Black Streaks on Your Roof: What Causes Them and How Soft Wash Roof Cleaning Removes Them

Drive through any neighborhood in Noblesville, Fishers or Carmel and look at the north facing slope of the roofs. On a house more than a few years old you will see them: dark streaks running from the ridge toward the gutter, darker in the shade, sometimes with a green or white fringe. Homeowners assume it is dirt, or tar bleeding from the shingles, or simply age. It is none of those. The black streaks are a living organism, and understanding that changes how they should be removed. Fresh Look Exterior cleans roofs across north central and central Indiana, and this is what the streaks are and how a soft wash takes them off.

What the black streaks actually are

The streaks are a blue green algae, Gloeocapsa magma, that feeds on the limestone filler in modern asphalt shingles. It arrives as airborne spores, takes hold where the roof stays damp longest, which is the north slope and anywhere trees shade the roof, and spreads downhill with the rain. The dark color is a protective coating the algae produces against ultraviolet light. Given enough time, moss and lichen move in on top of it, and those hold moisture against the shingle and lift its edges. The streaks are ugly first, and in the long run they shorten the life of the roof.

Why pressure washing is the wrong tool

An asphalt shingle is a mat of fiberglass coated in asphalt and covered in mineral granules. The granules are the roof's protection from the sun, and they are held on by the asphalt, not by much else. A pressure washer strips them. A roof that has been pressure washed often looks clean for a season and then ages years in a summer, because the bare asphalt underneath is now exposed to the weather. Fresh Look Exterior does not put high pressure on a roof, and the company turns down any job where high pressure would create an unreasonable risk of damaging the surface. That rule is the whole reason the method matters.

How a soft wash roof cleaning works

Soft wash roof cleaning uses low pressure, under 100 PSI, to apply a cleaning solution that kills the algae, moss and lichen at the root rather than blasting them off. The solution sits on the roof and does the work; the crew rinses at low pressure, the organism dies, and the roof comes back to its own color. Plants, beds and lawn below the roof edge are wetted down and protected before the application and rinsed after. The result is a roof that is clean because the organism is dead, not because the surface was scoured.

What to expect the day of the cleaning

The crew arrives, protects plants and surfaces below, applies the solution to the affected slopes, and lets it work. Every photograph on the Fresh Look Exterior site is the company's own crew on a real property.

How long a soft wash roof cleaning lasts

Because the soft wash kills the organism rather than removing the visible part, the roof stays clean far longer than a pressure washed roof. How long depends on shade, tree cover and how damp the slope stays; a roof under heavy trees will show streaks again sooner than an open one. Homeowners who want to extend the interval can ask about the roof rejuvenation service, which is offered on qualifying asphalt roofs after cleaning.

Signs it is time to clean, and when to wait

Streaks that are visible from the street are the obvious sign, and by the time they show the algae has usually been growing for a couple of seasons. Green patches or raised tufts along the shaded edges mean moss has arrived and the roof should be cleaned before winter, when frozen moss lifts shingle edges. A roof that is near the end of its life, with bare patches, curling or missing shingles, is a different conversation: cleaning will not fix structural wear, and Fresh Look Exterior will say so rather than sell a wash the roof will not benefit from. The company cleans roofs and does not replace them, so there is no incentive in the advice either way.

Where Fresh Look Exterior cleans roofs

Fresh Look Exterior is based in Eaton, north of Muncie, and cleans roofs across Noblesville, Fishers, Carmel, Westfield, Muncie, Anderson, McCordsville, Yorktown, Pendleton and Fortville, with the wider service area covering Hamilton, Madison, Delaware, Hancock and Jay counties. The company matches the cleaning method to the surface on every job, which is the approach it takes to house washing, concrete and gutters as well.

Frequently asked questions

Are the black streaks on my roof damaging it? Over time, yes. The algae feeds on the shingle filler, and the moss and lichen that follow hold moisture against the shingles.

Can I pressure wash my roof? It is not recommended. Pressure strips the granules that protect the shingle. Soft washing removes the streaks without that damage.

Do you clean roofs in Noblesville and Fishers? Yes, see roof cleaning in Noblesville and roof cleaning in Fishers. The company also cleans roofs across Hamilton County, Madison County and Delaware County.

Ready to see your roof without the streaks? Contact Fresh Look Exterior for a roof cleaning quote.

The EPA's page on mold and moisture explains why a damp, shaded surface grows what it grows.