Black or dark brown streaks on a roof are usually airborne algae, most often Gloeocapsa magma, not soot, dirt or rot. Algae is mainly cosmetic, and ARMA lists it as most widespread in humid regions, so on Colorado roofs it tends to show on shaded or north-facing slopes. Moss and lichen are less common here and usually local, under trees and debris. Moss is the bigger concern because it holds water against the shingles. Cleaning should be gentle: a diluted bleach solution per the shingle maker, never a pressure washer or brush. This page is general information; your roofer and shingle manufacturer make the final call.
On this page: Signs · Causes · What to check · Cleaning and fixes · Basis · When to call
What you are seeing
- Algae: Dark streaks running down the slope, or a general darkening, most visible on light shingles.
- Moss: Green, fuzzy clumps, usually in shaded spots, at the bottom of valleys or along the low edge.
- Lichen: Small, flat, crusty spots that grip the granules tightly.
- Tree droppings and debris: Spotty, local stains under overhanging branches.
Why streaks and moss grow
- Airborne spores land and grow where the roof stays damp longest.
- Shade and moisture. North slopes, tree cover and slow-drying areas stay wet after storms and snowmelt.
- Debris. Leaves, needles and dirt held on the roof feed growth and hold water.
- Light colors do not cause algae but make it far more visible.
- Low-slope sections that drain slowly; see shingles on low-pitch roofs.
Ground checks
- Look at the pattern with binoculars. Long vertical streaks are algae; clumps are moss or debris.
- Note where it grows. If it is only on one shaded slope, trimming trees may help as much as cleaning.
- Check valleys and gutters for packed debris; see overflowing gutters.
- Look for bare spots where granules are gone under moss. That is damage, not just color; see granule loss.
Cleaning and fixes
| Approach | When it fits |
|---|---|
| Trim branches and clear debris so the roof dries | Growth is local and shade-driven |
| Gentle cleaning with diluted bleach per the shingle maker, rinse lightly | Algae streaks you want lightened |
| Ask the shingle maker about moss removal methods | Moss clumps on asphalt shingles |
| Replace shingles where moss has stripped granules | Bare asphalt under the growth |
| Algae-resistant shingles at the next reroof | Shaded home with recurring streaks |
Fixes that fail: pressure washing, scrubbing or sweeping shingles, unapproved chemicals that can harm shingles and plants, and treating algae streaks as a reason to replace a sound roof.
Industry basis
ARMA’s algae discoloration guidance says the streaks are mostly Gloeocapsa magma, that discoloration is most widespread in the Gulf States and along the Northwest and Eastern seaboards, that diluted household bleach can lighten stains with temporary results, and that the solution should be applied gently, left 15 to 20 minutes and rinsed without scrubbing. It warns against power washers, brushes and brooms, recommends checking with the shingle maker on mixing and on moss, mentions algae-resistant shingles for reroofing, and advises that only trained professionals work on roofs. Owens Corning’s standard limited warranty covers algae staining only on shingles labeled algae resistant, ties 25-year algae coverage to using a qualifying Owens Corning hip and ridge product, and does not cover mold, lichen or green algae.
When to call
Call if moss is thick, if granules are missing under it, or if you want streaks cleaned without anyone walking a slick roof. Ask about algae-resistant shingles when you plan a replacement. The roof problems guide lists related problems.
Ask us about streaks or moss
Integrity Roofing and Painting is veteran-owned and family-run since 1990, an Owens Corning Platinum Preferred contractor with 650+ Google reviews. We will tell you whether the marks are cosmetic or hiding damage, and photograph any bare spots we find. Colorado Springs area: (719) 488-2800 or a free inspection. Denver metro: (303) 209-0152 or a free Denver inspection. Austin: (512) 610-5897 or a free Austin inspection. Same-business-day reply.
Sources: ARMA, Algae Discoloration; Owens Corning Standard Product Limited Warranty (Pub. 10023873, algae resistance section).
