We are bought a Hot Springs Spa (Vanguard model) hot tub, and are planning on having our landscape contractor sink it 18 inches.

One manager at a Hot Springs store says that the floor of the sunk vault for the tub should be a flat surface. Another Hot Springs spa manager says that the vault should be slightly graded and the spa can be shimed to level.

I want to be sure that the drainage is good and that my spa doesn't get flooded/ruined. Which advice should we take?