Black streaks on your roof: what they are and how to remove them

Black streaks on an asphalt shingle roof are almost always algae, most commonly a type called Gloeocapsa magma. It is not dirt, mold from inside the attic, or a sign the shingles are failing on their own. The safe way to remove it is a soft wash: a cleaning solution applied at low pressure that kills the algae, followed by a gentle rinse. High pressure should never go on a roof.

What the streaks actually are

Gloeocapsa magma is a hardy algae that spreads by airborne spores. Once it lands on a roof, it feeds on moisture and the limestone filler used in many asphalt shingles. As it grows, it forms a dark outer layer that protects it from the sun. That dark layer is what you see as black or blue-black streaks.

The streaks usually start near the top of a slope or on the north and shaded faces, then run downhill as rain carries spores across the shingles. That is why they often look like long drips rather than random spots.

You may also see other growth alongside it:

• Green moss, usually in thick patches where the roof stays wettest
• Lichen, which shows up as crusty gray, green or white spots that cling hard to the surface
• Mildew and general organic staining from leaves and debris that sit in valleys

Why roofs around The Woodlands get them

Algae needs shade, moisture and time. Homes in The Woodlands area have plenty of all three. Tall pines and oaks shade roofs for much of the day, humid Gulf Coast air keeps surfaces damp, and pine needles and leaves collect in valleys and behind chimneys where they hold water against the shingles.

A neighbor’s roof can also play a part. Spores travel on the wind, so once one roof on a street is streaked, others nearby often follow.

Is it hurting the roof?

The streaks are first a cosmetic problem, but they are worth dealing with. Algae holds moisture against the shingle surface. Moss and lichen go further: they root into the granules and can lift shingle edges as they grow. A dark, streaked roof can also absorb more heat than a clean one.

None of this happens overnight, which is why early treatment is easier on the roof than waiting years.

Why you should not pressure wash a roof

It is tempting to rent a pressure washer and blast the streaks off. Please do not. High pressure strips the protective granules from asphalt shingles, can force water underneath them, and can crack or loosen older shingles. It also tends to leave the algae’s roots behind, so the streaks return. And working on a wet, slick roof with a pressure wand is dangerous.

Scrubbing with a stiff brush has similar problems on a smaller scale.

How a soft wash removes black streaks

Soft washing does the work with chemistry instead of force. At Sebastian’s Washing Services, every roof is soft washed. The process looks like this:

1. Look over the roof and yard. We note the type of staining, check gutters and downspouts, and look at the plants below the drip line.
2. Protect the landscaping. Shrubs, beds and grass near the house are pre-wet, and sensitive plants are covered where needed.
3. Apply the cleaning solution at low pressure. It goes on across the whole roof surface, not only the worst patches, so the roof dries to an even color.
4. Let it work, then rinse. Roof edges, gutter faces, siding and landscaping are rinsed before we leave.

Most of the black streaking lightens while the solution is working, and the full result shows once the roof dries. Heavy lichen is different. It is killed by the treatment but may not rinse away the same day. It loosens and washes off with rain over the following weeks. You can read more on our roof cleaning page.

Keeping the streaks from coming back

No cleaning makes a roof permanently algae-proof in a humid climate, but you can slow the return:

• Trim branches that hang over the roof so more sun and air reach the shingles.
• Clear needles and leaves from valleys and gutters so water drains instead of sitting.
• Treat early. When you see the first faint streaks, book a cleaning before they spread across the whole slope.
• Wash the siding at the same time if it is also turning green. Many homeowners pair the two so the whole house looks clean together.

If your roof is showing dark streaks, moss or lichen, Sebastian’s Washing Services offers free quotes for soft wash roof cleaning across The Woodlands area. Call or email, and we will look at your roof and tell you what to expect.