweling, the SLU mix should be fairly flat. Distributing the mix must be done quickly. It’s a good idea to get at least one assistant to help distribute the mix. If you notice any stray lumps, mash them down with the trowel and then leave the SLU to seek its own level.

Some brands will cure and harden quickly – some in as little as two hours – but make sure you follow the brand’s curing instructions before installing membrane or tile. Don’t be too concerned if there are minor bumps in the surface. These can be ground down with hand or power tools.

Finally, to maximize adhesion between the hardened SLU surface and tiles or membrane, keep all foot traffic off the surface.