a drum for a roll - up closure , the closure having a door of hinged together slats , one end of the door adapted to be mounted on the drum , the door adapted to be wound on the drum as the drum is rotated . the drum has a wall defining its shape with a depression in the wall extending both inwardly and across the width of the drum . the cross - section of the wall defines an increasing spiral from one side of the depression to the other side of the depression . an extension of the wall extends from the one side of the depression towards the other side to partially close the depression . the partially closed depression is shaped to receive the one end of the door to mount the door on the drum .