Self-leveling underlayment and patching products are materials used to level out and repair uneven or damaged floors prior to installing new flooring. Self-leveling underlayment is a cement-based product that is poured onto the existing floor and is self-leveling, meaning it will flow and settle into any low spots or dips in the floor. It can be used to create a smooth and level surface for the installation of tile, hardwood, or other types of flooring.