Fascia Damage From Metal Flashing
The fascia board, isn't only
suffering from deteriorating metal flashing, this roof should have been
designed a little better. Most of the water damage, is from large
amounts of water, draining off of the roof, on a regular basis.
This section of the building could actually be out of level and this would force most of the water on the roof to drain to this corner.
If you look closely in the upper left-hand corner, you will notice the metal flashing and the wood fascia board is darker than the other areas in the picture. Why do you think it's darker? It looks like black mold, this area could be shaded for longer periods during the day, while the other areas are receiving direct sunlight. The direct sunlight will dry the moist areas faster than the areas located in the shade.
This damage wouldn't be hard to fix, and it should be fixed as soon as possible. If moisture is accumulating on the roofing material and draining off this corner, on a daily basis. There's a good chance that there's termites tearing up the wood framing, wood siding and even the wood fascia board.
Most architects are familiar with roofing problems like this and try to avoid them, when designing homes or other buildings. This is something that you might find on a building, that was built before the 1960s.
Home Repair Tip: The roof edge metal will need to be removed and replaced. The only way that this will be able to be done, will be to remove some of the roofing and replace it. This repair is going to be expensive, but should be done as soon as possible.
The longer you leave the wood to rot, the bigger the problem you will have later with mold, mildew and possibly termites. Don't forget that termites love moist wood.