[Zeke]

Flat roofs, well low pitched roofs, never stay flat. After awhile the joists deflect and make for a shallow pond. Some of these big shallow ponds have caved in, like stores and warehouses. My garage roof has this problem, but it's only 18 x 18. Once the water stands for any length of time, it works its way down thru the laps of the sheets.

One thing you can do is use foam to reshape the slope before a new roof system is installed. I've seen it done with a spray on foam, but I bet a hot wire cutter could slope 4 foot sheets.