Positive drainage is the intentional sloping of a roof deck, plaza, or waterproofing assembly to direct water toward drains, scuppers, or gutters. A system achieves positive drainage if it accounts for structural deflections and prevents any standing water from remaining 48 hours after a rain event. Tapered insulation or sloped concrete are common methods to achieve this slope.
Even with the deck sag between the joists, the tapered insulation gives us enough pitch for positive drainage.
Standard flat roofs require a minimum slope of one-quarter inch per foot to guarantee positive drainage.
Related terms
Ponding
The accumulation of standing water on a flat roof that fails to drain or evaporate within 48 hours.
Plaza deck
A waterproofed structural slab over occupied space, topped with pavers, soil, planters or a wearing slab.
Sump
A built-in depression around a roof drain that accelerates water flow and prevents ponding.