The roof of a structure shall be designed for the storage of water where the storm drainage system is engineered for controlled flow. The controlled flow roof drain system shall be an engineered system in accordance with this section and the design, submittal, approval, inspection and testing requirements of Section 18-29-1105.4. In lieu of the conventional downspout requirements of Section 18-29-1106, controlled flow roof drainage may be used. The water of a 25 year frequency shall not be stored on the roof for more than 24 hours. The water depth on flat roofs shall not exceed 3 inches (75 mm) during the above storm and 3 inches (75 mm) average depth on sloped roofs not exceeding 6 inches (32 mm) rise or depth of slope.