A low cost method and device for diverting water away from a flat roof's low spots, thus preventing it from puddling there during a rainstorm. Such puddles, if allowed to stand, can damage a roof deck structurally. The device includes a flexible membrane, an air valve mounted thereon, and mechanical fasteners for attaching the membrane along its periphery, to both the roof deck and its existing covering. Ideally, the membrane can be heat welded, taped or glued to the roof covering and joined thereto to form an airtight seal, thus simplifying construction of inflatable barriers for the roof's low spots. For each such barrier, a membrane, sized and shaped to cover at least one low spot, substantially overlaps the latter's edges and defines, in combination with the roof covering, an air pocket which, when inflated via the air valve, causes the membrane to protrude upwardly, effectively elevating the covered spot.