Containment tank assembly

A containment tank assembly for storing fluid material comprising, a primary storage tank having a bottom wall, a side wall integral with the bottom wall and a closure integral with the top edge of the side wall, a secondary containment tank having a bottom wall and a side wall integral with the bottom wall, said secondary containment tank being open at the top, said primary storage tank being positioned within said secondary containment tank with the bottom wall of the primary storage tank being seated on the bottom wall of the secondary containment tank, the side wall width of said primary storage tank being less than the side wall width of said secondary containment tank to thereby provide a space between the adjacent side walls of the respective tanks, said space receiving any leakage of fluid from the primary storage tank, and a roof over said space to prevent any leakage therein from becoming contaminated by rain water, said roof comprising, a first lip integral with a perimeter of the closure in proximity to the top edge of the secondary containment tank, said first lip extending inwardly to the sidewall of said primary storage tank over said space, and a second lip integral with the top edge of the secondary containment tank, said second lip being positioned under said first lip and extending inwardly over said space.