Moss, lichen or algae? Three different things on your roof
Growth that looks similar from the driveway can be very different up close. Start with an observation, not a guess.
What are we looking at?
Moss is a small plant that absorbs moisture across its surface. It often forms raised tufts or cushions. Lichens are partnerships between a fungus and a photosynthetic organism; they may form pale crusts or leafy patches. Roof algae can appear as dark staining or streaks rather than a raised mat.
A photograph can help describe the appearance, but not every patch can be reliably identified to species without closer examination. A grey patch is not automatically a particular lichen, and black staining is not proof of mould.
Why identification matters
ARMA distinguishes algae discoloration from moss growth on asphalt shingles. It describes algae as largely an appearance issue on ordinary shingles, while heavy moss can lift shingle edges and interfere with water shedding.
Treating all visible growth as the same problem can lead to unnecessary work. Material, condition, moisture and the type of growth matter.
A safe first observation
Photograph the whole slope and the affected area from a safe location. Notice shade, leaf litter, downspouts and nearby trees. Do not climb onto a slippery roof to get a close-up.
The photographs in Valley Wise's field archive document actual roof conditions, but a field photo should never be presented as laboratory species identification.
Sources and further reading
- U.S. Geological Survey — Bryophytes and Lichens
- Asphalt Roofing Manufacturers Association — Algae & Moss Prevention and Cleaning
- ARMA — Algae Discoloration of Roofs
This educational guide does not replace roof manufacturer directions or a qualified inspection. External references are not endorsements of Valley Wise.
Related Valley Wise service · All Moss Guide articles
Ready to discuss a property? Explore services and request an assessment.