I found some houses in my neighbourhood had this issues. Please see the pictures.
http://www.flickr.com/photos/80011010@N07/ I didn’t have any chance to get into the attics. Can you please tell me what causing this happened? High humidity in the attics? lack of attic ventilation?
And, cheap roofing shingles. They leave out ingredients to save money. Warranties on these roofs are maybe 5 years, and after that, this happens. Common here. The owner got what he paid for, or did not pay for.
Mike’s post #2.
Ugly, but generally not a big deal. It will retain moisture longer as it gets thicker, which isn’t good for the roof, but mostly a cosmetic issue.
Slippery when wet!