Flat Roof Dirt Staining
Here's a good picture of a roof that doesn't have enough
slope in it to drain the water off without ponding. In order for the
water ponding to stain the roof, there has to be a section of the roof
Flat roof dirt staining is common and should be repaired to prevent further damage to the structural framing and to the roofing materials. The larger this area becomes, the more weight it will put on the structural roof framing.
If this was ever to become too big and hold too much water, the entire roof could collapse. If you have a situation like this on your roof, there are a few things that you can do about it.
Home Repair Tip: The first thing that I would do, if possible would be to inspect the attic area, to see if there is actually any structural roof framing damage. Look for any cracked or broken roof rafters, beams and structural supports.
These areas can usually be raised if their sagging. You could add additional roof rafters, scab on extra lumber or put a support beam underneath the sagging area, to raise it up enough so that the water won't accumulate in this area anymore.
If you can't access the attic, you can always contact a roofing contractor for some more advice. Some roofing contractors recommend that you add more roofing materials to these areas to force the water to go around them and off the roof safely.
By adding more roofing materials, you're adding more weight to the roof framing, so be careful when choosing this route.