Water Damage Not Caused by Drain Overflow

Excessive rain from thunderstorms caused interior water damage to a building through the roof. The building has a flat roof with a drain. Carrier denied the claim, citing exclusion for water that backs up from a sewer or drain. Whether the drain is not clogged, partially clogged, or completely clogged, I do not believe the exclusion applies. The water entered the building because it never reached the drain and traveled the wrong way on the roof. The water came through a seam or other opening. Is this caused by overflow or back-up? Is the loss covered or excluded?

New York Subscriber