Self-Leveling Compound for Flooring

Self-leveling underlayment can be useful for correcting certain substrate conditions, but it is not automatically the right solution for every uneven floor. It excels at broad or multiple low areas on a sound, stable substrate. It does nothing useful about movement, and it is an expensive way to deal with a single localized dip.

Self-leveling underlayment is a cement-based material mixed with water to a flowable consistency and poured onto a prepared substrate, where it spreads and settles into a common surface plane before curing. Its purpose is to establish a suitable surface for the flooring that goes over it.

Two things follow from how it works. First, the poured depth varies across the floor — deep in the low areas, thin over the high ones — which is why estimating uses an average depth. Second, it fills to a plane; it is not a structural layer, and it does not make a weak or moving substrate sound.

Most self-leveler problems are decided before the first bag is opened. Work through these while the floor is still dry and empty.

• Is the substrate structurally sound, and is it staying put? A pour will not hold a moving floor together.
• Does the substrate flex or deflect underfoot? Correct that first — the cause, not the symptom.
• Is the product approved for this substrate — this type of wood panel, this slab, this existing surface, this residue?
• What does the product require for primer, and is the primer compatible with the substrate?
• What are the product's minimum and maximum thicknesses, and does your floor fall inside them everywhere?
• Where can material escape — perimeter gaps, doorways, penetrations, stair openings, duct boots, plumbing?
• What does the added thickness do to finished height at doors, appliances, cabinets, stairs, transitions and adjoining rooms?
• What are the moisture conditions and requirements for the substrate and for the products going over it?
• Will the flooring you chose be installed over this underlayment in a way its manufacturer permits?

Preparation for a pour is more involved than for patching, because the material will go wherever it can.

Once mixing starts, the clock is set by the product, and it is usually shorter than a first-timer expects. Everything should be ready before the first bag is opened.

• Area fully prepared, primed as required, contained, and clear of anything you do not want cemented down.
• Mixing equipment, buckets, water and gauge rake or smoother at hand, and a helper if the area warrants one.
• Water measured exactly as the product instructions state — this is not a by-eye step.
• Bags staged near the mixing station in the order they will be used.
• The pour sequence planned: where you start, which direction you work, and where you finish so you are not standing in it.
• Site conditions checked — temperature, ventilation and traffic all affect how a pour behaves and cures.

The quantity depends on your area, the average depth you are filling to, and the coverage your specific product states. Bag yield is not standard across products, and stated coverage is usually quoted at one particular depth.

Average depth is genuinely an estimate on an uneven floor. Deciding where the finished surface plane sits, then estimating the mean fill below it, is closer to how the material is actually consumed than measuring the deepest point.

After the pour, two things remain: satisfying the product's curing and drying requirements, and confirming the surface you now have is the surface you wanted.

• Observe the product's cure and drying requirements before walking on it, prepping further or installing over it.
• Recheck the surface with a straightedge in several directions once it has cured.
• Look at the edges of the pour, at doorways and where the pour meets an uncovered area — those are where ridges and abrupt changes show up.
• Address any remaining high points or transitions as the product permits.
• Confirm the flooring manufacturer permits installation over a cementitious underlayment, and meet any moisture or condition requirements for the new surface.
• Clean again before the flooring goes down.

A pour is not the default. For a single shallow dip, floor patch is usually faster, cheaper and less disruptive. Where the main problem is a hump, removing the high spot may reduce or eliminate the need to fill everything around it. Where the floor moves, the fix is structural. The inspection order that leads to this decision is in how to prepare a subfloor, and the measurement behind it in flatness vs. level.