Roof Drain Connection and Secondary Drainage for a Commercial Addition
A 3,000-square-foot commercial building has a flat roof and an existing 6-inch combined building drain that carries sanitary sewage to the city combined sewer. The owner wants to add two 4-inch roof drains, each serving 1,500 square feet, and connect them through a 4-inch leader that runs 90 feet at 1/8 inch per foot to the existing building drain. The local one-hour rainfall intensity for the 100-year storm is 3.5 inches per hour. A 4-inch overflow scupper on the opposite side currently discharges onto a public sidewalk and is the only secondary drainage. The building official has flagged the connection and the secondary drainage. The contractor argues that the combined sewer accepts storm water and that the scupper is adequate because it is 4 inches wide.
1. Can the roof drains legally connect to the existing sanitary building drain? Explain.
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Roof drainage should not be connected to the sanitary building drain. Storm water and sanitary sewage are separate systems; connecting roof leaders to a sanitary drain adds clear water to the sewer and is prohibited unless the authority specifically approves a combined system. Even where a combined public sewer exists, the building's sanitary drain is not automatically a legal storm receptor. The roof drains need a separate storm line to the city storm main or an approved detention or infiltration system. The 4-inch leader must connect to the storm line with a wye or other approved fitting, not a sanitary tee, and the storm line must be sized for the roof area.
2. Calculate the design flow for each roof drain and for the total roof, then assess the 4-inch leader.
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Design flow is calculated as area × rainfall intensity × 0.0104 when area is in square feet and intensity is in inches per hour. For each 1,500-square-foot drain: 1,500 × 3.5 × 0.0104 = 54.6 gallons per minute. Two drains give about 109 gpm. A 4-inch leader at 1/8 inch per foot has a table capacity well above that in many plumbing codes, so the pipe size may be adequate for flow, but the table must be checked for the actual slope and the local code's rainfall table. The 90-foot run falls 90 × 0.125 = 11.25 inches, so the building drain invert must be at least that far below the roof drain outlet plus the leader connection depth. If the invert is higher, the leader cannot be run at 1/8 inch per foot and the design must change.
3. Is the existing 4-inch scupper acceptable as secondary drainage? What must be corrected?
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A single 4-inch scupper is not adequate secondary drainage just because it is 4 inches wide. Secondary drainage must be sized to handle the same design flow as the primary system, or the percentage the local code requires, and it must discharge to an approved location. Discharging onto a public sidewalk creates a hazard and may violate the code. The scupper's capacity depends on its width, depth, and head above the roof, not width alone. Provide secondary roof drains or scuppers sized for at least the primary design flow, set at an elevation above the primary drains so they operate only when the primary system is blocked, and route the discharge to a safe, approved location. The building official is correct to flag both the connection and the secondary drainage.