Moss and Lichen on a Roof
Black streaking is a surface organism. Moss and lichen are not — they root into the shingle, and by the time you can see them clearly they have already begun causing physical damage.
Telling the three apart
Algae is the familiar flat black streaking, running vertically down the slope. It sits on the surface.
Moss is green, three-dimensional and spongy, and grows in mats — usually along the shaded lower courses and in valleys where debris collects and holds moisture.
Lichen is the difficult one: flat, crusty, pale green or grey-white patches that look almost painted on. It is a symbiosis of fungus and algae, and it anchors into the shingle surface with root-like structures rather than sitting on top.
Why they matter more than algae
Moss holds water directly against the shingle continuously, rather than letting the roof dry between rains. That constant saturation degrades the asphalt and works into the courses, and in a mat thick enough it can lift the shingle edge and let water track underneath.
Lichen is worse in a different way. Its anchoring structures penetrate the shingle surface, and when lichen is removed it takes granules with it — not because of how it was cleaned, but because it had grown into the material.
Both indicate a roof plane that stays wet far longer than it should, usually from overhanging canopy and debris accumulation.
What not to do
Do not scrape or brush them off. Physically removing lichen strips granules wholesale, and scraping moss lifts shingle edges. This does more damage than the organisms were doing.
Do not pressure wash. All the usual reasons apply, and with moss present you will also drive water under the lifted courses.
Do not ignore the overhanging branches. Moss and lichen are symptoms of a plane that never dries. Trimming back canopy and keeping valleys clear addresses the cause.
What removal realistically achieves
We soft wash, exactly as with algae — low pressure and a solution that kills the organism where it sits. Moss and lichen die and then weather off gradually over subsequent rains rather than disappearing on the day.
That gradual release is the point. Anything that removes lichen instantly is removing shingle surface along with it.
Expect the roof to keep improving for weeks after the visit. Where lichen was well established you may see slightly lighter patches once it has fully weathered away, because it had been growing into the granule layer. That is pre-existing damage becoming visible, not damage from the cleaning.
Questions we get about this
Is moss worse than black algae streaking?
Why is my roof lighter where the lichen was?
Will it all come off on the day?
How do I stop it coming back?
Related: Black streaks on your roof · Shingle granules in your gutters · all FAQs