Black Streaks on Asphalt Roof Shingles: What They Are and How to Soft Wash Them Safely

If you searched for roof cleaning New York because dark stripes are running down your shingles, you are not alone. On many houses around the city and the suburbs, the north side of the roof slowly turns from gray to streaky black, and the stains seem to grow a little more every year.

Most people know Green Choice for eco-safe green rug cleaning in NYC. The same family-owned company, in business since 2005, also washes roofs, siding, decks and driveways. Here is what the streaks are, what never to do to a shingle roof and how a soft wash works.

01. What the black streaks on shingles really are

Those dark lines are not soot from the chimney and not a sign that the roof is rotting. According to the Asphalt Roofing Manufacturers Association, Gloeocapsa magma is probably the most common of several algae types behind roof discoloration. The spores travel through the air and settle on roofs and other outdoor surfaces.

The algae make a dark pigment that turns light shingles brown or black. The association notes this staining is most widespread along the Eastern Seaboard, among other regions, so our area is right in the zone.

02. Why some roofs, and some sides, streak first

The roof that stays damp the longest streaks first. Look for:

• North-facing slopes that get little direct sun and hold morning dew for hours.
• Roof sections under oak, maple or pine branches, where leaves and shade keep the surface wet.
• Houses near the water on the South Shore, the North Shore and the East End, where the air is humid most of the year.

Moss is different: green, thick clumps between shingles, often near gutters and in shady valleys. It holds water against the roof, so it needs gentle care too.

03. Why pressure washing is the wrong tool for shingles

Asphalt shingles are covered with small mineral granules that protect the asphalt from the sun. A hard stream of water blasts granules away along with the algae.

The Asphalt Roofing Manufacturers Association is very direct about this. It says not to use a power washer or any type of brush or broom to clean algae from the roof, warns that high-pressure washing systems are likely to damage asphalt roofing, and advises against scrubbing because friction can loosen and remove granules. It also says other cleaning chemicals or methods should not be used without the approval of the shingle manufacturer.

04. What you can safely check from the ground

You do not need a ladder. On a dry, bright day, walk around the house with binoculars or your phone camera zoom.

• Note which slopes have streaks and how far down they reach.
• Check the gutters and the splash area under the downspouts for piles of loose granules, which can look like coarse dark sand.
• Look for curled, cracked or missing shingles. Those are a job for a roofer, not a cleaner.

The association encourages using professionals to clean shingles. Steep, wet, two-story roofs are not a weekend project.

05. Soft washing, pressure washing and scrubbing compared

People often say “power wash the roof,” but the methods are very different. Here is how they compare on asphalt shingles:

06. How a professional roof soft wash works

Our crews treat a shingle roof as a gentle wash, using the same care we bring to every power washing service job. A typical visit looks like this:

1. Walk around and inspect. We check streaks, moss, gutters and damaged shingles, and tell you if something looks like a roofing repair instead.
2. Protect the yard. Plants, shrubs and lawn under the drip line are soaked with clean water first, and doors and windows are closed.
3. Apply the solution at low pressure. A cleaning solution is misted onto the shingles from the bottom up, with no hard stream and no brushing.
4. Let it work, briefly. The association’s guidance is to leave a bleach-and-water solution on the roof for at least 15 minutes but no more than 20.
5. Rinse gently. The roof gets a soft, low-pressure rinse, then gutters, plants and walkways below are rinsed too.

07. Protecting plants, gutters and the walls below

Everything that comes off the roof runs through the gutters and over the edges, so shrubs and beds near the drip line are wet before the wash and rinsed after, and gutters are flushed.

The walls under the roof often need help too. If the siding below has the same green film, our soft washing for house siding can be done on the same visit, and our guide to green streaks and mildew on vinyl siding explains why. A slippery, gray deck in the same shade usually gets deck and fence washing at the same time.

08. When to wash a roof: late fall or spring

A roof wash needs a few dry hours and temperatures above freezing, so the solution can work and the roof can dry. In our area that usually means spring after the last frost, or the mild days of fall before the first hard freeze.

In late fall most leaves are already down, so gutters can be cleared in the same visit. Windy or rainy days are skipped.

09. How to keep the streaks away longer

Since the algae arrive through the air, no wash keeps them away forever. A few habits slow them down:

• Trim branches that hang over the roof so the sun can dry the shingles.
• Keep gutters clear so water and leaves do not sit at the roof edge.
• When it is time to replace the roof, ask your roofer about algae-resistant shingles. The association notes several types are now on the market, designed to slow algae growth for a long time.

10. Roof washing in your area

On Long Island we serve homes across Suffolk County, including power washing in Smithtown and Southampton roof and house washing out east, and across the Nassau County service area, from Great Neck power washing on the North Shore to power washing in Long Beach by the ocean.

North of the city, our Westchester County service area includes New Rochelle power washing, and in Queens we serve detached houses with power washing in Bayside.

Inside work goes on all season too. In early November our crews cleaned carpets in Sag Harbor and picked up area rugs in Brentwood for a full wash. Our carpet cleaning service can be booked for the same week as the roof, with non-toxic products safe for kids and pets. You can read more about our family business.

Every job starts with a free quote, the price is confirmed with you before any work begins, and you pay by card or cash when the job is done. Call 1-800-589-4364 or use the form below and tell us how many stories the house has and which sides of the roof are streaked.