A roof drain that overflows is usually blocked or overwhelmed, which is a drain and roof-maintenance problem. A drain that leaks inside the building may be the drain body, its flashing or the interior pipe it connects to. Roofers and plumbers each own part of it, and your lease decides who pays.
What causes an overflow?
EMC Insurance's guidance lists clogging hazards, such as debris, leaves and branches, as a common problem, especially in a heavy storm. It also warns that one inch of standing water adds about five pounds per square foot to a roof. Standing water can degrade roofing materials and work its way under flashing and other sealants.
Phoenix amends the Uniform Plumbing Code so that primary roof drains, scuppers and conductors are sized on a rainfall rate of 3 inches per hour for a 60-minute, 100-year storm unless the authority having jurisdiction requires otherwise (Ordinance G-7397, effective August 1, 2025). That rate applies to new design. An older building may have drains that were sized for less, which is why a monsoon cloudburst can overwhelm them. What code requires goes further.
An IIBEC technical article argues that all roofs should have secondary drainage to prevent water accumulating on the roof if the primary drains are blocked. That is the author's position, but it explains why overflow drains exist.
Where is the leak coming from?
- Water only during rain, near the drain: a blocked or overloaded drain, or flashing around the bowl. Roofer first, then plumber if the interior pipe is involved.
- Water inside after rain, away from the drain: interior storm piping or a joint above the ceiling. Plumber.
- Water on the roof that won't drain: a blocked drain line or low spot. Clearing the line is plumbing; the low spot is roofing.
EMC cites the Insurance Institute for Business and Home Safety in recommending that roof drainage systems be inspected and cleaned regularly, ideally every spring and fall and after severe weather. Why drains and vents need cleaning is a good routine.
Whose problem and whose cost?
The roof is usually the building owner's, but a lease may shift maintenance to a tenant, and a tenant's own equipment can drain onto the roof. Read the lease before choosing who calls whom. Tenant versus landlord lays out how to read it. For the first hour, stopping the damage comes before the argument.