Warehouse Roof Ponding and Undersized Secondary Drainage
A 1,200 m² warehouse roof drains through four 150 mm roof drains and one 100 mm overflow scupper. The roof slope is 0.5% toward the drains. Design rainfall intensity is 100 mm/h. The primary leaders run horizontally 30 m at 0.5% before turning down. After a 30-minute storm, ponding 75 mm deep remains near the scupper, and one interior column shows water staining. The owner has added a new mezzanine, reducing roof drain access. The contractor proposes adding two more 100 mm drains connected to the same leader. The building has no separate secondary drainage system other than the single scupper, and the scupper invert is only 25 mm above the roof surface at the low point.
1. Calculate the approximate primary roof flow and use it to assess whether the four 150 mm drains are obviously undersized.
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The primary flow is roof area times rainfall intensity. With 1,200 m² and 100 mm/h, the volume rate is 1,200 × 0.1 = 120 m³/h, which is 33.3 L/s. Spread over four drains, that is about 8.3 L/s per drain. A 150 mm leader at 0.5% slope has a finite capacity that depends on Manning's roughness and full-flow conditions, but it is not obviously below 8.3 L/s when new and clean. The ponding and staining therefore do not automatically prove the primary drains are too small. They suggest a secondary drainage or blockage problem instead. The 30 m horizontal leader and the 0.5% slope should be checked for actual capacity, debris, and settlement. The calculation shows the primary system is close enough to its limit that any blockage or added flow could cause ponding.
2. Evaluate the contractor's proposal to add two more 100 mm drains connected to the same leader.
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Adding two 100 mm drains to the same leader increases the collected flow but does not increase the leader's capacity in proportion. It may simply overload the shared leader and cause backups at the low point. More importantly, it does not provide the independent secondary drainage that the roof needs. The existing single 100 mm scupper with its invert only 25 mm above the roof is too small and too low to act as reliable emergency overflow for a 1,200 m² roof. The proposal also worsens access problems created by the new mezzanine. A proper design would keep the primary and secondary drainage separate, size the secondary system for the emergency storm, and ensure the scupper can pass the required flow without ponding beyond the structural limit.
3. Recommend the corrective work and explain how ponding depth relates to structural loading.
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Clear and inspect the four primary drains and leaders, verify the 0.5% slope, and repair any settled or blocked sections. Provide a separate secondary roof drainage system, such as overflow scuppers or overflow drains, sized for the emergency rainfall intensity required by the code. The secondary system should discharge independently and its inlet should be set at an elevation that limits ponding to the structural design allowance. Check the 75 mm ponding load: water weighs about 1 kg per mm per square metre, so 75 mm represents roughly 75 kg/m², which may be significant on a long-span roof. The interior column staining should be investigated as a possible leakage or structural movement symptom. Do not connect new drains to the existing leader as a substitute for secondary drainage. After corrections, water-test the roof or observe a controlled storm to confirm the scupper and overflows work.