Black Streaks, Moss and Algae on Your Roof: What to Do
Black streaks rarely hurt how a roof works. Moss is a different matter. Here is how to tell them apart and what each one actually needs.
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Black streaks on asphalt shingles are usually algae, most often a species called Gloeocapsa magma, and they are mainly a cosmetic problem. Moss is more serious because it holds water against the roof and lifts shingle edges as it grows. Both are cleaned with a gentle, low-pressure treatment, never a pressure washer, which strips the protective granules off the shingles.
Algae, moss or lichen: telling them apart
From the ground these can look alike. A pair of binoculars and the clues below usually settle which one you have.
Algae: dark streaks
Black or dark green streaks running down the slope, heaviest on north-facing sides and under trees. The giveaway is a clean stripe just below copper or zinc flashing, where metal washed down by rain keeps the algae from growing.
Moss: green, raised clumps
Soft, cushion-like growth that starts in the gaps between shingle tabs and along the bottom edges. It grows thickest in steady shade and stays wet long after rain.
Lichen: crusty gray-green circles
Flat, round patches bonded tightly to the shingle surface. Lichen is a partnership of a fungus and an alga, and its grip is strong enough that scraping it off tears away the granules underneath.
Dirt, soot or tree debris
Staining that sits in one area, around a chimney top or under one particular tree, rather than streaking evenly down the slope. It usually washes off or weathers away on its own.
Why Connecticut roofs grow it
Algae and moss need moisture, shade and time, and many Connecticut houses offer all three. Humid summers, mature oaks and maples overhanging the roof, and north-facing slopes that stay damp from dew until midday give spores everything they need. Wooded lots and tree-lined streets tend to show it house after house, because the spores travel on the wind.
Streaks often appear first on the lower part of a slope, where water from the rest of the roof runs over the shingles and they stay wet longest. Moss usually starts where debris collects: in valleys, behind chimneys, along the bottom edges of shaded shingles and in gutters that rarely get cleaned.
Shingle color changes how much you notice it. The same algae that barely shows on charcoal shingles stands out sharply on light gray, tan or white ones, which is why two neighboring roofs with identical growth can look so different from the street.
Roof problem checker
Tick what you have noticed and see what it usually means.
How much harm each one does
Use this to decide whether you are looking at a cleaning job, a repair or simply something to watch.
| What you have | Effect on the roof | What to do | Urgency |
|---|---|---|---|
| Algae streaks only | Mostly cosmetic; holds some moisture on the surface | Gentle cleaning if the look bothers you; algae-resistant shingles at the next reroof | Low |
| Thin moss in a few spots | Starts holding water at the tab edges | Have it removed gently and trim back the shade | Moderate |
| Thick moss lifting shingles | Water reaches under the shingles; leaks and rot follow | Removal, then repair of lifted or cracked shingles | High |
| Lichen | Bonds to the surface; removal by force damages it | Treat it and let it weather off; do not scrape | Low to moderate |
| Moss on cedar shake | Keeps the wood wet and speeds rot | Clean, open up airflow and check for soft shakes | High |
Cleaning: what to do and what to avoid
Moss and algae make shingles slick even when dry, and cleaning solutions make them slicker, so this is work for someone with harnesses and roof anchors. These are the rules a careful contractor follows, and the ones to ask about before you hire anyone.
- No pressure washing on asphalt shingles; the spray blasts away the granules that protect the asphalt from sunlight
- No scraping or wire brushes; moss should be loosened gently and lifted off, working down the slope
- A cleaner the shingle maker approves, applied at low pressure; the asphalt roofing industry has long pointed to a diluted chlorine bleach solution, rinsed off gently
- Plants, shrubs and lawn below wetted down and covered so runoff does not burn them
- Gutters and downspouts cleared afterward, since loosened moss and debris end up there
- No one walking on a roof that is still wet from treatment
- Realistic expectations: streaks can return within a few years unless shade and moisture change
Keeping it from coming back
Sun and airflow are the cheapest cures. Cutting back branches that hang over the roof lets slopes dry after rain and cuts down the leaves that collect in valleys. Clean gutters matter for the same reason; if yours overflow or sag, our page on gutter installation and repair covers the fixes, including guards.
Copper or zinc strips installed near the ridge release small amounts of metal each time it rains, which slows regrowth below them. The effect is strongest close to the strip and fades on long slopes, so treat strips as a help rather than a cure.
Preventive sprays sold for roofs are another option some homeowners ask about. Confirm the shingle maker allows the product, keep runoff away from vegetable beds, and remember that anything applied to the roof surface brings back the ladder and the fall risk. Having valleys and the area behind the chimney cleared of leaves each fall removes the debris where moss usually gets its start.
When the roof is eventually replaced, algae-resistant shingles are worth asking about. They contain granules treated with copper that discourage growth for years, though the protection does wear down over time. Our asphalt shingle roofing page explains the main shingle choices.
Clean it now or live with it?
For streaks alone, cleaning is a choice, not a repair. Three questions usually decide it. First, are you selling soon? Streaked roofs photograph badly, and buyers sometimes read dark stains as a worn-out roof, so cleaning before listing photos can be worth it.
Second, how old is the roof? If it is within a few years of replacement anyway, the money spent cleaning it may be better put toward algae-resistant shingles on the new one. Our guide to the signs you need a new roof helps you judge where yours stands.
Third, is there moss mixed in with the streaks? Moss changes the answer from optional to necessary, because it does physical damage. Algae alone can wait for a convenient season; moss should come off before winter.
When moss means repair, not just cleaning
Once moss is off, look at what it left behind. Shingle edges that stay lifted, cracked tabs, missing granules in patches and soft decking near the eaves are damage, and cleaning alone will not stop water getting through. Those spots need roof repair before the next winter, when trapped water freezes and widens every gap.
On a cedar shake roof, moss is a bigger warning. Wood that stays wet softens and splits, and shakes that crumble when pressed need replacing. If moss has spread across most of an older asphalt roof and lifted shingles everywhere, the honest answer may be a replacement rather than a cleaning bill.
What A Class does when you see growth on the roof
Send us photos or describe what you see when you call (203) 779-6787, and we will arrange a visit. A drone survey lets you see every slope without anyone stepping onto slick moss, and where it is safe we check suspect areas by hand. Morning visits on shaded slopes are avoided when dew makes the surface too slick.
You then know which of three situations you are in: cosmetic streaks, moss that has started to damage shingles, or a roof worn out underneath the growth. If cleaning alone is enough, the photos will show that and you can hire a soft-wash cleaner with confidence that the shingles are sound. If shingles, flashing or gutters need work, the price comes in writing before anything is scheduled, the inspection costs nothing, and the crew cleans up when the work is finished.
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