Why do you need two layers of leveling layer for planting roof in basement?

Ensure the flatness and stability of the basement roof. The first leveling layer can fill the uneven surface and increase the thickness of the top of the substrate; The second leveling layer can further modify the surface to ensure the flatness and firmness of the planting roof to support the vegetation above. Basement refers to one or more floors below the ground of a building, which is usually used for storage, equipment room, gas room, garage, office, game room, gym and other purposes.