A self-evacuating vertical downspout adapter includes an elongated adapter body defining open top and bottom ends, and an enlarged-volume debris trap with a debris outlet between the top and bottom ends. A drainage grate is located inside the adapter body and defines a drain-through floor of the debris trap angled downwardly towards the debris outlet. A hinged outlet door is attached to the adapter body adjacent the debris outlet, and is urged in a normally closed position to prevent substantial escape of rainwater through the debris outlet. The outlet door is adapted for being pivoted on-demand to a temporarily open position by debris captured on the drainage grate and gravity-fed towards the debris outlet for automatic evacuation.