You pull into the driveway one afternoon and notice it: dark streaks running down your roof that weren’t there a few years ago. They make the whole house look older than it is. Is it dirt? Mold? Damage?
In most cases, those black streaks are algae, most commonly a type called Gloeocapsa magma. It grows on roofing surfaces, especially on shaded or north-facing sections, and spreads slowly over time. As it grows, it develops a dark outer coating that protects it from the sun, which is why it shows up as black or dark gray streaks instead of green. The good news is that it can be removed safely with a soft wash roof cleaning, without high pressure.
What causes algae to grow on roofs?
Algae spores are carried by wind, birds and animals, so they can land on almost any roof. Whether they take hold depends on the conditions. Algae tends to grow where roofs stay damp or shaded longer:
- North-facing roof sections that get less direct sun
- Areas shaded by trees, neighboring homes or a second story
- Spots where sprinklers or drip lines regularly hit the roof edge
- Sections below overhanging branches that hold moisture and drop debris
- Roof areas around clogged gutters, where water and debris sit
On asphalt shingles, algae is commonly said to feed on the limestone filler found in many shingles. On tile roofs, which are very common in Fresno and Clovis, algae, moss and lichen can still grow in the shaded, textured surface of concrete and clay tiles.
The streaks usually start near the top of a roof section and run downward because rainwater carries spores down the slope, spreading the growth as it goes.
Could it be something else?
Algae is the most common cause, but it’s not the only possibility. It’s worth taking a closer look, from the ground, before assuming:
- Dirt and debris: Dust, leaves and organic material can leave dark stains, especially below trees.
- Ash and soot: During fire season, ash can settle on roofs and leave gray or dark residue.
- Moss and lichen: These look more raised, fuzzy or crusty than flat streaks, and are often green, gray or yellowish.
- Granule loss on shingles: Older asphalt shingles can lose their protective granules, exposing darker material underneath. This is a wear issue, not a cleaning issue.
- Metal runoff or rust: Streaks below vents, flashing or satellite mounts can come from metal parts.
If you’re not sure what you’re looking at, or if you’re seeing cracked tiles, curling shingles or bald spots, it’s a good idea to have a roofer inspect it. A professional roof cleaning also makes it much easier to spot damage that’s been hiding under the grime.
One clue that points toward algae: the streaks are usually worst on the shadiest parts of the roof and lighter or missing on the sunniest slopes. Dirt and ash tend to spread more evenly across the whole roof.
Is algae bad for your roof?
At minimum, roof algae is a curb appeal problem. Those streaks can make a well-kept home look neglected, and they’re one of the first things people notice from the street, including potential buyers. Algae also holds onto moisture, and over time moss and lichen growth can work into the surface of roofing materials. Removing it as part of regular home maintenance helps keep your roof looking good and makes problems easier to spot early.
Why you shouldn’t pressure wash your roof
It’s tempting to grab a pressure washer, but high pressure is one of the worst things you can point at a roof. On asphalt shingles, it can strip away the granules that protect them from the sun. On concrete and clay tile roofs, it can crack or chip tiles, and walking on the roof to do it can break even more. High pressure can also force water up and under roofing materials.
It also just doesn’t work very well. Pressure washing knocks off the visible growth but often doesn’t treat what’s left behind, so the streaks can come back sooner.
For your own safety, don’t climb onto your roof to inspect or clean it. Roofs are steep, tile can break under your weight, and surfaces get slippery fast once water is involved. Binoculars from the yard, or photos from a second-story window, will show you plenty. Leave the roof work to a professional crew.
How soft washing removes black streaks
Soft washing is the preferred method for cleaning roofs because it treats the algae instead of blasting it. A cleaning solution is applied at low pressure, similar to a garden hose. The solution kills the algae, moss and lichen and breaks down the dark staining. Then the roof is rinsed gently.
Gutters and downspouts get a rinse as part of the process, and the solution is applied carefully around windows, doors and outdoor fixtures. It’s a methodical job, but it’s far gentler on your roof than any high-pressure approach.
Because the growth is treated at the source, a soft-washed roof tends to stay clean for a long time. How long depends on shade, trees and weather, but it’s typically much longer than a roof that was simply pressure washed.
At Ducky’s, our soft wash roof cleaning works on concrete tile, clay tile and asphalt shingle roofs. We pre-rinse and after-rinse your landscaping to protect your plants, and we rinse the gutters too. Many homeowners pair a roof cleaning with a house wash and gutter cleaning for a complete exterior refresh.
Ways to help keep streaks from coming back
- Trim tree branches that hang over the roof to let in more sun and reduce debris.
- Keep gutters clean so water and debris don’t sit along the roof edge.
- Adjust sprinklers so they don’t spray the roofline or eaves.
- Schedule regular soft washing before streaks become heavy and widespread.
Need help? Call Ducky’s
If black streaks are making your roof look older than it is, we can help. Ducky’s Exterior Cleaning offers soft wash roof cleaning for homes and businesses across Fresno, Clovis and the Central Valley. We’re licensed and insured, open 7 days a week from 5am to 10pm, and every job is backed by our 100% satisfaction guarantee. Call (559) 492-0155 for a free estimate.