A self-actuating drainage device for placement on a structure to drain water accumulated on the structure includes a collection funnel having a bottom with a funnel hole extending therethrough, a siphon conduit contiguous with the funnel hole, the siphon conduit having a first end disposed on the structure and an opposite end disposed at a position lower than the first end. The device further includes a valve mechanism which includes a float maintained within the collection funnel. The valve mechanism opens when accumulated water raises the float so that the water collected in the collection funnel is released through the siphon conduit and starts a siphoning action from the first end to the opposite end when the valve mechanism closes.