An easy way to determine how much leveling is needed is to place a carpenter’s level on a long straight edge and then measure the space (depth) between the straight edge and the substrate. This will pinpoint the deep spots and help you to estimate the need for leveling in the lowest areas of the room.
A self-leveling underlayment such as MAPEI Self-Leveler Plus is a pourable product that helps correct a substrate that is out of level. Self-leveling underlayments require very structurally sound, solid and stable substrates. Because self-leveling underlayments have short working times, it is a good idea to have at least three people working together when pouring a self-leveler – one to mix the product, one to carry the buckets and pour out the self-leveler, and one to spread the self-leveler, and guide it into the desired areas as it is poured.