Abstract: The present invention relates to a roof drain structure comprised of a drain plate having an upstanding overflow port and a clamp ring used to secure an edge of the roofing substrate against the drain plate and around the overflow port. The clamp ring surrounds and physically cooperates with the overflow port to align several screw apertures and the clamp ring with several corresponding apertures in the drain plate. There are preferably at least two alternative drain plates having different size overflow ports, and a clamp ring that may be modified to fit either of the ports and still align the screw apertures. The preferred clamp ring has removable spokes that are left in place for use with a smaller overflow port and that are removed for use with a larger overflow port. The overflow port is preferably stepped to have a lower larger portion and an upper smaller portion so that the contractor can modify the overflow port in the field to accommodate different sized drain pipes.