North-facing slopes stay damp longest, which is why growth starts there. Treat moss before it spreads. Treat streaking when you care about how the roof looks.
This is most common in homes built roughly 1980 to 2000. The polybutylene and aluminum-wiring window, 150 amp panels, R-22 systems and first-cycle architectural shingle.
1Algae versus moss
| Dark streaks (algae) | Moss | |
|---|---|---|
| Looks like | Black or dark gray streaks running down the slope | Green, raised, fuzzy clumps, often along shingle edges |
| Harm to roof | Cosmetic | Holds water, lifts shingles, speeds up wear |
| Urgency | Low | Deal with it before it spreads |
Shade from trees, a north exposure and humid weather all keep the surface damp enough for both to grow.
2How to check it
- Look from the groundUse binoculars. Streaks are flat. Moss is raised and green.
- Check the shingle edgesMoss under the edges of a course can lift shingles and let water in.
- Look at what shades the slopeOverhanging limbs keep the roof damp and drop debris that feeds growth.
Pressure washing strips granules and can drive water under the shingles. Moss also makes a roof slippery. Work from a ladder at the eave or from a safe position, and never work at the roof edge alone.
3Fixing it
Do it yourself
- Apply a cleaner labeled for roofs, following its directions, and let rain rinse it
- Gently brush loosened moss downslope with a soft brush, never up under the shingles
- Trim limbs that shade the slope and keep it damp
- Keep gutters clear so the eaves dry out
Hire a pro
- Steep or high roof
- Heavy moss that has lifted shingles
- Installing zinc or copper strips at the ridge
Zinc or copper strips work best on a clean roof and are easiest to add during other roof work. If you’re reroofing, ask about algae-resistant shingles.
4What drives the cost
A cleaner and a soft brush cost little. Hired cleaning depends on roof size, pitch and how heavy the moss is. Ridge strips are a modest addition when a roofer is already on the roof. See roofing cost.
5Questions
Do I need to remove algae streaks?
Only if you care about the look. They are cosmetic. Moss is the one to deal with.
Will the moss come back?
Often, if the slope stays shaded and damp. Trimming trees and adding zinc or copper strips help keep it away.
Can I scrape moss off?
Brush gently downslope. Scraping or prying lifts and breaks shingles.
Written by Jaylan Jones. I Like To Exterior Services earns a referral fee when a call connects to a contractor. See our business-model disclosure.
