The roof drain overflow cover with sensor cooperatively engages and covers a conventional roof drain cap cover to prevent trash from reaching and clogging the conventional roof basket cover and drain. Accumulating trash builds up on the screen of the overflow prevention device causing a doughnut shaped buoy connected to a trash tray to rise. The trash tray is retained to a base with magnets. When the water reaches a predetermined level, the doughnut shaped buoy rises pulling the trash tray away from the base allowing water to flow to the conventional roof drain basket and activating an overflow sensor and a control module. The control module is in communication with the water flow sensor and includes an indicator which is activated when the water flow sensor detects the detachment of the trash tray from the base indicating a potential flooding situation with the roof drainage system.