Does Roof Drainage Need to Be Atmospheric?
Obviously, proper drainage of waste from plumbing fixtures in a building is achieved only if atmospheric pressures are maintained within the piping system. Venting and pipe sizing requirements in plumbing codes ensure this condition is maintained so fixture traps do not get siphoned off under negative pressures. These installation requirements also ensure sewer gas does not get blown through fixture trap seals under positive pressures. With roof drainage, however, the pipe interior pressures can be positive or negative since trap seals are not installed in storm water systems (apart from old systems tied to a combined sewer). Pipe systems may also run completely surcharged during extreme rainfall conditions, something not desirable in a sanitary waste system. It is therefore possible to increase the efficiency of roof drainage by designing the piping to operate completely charged with water. In this condition, however, different assumptions about the hydraulics must be made to predict
