The PVC standpipe type water dam of this 15-1/4" (390) diam. Duco coated cast iron drain can be trimmed to retain up to 5" (125) of standing water on flat roofs where rooftop water is used to insulate the building's interior. This drain is also installed as a safety overflow drain in conventional roofs. To protect the drainage system, a polyethylene dome strainer restricts debris from entering the drainage system.