The term SLU refers to Self- Leveling Underlayment. This product is a great and quick way to flatten most horizontal surfaces prior to installing large format tile (LFT). As you probably already know, large format tile installations (a tile with any one side greater than 15" in length) require the floor to have no more than 1/8" in 10' deviation. This means that the floor will not have any low spots or “bird bath” areas greater than 1/8" deep when measured from the high points on the floor with a 10' straight edge.
Most of us know that the floors over which we are asked to set tile are nowhere near 1/8". Being given a floor within 1/4" would be a gift, but most times the floor surface (substrate) is a mess. If these areas are small, a fast-setting patch works really well. But when the area to be flattened is large or looks like the ocean on a rough day, self-leveling underlayments can save you time and money.