Warehouse Roof Drainage, Secondary Overflow and a Dock Area Drain
A 22,000 sq ft warehouse addition has a flat roof with a 24-inch parapet on all sides. Four 4-inch roof drains, one per 5,500 sq ft of roof area, connect to 4-inch leaders that run inside the building and discharge at grade at the base of the wall. There are no overflow drains and no scuppers. The site design rainfall rate is 3.3 inches per hour. At the loading dock, a new 12-inch area drain at the bottom of a depressed ramp is proposed to connect to the sanitary sewer, because the site storm main is 300 feet away and its invert sits 1 foot below the dock drain outlet. The contractor argues that the roof drains are "oversized, so secondary drainage isn't needed," and proposes to daylight one leader 6 inches above the sidewalk with no splash protection.
1. Is secondary roof drainage required here? Explain the basis and how it should be designed.
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Yes. The governing condition is entrapment: a 24-inch parapet surrounds the entire roof, so if the primary drains block, water cannot escape over the edge and will pond until it finds a weakness in the structure. The code requires secondary drainage, in the form of overflow scuppers or overflow drains, on roofs where the perimeter construction entraps water, and the contractor's "oversized primary" argument fails because a single blocked strainer or leader defeats the entire primary system regardless of its capacity.
Design points to require: the secondary system must be independent of the primary, typically overflow scuppers through the parapet or overflow drains with leaders separate from the primary below the roof; the overflow elevation must be set so the ponding depth is within what the structural design allows, which means the structural engineer confirms the depth from the roof surface to the overflow sill; and the secondary system must be sized for the same design rainfall over the area it protects.
Require the design before the roof is closed in, and inspect the installed overflow elevations against the approved detail.
2. Check the roof drain loading. How much flow must each drain handle, and what must be verified against the code tables?
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Start from the design rainfall and the area per drain. A rainfall intensity of 1 inch per hour over 1 square foot is roughly 0.0104 gallons per minute, so 5,500 sq ft at 3.3 inches per hour yields 5,500 x 3.3 x 0.0104, about 189 gallons per minute per drain. Each 4-inch drain and its leader must be capable of that flow.
Verification: look up the roof drain and leader capacity in the adopted code's tables for the pipe size, the rainfall rate and the slope, and account for the leader's developed length, fittings and any horizontal run. A 4-inch vertical leader with a short horizontal offset may or may not carry about 190 gpm depending on the table's basis, so the number must be checked rather than assumed. Also verify the strainer or dome size, that the leader is supported and has expansion provisions where it passes through floors, that roof drains are not connected to the sanitary system, and that the leader discharge at grade goes to an approved point of disposal rather than onto the sidewalk or the foundation.
3. What do you do with the proposed dock area drain connection to the sanitary sewer and the proposed daylight discharge?
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The sanitary connection is not approvable as proposed. Clear-water and storm drainage, including surface drainage from a ramp and loading dock, must go to a storm drainage system where one exists or can reasonably be reached; connecting it to the sanitary sewer adds clean water to a system sized for sewage. The 300-foot distance and the 1-foot invert difference are cost and design problems, not code waivers. If a combined or joint connection is allowed at all in that jurisdiction, it requires the sewer authority's and the code official's written approval, and once the drain is part of the sanitary system it needs a trap, a vent and the same testing as the rest of the sanitary drainage. A loading dock also merits review for sand or oil separation if vehicles park or idle there.
The daylight discharge is also deficient. A leader ending 6 inches above a sidewalk will erode, pond, ice over in winter and possibly discharge onto neighboring property. Require a splash block or concrete apron, a discharge point that does not create a nuisance or a hazard, and routing to a storm structure, swale or approved point of disposal. Record the approved discharge location so a later inspection can confirm it.