I live in the Seattle area. Below is a picture my roof. What are the green traces on the roof.

Is this algae or moss? What is the easiest way to remove it without damaging the shingles and the paint?

2 Answers 2


It's algae now & will become moss later & it will come back until you properly pitch the gutters &/or clean the gutters. Any wetness that continues long after everything else has dried, like you have, indicates a drainage problem. However, a tablespoon of bleach to a cup of water with a gentle & light brushing (don't scrub the shingle granules off the shingles) will remove it. Bad gutter installer, as indicated by lower gutter.

  • Bleach is only a short term solution, sprinkle some copper sulphate granules or get one of those zinc/copper based Moss-Away type roof treatments and sprinkle it on wherever you have moss issues. Mar 18, 2016 at 2:27
  • Good point, they will last longer but they won't be permanent. This is from water & debris stagnating in the gutter. The algae is washing down from the algae in the gutter.
    – Iggy
    Mar 18, 2016 at 2:35
  • Thank you @Iggy. I am curious to know why you say that it is a bad gutters. I've been looking at houses around my area and even the very high end ones have similar gutter set up...
    – Martin
    Mar 19, 2016 at 19:42
  • I was hoping you'd catch that. The lower gutter stops quite a few inches short of the roof edge on the right. That's very improper & is a big indicator of the gutters holding water & debris from not being tilted down toward their drains.
    – Iggy
    Mar 19, 2016 at 19:49

Malarkey and Atlas have Scotchguard lifetime algae resistant shingles. If your shingles are either of those brands you may have a warranty claim. Other manufacturers have 10-15 yr algae resistant warranties where they might cover a cleaning. They recommend TSP, Bleach and Water mix. Atlas

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