That dark growth on your roof isn't all the same thing. Algae, moss, and lichen need different approaches — and one of them is significantly harder to remove.
If you've noticed dark streaks, green patches, or rough crusty spots on your roof, you're looking at biological growth — and not all of it is the same. Algae, moss, and lichen each behave differently, spread differently, and require different approaches to remove. Here's how to tell them apart and what to do about each.
Black Streaks: Roof Algae (Gloeocapsa Magma)
The most common roof growth in Tennessee is Gloeocapsa magma, a cyanobacteria that appears as dark gray or black streaks running down your shingles. It travels by spore, which is why it spreads across rooflines and from house to house in neighborhoods.
Algae is the easiest of the three to treat. A soft wash application — low-pressure water combined with the right cleaning solution — kills it at the root and rinses it away. ARMA (the Asphalt Roofing Manufacturers Association) recommends soft washing as the approved cleaning method. High-pressure washing is not recommended — it removes the granules from shingles and accelerates wear.
Green Patches: Roof Moss
Moss grows in thick, green cushion-like patches, usually on north-facing sections of the roof where shade and moisture accumulate. Unlike algae, moss has root-like structures called rhizoids that grip the surface of the shingles. As moss retains moisture, it can work under shingles and lift them over time.
Moss responds to soft washing but may require a longer dwell time for the solution to penetrate the thickness. Dead moss doesn't fall off immediately — it typically releases over the following weeks after treatment as rain rinses it away. Never pressure wash moss off a roof; the force required to remove it will damage the shingles.
Crusty Gray or Orange Patches: Lichen
Lichen is a composite organism — part fungus, part algae — that bonds to roof surfaces more aggressively than either moss or algae alone. It appears as flat, crusty patches in gray, yellow, orange, or greenish tones. The holdfast structures penetrate the shingle surface itself.
Lichen is the hardest to remove and the most likely to leave permanent etching on shingles even after treatment. A soft wash application can kill it, but the dead organism often remains attached for months and may leave marks behind. Early treatment before lichen establishes is the best approach.
Side-by-Side Comparison
| Appearance | Algae: dark streaks | Moss: green patches | Lichen: crusty flat spots |
| Spread method | Algae: airborne spores | Moss: spores + moisture | Lichen: spores |
| Removal difficulty | Algae: easy | Moss: moderate | Lichen: hardest |
| Roof damage potential | Algae: low | Moss: moderate | Lichen: high |
| Recommended treatment | All three: soft wash (low-pressure + cleaning solution) |
What to Do
For any of the three, soft washing is the correct approach. Don't attempt to power wash biological growth off your roof — the risk to shingles isn't worth it. The correct process is: apply cleaning solution at low pressure, let it dwell, rinse gently. The growth dies and washes away over time.
If you're seeing growth on your Cookeville home, the earlier you treat it the better. Algae spreads fast in Tennessee's humid summers. Moss and lichen only get harder to remove as they establish.
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