Preparation is Key to Successful Self-leveling Underlayment Use
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.
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.
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.