A recessed area in a roof deck or plaza slab that drops the surface elevation immediately surrounding a drain. This depression forces water to accelerate into the drain bowl, overcoming the slight lip of the drain ring and preventing standing water on the adjacent flat membrane. Sumps are typically formed using tapered insulation boards or by depressing the structural deck itself.
Make sure you cut the tapered insulation correctly so we've a proper sump at every drain location.
A drain without a sump will almost always leave a ring of standing water because the clamping ring sits slightly higher than the roof membrane.
Related terms
Ponding
The accumulation of standing water on a flat roof that fails to drain or evaporate within 48 hours.
Positive drainage
A design and construction condition ensuring that all water flows away from surfaces and drains completely within 48 hours.
Plaza deck
A waterproofed structural slab over occupied space, topped with pavers, soil, planters or a wearing slab.