Lichen vs Moss vs Algae on Your Roof: How to Tell Them Apart
People lump all roof growth together as moss, but a Lower Mainland roof usually hosts three different things: algae, moss and lichen. They look different, behave differently, and matter differently for your roof. Telling them apart helps you understand what you are dealing with and why some of it is harder to remove than others.
Algae: the dark streaks
Algae is the flattest and earliest of the three. It shows as dark streaks or a black film running down the shaded slopes, and it is the growth behind the classic black roof streaks. It feeds on the shingle surface and holds moisture, and it spreads easily by spores from roof to roof.
It is cosmetic at first and the least physically damaging of the three, but it is usually the sign that conditions are right for moss and lichen to follow.
Moss: the green cushions
Moss is the green, spongy growth that forms cushions along shingle edges and in the valleys. Unlike flat algae, it has body and it holds a lot of water, and its structure lifts shingle edges as it thickens, which is what lets water get underneath. Moss is the one that does the most structural harm if left, and it is why gutters fill with green debris.
It is also the most visible, so it is usually what prompts people to call, though the algae and lichen are often there too.
Lichen: the toughest to remove
Lichen is the crusty, often pale grey-green or mustard growth that clings tightly in patches. It is a combination of algae and fungus, and it anchors into the shingle surface far more stubbornly than moss or algae sit on it. That makes it the hardest of the three to remove and the one most likely to need a proper treatment and time to fully release.
All three respond to soft washing, which kills them at the root, but lichen in particular is why a soft-wash result keeps improving over the following weeks as the killed growth gradually lets go, rather than all coming off on the day.