marks on roof shingles

Hi all,

I found some houses in my neighbourhood had this issues. Please see the pictures. 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?


Algae growth.

Difficult to clean.

Easier to prevent by adding a zinc strip near the top.

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.

Something interesting I saw a few weeks ago

BINGO, and tell them to extend that downspout.

So if you guys see this algae on roofing, will you recommend replacing the roofing or cleaning? How do you write it up?

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!