Moss and Black Streaks on a Roof: What Each One Means
Black streaks on a roof — the dark lines running down the north-facing slope of so many Schaumburg houses — are one of the most common questions homeowners here ask, and the answer is a relief to most people: it is algae, it is cosmetic, and the roof does not care. Moss is the one to watch. Here is how to tell them apart, why this climate favors one over the other, and what to do about each.
What causes black streaks on a roof
The streaks are a blue-green algae, Gloeocapsa magma, that feeds on the limestone filler in asphalt shingles. It arrives as airborne spores, takes hold on the slope that stays damp longest — north-facing, shaded by a silver maple, under a tree line — and grows in vertical streaks following the water. It is dark because it builds a UV-protective coat. It spread across the Midwest as manufacturers put more limestone in shingles.
What roof algae does to the roof: very little. It holds a little more moisture and absorbs a little more heat, and on a 25-year roof that is a rounding error. What it does to the house: makes it look older than it is, which matters if you are selling. That is the whole case for cleaning it, and it is a fair case; see soft wash vs pressure for the method.
Moss on a roof: rarer here, and worse
Moss needs shade and near-constant moisture. Schaumburg gets around 38 inches of rain a year and a lot of dry, cold winter and hot, dry July; it is not the Pacific Northwest. So moss here is mostly confined to the north eave under a dense tree, the bottom three courses where the shingles stay wet after a rain, and the shaded valleys. When it does take hold, it matters:
- Moss holds water against the shingle for days, and against the deck at the eave.
- It grows under the tab edges and lifts them, breaking the seal.
- In winter, the water it holds freezes and expands, and the freeze-thaw shears granules and cracks the mat.
- At the eave it dams meltwater, a small version of an ice dam.
A roof with a thick green band along the north eave has been holding water at exactly the spot ice dams form. On a roof under 15 years, kill the moss and it will recover. On a roof over 20, the moss is usually growing on shingles that had already lost their granules and were holding water anyway, and the moss is a symptom.
Lichen
Gray-green circular crusts, usually on older roofs. Lichen roots into the shingle surface and takes granules with it when removed. Kill it with the same soft-wash solution; do not scrape it. Heavy lichen on a roof is a sign the granules are already gone in those spots.
Telling them apart
| What you see | What it is | Does it matter |
|---|---|---|
| Dark vertical streaks, north slope | Algae | Cosmetic |
| Green cushions along the eave or in a valley | Moss | Yes, treat it |
| Gray-green round crusts | Lichen | Sign of an old roof |
| Black streaks with bald patches beside them | Algae on a worn roof | The roof, not the algae |
| Dark stain around a pipe or chimney only | A leak or rust | Call about it |
Treatment
Both algae and moss die with a soft-wash solution of diluted sodium hypochlorite and surfactant applied at low pressure and rinsed — never pressure-washed. Expect $300–$800 per roof. Dead moss stays put for a few weeks and then blows or brushes off. Cover the plants below and run the downspouts away from beds; the runoff kills what it sits on.
Prevention
Algae-resistant shingles. Every major manufacturer now makes an AR line with copper-coated granules blended in. They cost a few percent more, and the streaks typically stay away for the length of the algae warranty, often about 10 years. If you are replacing the roof, this is the answer, and it costs almost nothing. The replacement page covers shingle choices.
Zinc or copper strip. A strip of metal along the ridge, or under the ridge cap, on an existing roof. Rain washes metal ions down the slope and kills algae and moss in a band below it — about 15 feet reliably. $150–$400 installed. It works on the streaks and on moss in the top half of the slope; it does not reach the eave on a deep roof, which is where the moss is.
Trimming. The silver maple or the spruce that shades the north slope is why it stays damp. Cutting it back six to ten feet from the roof changes the moisture on the slope more than anything else, and it takes the limbs off the roof before a February ice storm does.
Gutters. A gutter full of leaves keeps the eave wet; a clean one lets it dry.
When the streaks mean something else
If the north slope has streaks and the south slope has bald patches, curling and granules in the gutter, the streaks are the least of it. The algae is on a roof that is at the end, and cleaning it is money better put toward the replacement. The signs a roof is near the end post covers what to look for. And a dark patch that is not a streak — a stain that spreads around a chimney or below a skylight — is water, not algae, and belongs on the repair page.
Attic ventilation also plays a part: a roof deck that stays warm and damp from below feeds moss at the eave from the underside. Opening the soffits and adding a ridge vent dries the deck, and it is the same work that stops the ice dams. On most houses here, the streaks are the least interesting thing about the north slope. The eave under them is the part to look at.
A five-minute look at the north slope
You can learn most of what matters from the ground, with binoculars, a day after a rain:
- Find the slope that stays dark longest. That is the damp one, and it is where growth starts.
- Look at the bottom three courses. A green band there is moss; a dark band with no texture is usually algae.
- Check the tab edges. Moss pushing up under the edges makes the courses look wavy instead of flat.
- Look in the gutter. Green clumps mean moss is shedding; a layer of granules means the shingles are wearing out.
- Compare the south slope. If it is bald or curling, the age of the roof is the bigger story.
- Check the attic at the north eave in winter. Frost or damp sheathing there means the deck is wet from below too.
Mistakes that cost more than the moss
- Pressure-washing. It strips granules and can shorten the life of the shingles faster than anything growing on them.
- Walking a mossy roof. Wet moss is slick. Leave roof work to someone with fall protection.
- Mixing the cleaner strong and skipping the rinse. Stronger is not better; it burns plants below and can stain metal.
- Cleaning a roof that is due for replacement. That $300–$800 is better spent on the new roof with algae-resistant shingles.
- Ignoring the cause. Without trimming, clean gutters and a drier attic, the growth comes back on the same slope.
Common questions
Do black streaks on shingles damage the roof?
Not in any way that shortens its life much. The algae holds slightly more moisture and heat, which on a sound roof is minor. The real reasons to deal with it are looks and resale. If the streaks sit next to bald patches or curling, the roof itself is the issue, not the algae.
Do zinc or copper strips actually stop roof algae?
They help on the part of the slope they reach. Rain carries metal ions down from the strip and keeps algae and moss from growing in a band below it, roughly the top 15 feet. On a long slope the lower courses and the eave can still streak, so a strip works best on shorter slopes or alongside trimming and clean gutters.
Will moss come back after the roof is cleaned?
Usually, if nothing about the slope changes. The same shade, the same wet eave and the same full gutter will grow it again within a few seasons. Trimming the tree back, keeping the gutters clear and adding a zinc or copper strip near the ridge are what keep a cleaned roof clean.
Should moss be scraped off shingles?
No. Live moss is rooted into the granules, and scraping or pressure-washing takes the granules with it. Kill it with a low-pressure soft-wash solution, let it dry for a few weeks, and let rain and a soft hand brush clear it. Then fix whatever keeps that part of the roof damp.
Need roof work in Schaumburg?
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