Warehouse Roof Drain and Secondary Drainage
A 10,000-square-foot warehouse has a flat roof with a 24-inch parapet. The local 100-year, 1-hour rainfall is 3.2 inches. The roof has one 4-inch roof drain connected to a 4-inch leader; there is no secondary drainage. The roof slope is 1/8 inch per foot toward the drain, and the drain is at the low point. The owner wants to add storage mezzanines and asks whether the existing roof drainage is adequate. The building official has asked for calculations before approving the permit.
1. Calculate the primary roof drainage flow for the 10,000-square-foot roof at the design rainfall.
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Use the roof area times the design rainfall: 10,000 square feet x 3.2 inches per hour = 32,000 square-foot-inches per hour. Convert to gallons per minute using the standard factor of 0.0104 gpm per square foot per inch per hour:
32,000 x 0.0104 = 333 gpm.
The primary drainage system must be sized to handle approximately 333 gpm at the design rainfall. This is the flow the roof drain and leader must carry, and it is also the flow the secondary system must be able to handle independently.
2. Is the single 4-inch roof drain adequate, and is secondary drainage required?
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A single 4-inch roof drain is not adequate for 333 gpm. A 4-inch roof drain at the code's reference slope is typically rated in the range of about 180 to 200 gpm, well below the required flow. The drain and leader are undersized.
Secondary drainage is also required. With a 24-inch parapet, the roof is enclosed well above the roof surface, and the code requires secondary drainage so that water can escape if the primary drain clogs. The secondary system must be independent of the primary and sized to handle the same design flow. Adding mezzanines increases the importance of reliable drainage because ponding can affect structural loading.
3. What secondary drainage arrangements are acceptable, and what must be verified at inspection?
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Acceptable arrangements include a second roof drain or scuppers through the parapet, provided the secondary system is independent and sized for the design flow. The secondary inlet should be set above the primary inlet so it operates only when the primary is blocked, and the secondary discharge must be routed to a safe location. If scuppers are used, they must be large enough to pass the secondary flow without allowing water to rise above the intended level.
At inspection, verify the drain and leader sizes against the calculated flow, the secondary inlet elevation, the independence of the secondary path, and the structural support for any new mezzanine loads. The building official should receive the rainfall rate, roof area, conversion, and manufacturer's drain capacity data before approval.