Black Streaks and Moss on a Sioux Falls Roof: What to Do

Black streaks on shingles are algae, most often a kind called Gloeocapsa magma. The asphalt roofing makers’ trade group, ARMA, treats them as a stain, and a mild bleach rinse lightens them for a while.

Moss and lichen are a bigger worry, because they hold water against the shingles. Whatever is growing up there, never pressure wash an asphalt roof.

What are the black streaks on my roof?

Algae. ARMA’s technical bulletin names Gloeocapsa magma as probably the most common of several species. The spores travel on the air, settle on roofs, and their natural pigment turns a light roof dark brown or black, in streaks or as an even stain.

It shows most on light shingles. On a dark roof the same growth is hard to see.

People mistake it for soot or dirt. Moss and tree droppings leave patches in one spot. Algae shows as streaks or an even stain.

Does roof algae grow in a climate as dry as Sioux Falls?

Yes, though less than on the coasts. ARMA says algae is worst in the Gulf States and along the Northwest and Eastern Seaboards, and grows to some degree in every region, most where it is warm and humid.

Sioux Falls is drier than any of those. The airport averages 27.85 inches of rain and melted snow a year, and 45.3 inches of snow (NOAA, 1991 to 2020).

We found no study of how often algae, moss or lichen grow on South Dakota roofs. Shade helps them. Oregon State University Extension says the lasting fix for moss is to let the sun in by trimming trees, because otherwise it comes back.

Do algae, moss and lichen damage shingles?

Algae, by the evidence we found, no. ARMA describes it as discoloration, and a GAF article reports ARMA saying there is no scientific evidence it damages asphalt shingles. It still makes a house look tired.

Moss is different. Oregon State University Extension says moss does not eat the roof itself, but it traps debris and holds moisture, which shortens roof life, and on asphalt shingles it can lift the edges so water gets underneath.

Lichen grips hardest. Home inspectors’ training material says lichen bonds tightly to shingles and takes granules with it when pulled off, leaving damage that can look like blistering. InspectAPedia adds that moss and lichen wear shingles faster in freezing climates by holding water on the surface.

One insurer’s guide lists moss among the signs a roof is near the end. If yours has a thick mat, get a roof inspection with dated photos before you clean it.

How do I clean black streaks off a roof safely?

With bleach and water, gently, and no pressure. ARMA’s method:

1. Mix liquid household chlorine bleach with water. ARMA’s bulletin gives one part bleach to one part water as typical, and its current web page says mixing directions vary by shingle maker, so check yours first.
2. Wear eye protection and rubber gloves, and wet or cover plants below the eaves.
3. Spray it on gently. Do not scrub, because friction loosens the granules.
4. Leave it on for at least 15 minutes and no more than 20.
5. Rinse with a gentle spray of water.

ARMA warns that high pressure washing is likely to damage asphalt roofing and should not be used for any purpose. Its current page also rules out any brush or broom. Other chemicals need the shingle maker’s approval.

The roof turns slippery once it is wet. ARMA recommends trained professionals do roof work, and says to work from a ladder or walkboards where you can.

Expect the streaks back. ARMA says cleaning only lasts a while.

How do I get rid of moss and lichen?

Kill it in place, let it fall off, then let the sun in. Oregon State University Extension suggests a zinc sulfate spray or a soap-based moss killer made from potassium salts of fatty acids, applied with a pump sprayer.

Do not use zinc sulfate if your gutters, downspouts or flashing are copper. Zinc corrodes copper.

The sources split on brushing. Oregon State calls a long-handled scrub brush safer than a pressure washer, while ARMA says no brush on shingles at all. Ask your shingle maker, as ARMA advises for moss.

Lichen takes patience. One builder writing in Fine Homebuilding treated his roof with a spray and waited more than a year for the growth to die and drop. Pulling it off strips the granules.

Do zinc or copper strips stop algae and moss?

They help on the shingles just below them. The strip slips under a row of shingles near the ridge with a band left showing. Rain washes metal off the strip and down the slope, and that slows growth.

Fine Homebuilding says strips usually go in every 5 to 8 feet up the slope where growth is bad. Patents for these strips note they protect only a few rows below and work slowly on growth that is already there.

We found no university test of how well they work. Two cautions: a zinc strip above copper gutters or flashing puts zinc where it corrodes copper, and an Oregon county guide notes zinc is toxic to fish and other water life.

Are algae-resistant shingles worth it?

If streaks bother you, they are the only fix that lasts. ARMA says these shingles are designed to hold off algae for extended periods. Every major maker sells a line with an algae warranty:

Read what that buys. GAF’s warranty covers blue-green algae only, on shingles from packages with the logo. In the first 10 years it pays the cost of a professional cleaning or new shingles, whichever is less, and after that the payout shrinks with age. Stains from moss, lichen, fungus or mold are excluded.

Compare the full warranties, wind and impact ratings in how shingle lines differ.

Questions about black streaks and moss on a roof

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