Scuppers As Primary Roof Drainage

Firstly there is an explanation in the above referenced section for where and why a secondary roof drainage system must be provided.

Scuppers as primary roof drainage.

Section 1503 4 2 scuppers when scuppers are used for secondary emergency overflow roof drainage the quantity size location and inlet elevation of the scuppers shall be sized to prevent the depth of ponding water from exceeding that for which the roof was designed as determined by section 1611 1. An overflow drainage system will be designed to function independently of the primary system to drain water from the roof if the primary drains or scuppers become blocked and to prevent overloading of the roof. The factors which affect proper roof drainage are deck slope drain flashing size and placement of drains. Scuppers are used to provide an outlet through parapet walls or gravel stops on flat and built up roofs to allow drainage of excess water.

See section 1107 for yourself. When scuppers are used for secondary emergency overflow roof drainage the quantity size location and inlet elevation of the scuppers shall be sized to prevent the depth of ponding water from exceeding that for which the roof was designed as determined by section 1611 1. A primary drainage system is designed to remove water from the roof as quickly as possible. Scuppers shall not have an opening dimension of less than 4 inches.

The full flow capacity of a drain is not realized until the water reaches approximately 2 5 inches above the inlet in a four inch drain and 3 5 inches in a six inch drain. Scuppers can be installed to carry water into gutters or directly into downspouts through conductor heads.