Roof discoloration

[StevenT]

Could ventilation blockage be the cause of this discoloration.

Attic is ventilated by eaves vents/exhaust fan. The eaves are blocked with insulation. Under the small vent cover, there is a 6" x 10" vent hole cut into the roof, next to the ex. fan.

Because they are so close to each other, combined with the blockage causing neg. pressure be the cause?

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[hausdok]

It's alga and dirt,

Moisture-laden air leaves those vents and collects on the surface in the evenings and cool mornings before the sun comes up to evaporate it. It feeds alga spore that are also captured in the damp spots. Dust in the wind as it passes over the ridge also collects on the damp area. Over time it builds up and darkens the shingles. Mix a weak bleach/water solution with a little liquid detergent, apply it and rinse it off with a garden hose and it's gone.

I won't even go into how I feel about substituting properly balanced passive roof plane ventilation with those friggin' roof fans.

ONE TEAM - ONE FIGHT!!!

Mike