FLat roofs are not very flat at all. I did a cabin not to long ago that had a very low pitched roof (3 12) up near tahoe. The roof was steal and had both passive and active heating systems. To keep the snow off. it was pretty cool, but the system would be $$$$$$ to replace if it fails. one of those clients that wanted something, and do to codes it ment doing something "out side of the box". Which is always fun, but can be a pita if the client is a bitch.