The insured has a Special Causes of Loss form, CP 10 30 04 02, with a Manufacturers Property Extension Plus endorsement that includes $25,000 in coverage for backup of sewers or drains. 

During an extended period of heavy rain, about eight inches of water accumulated on the insured's flat roof. The roof membrane did not leak, but water spilled over the curb where the air conditioning unit sits, which in turn caused interior damage to the insured's offices. 

The building has two four-inch drain pipes for the purpose of removing water from the roof. The pipes drain to a nearby sewer/drainage ditch. When the insured was notified of the problem, he went up on the roof and there was minimal debris around the drain, but even when removed, the flow did not increase. Each pipe does have a cleanout at the bottom of the building (the pipes run down the sides of the building), and once the insured removed the caps on the cleanouts, essentially doubling the drainage capacity of the pipes, the water began to drain from the roof.