Abstract: This disclosure relates to a closure device for a roof drain comprising a generally inverted cup-shaped cap having an apex portion and a peripheral skirt terminating in a peripheral edge carrying peripheral sealing means for engaging an area outboard of a roof drain, and a threaded rod passing through the apex portion for rotationally connecting the closure device to a roof drain closure whereby upon rotation of the threaded rod the peripheral sealing means will be brought into contact with a peripheral area outboard of a roof drain to form a seal for retaining a predetermined quantity of water upon a roof for fire prevention purposes.