Black Streaks, Moss, and Lichen on a Roof: Causes and Prevention

Moss does not directly damage roofing, Oregon State University Extension notes, but it traps moisture and debris and can lift asphalt shingles. Black streaks, moss and lichen are three different growths with different risks, so identify yours before you buy anything, and keep zinc sulfate moss products away from roofs with copper flashing, gutters or downspouts, because zinc corrodes copper.

Identification also decides the timing. Some prevention steps belong to the day the roof is replaced, and some can start this weekend from the ground. Removal methods are a separate subject, covered in how to remove roof growth without damaging the shingles.

Streaks, Moss, or Lichen: What You Are Looking At

Look from the ground with binoculars or a zoom lens, or from a ladder at the eave if you follow ladder safety rules. Each growth has its own look.

The Asphalt Roofing Manufacturers Association’s technical bulletin on algae discoloration, revised June 2024, names Gloeocapsa magma as “probably the most prevalent of several algae species that contribute to discoloration,” and says the problem is most widespread in the Gulf States and along the Northwest and Eastern Seaboards. The bulletin describes the problem as discoloration and does not call it purely cosmetic, so look at the shingles under the streaks for the wear signs covered below instead of assuming the streaks are only a stain.

A roof can carry more than one growth at once, for example streaks on an open slope and moss along a shaded eave, and each gets its own answer.

Record what you find before you decide anything. Take a close-up of each growth, from the eave on a ladder or with a zoom lens from the ground, plus a wider shot showing which slope it is on and what shades it. Those photos let a roofer or cleaner judge the growth without a separate trip up.

Which Growth Harms a Roof

Moss is the growth to take seriously. Oregon State University Extension’s guide to controlling roof moss, reviewed in 2025, says the moss itself does not directly damage roof materials, but it traps moisture and debris and can lift asphalt shingles. A lifted shingle edge lets wind-driven rain under the lap and leaves the shingle easier to tear off in wind. Thick moss in the gaps between tabs also slows drainage, so water sits on the roof longer.

Signs that moss has moved past a monitoring item: shingle edges visibly raised above the course below, moss filling the gaps between tabs, clumps washing into the gutters, or debris collecting behind the growth. Any of those calls for removal and a check of the shingles underneath.

Lichen holds on hard. Its main risk to a shingle roof comes when someone scrapes or pries it off and takes the granule surface with it. Left alone, a few patches are a monitoring item.

Algae streaks change how the roof looks. On their own, streaks do not show the curling or bare asphalt that signal wear; if you see those as well, they are a separate question for telling replacement-level wear from local problems.

Algae-Resistant Shingles: What to Ask at Replacement

Prevention for algae is built into the shingle. ARMA’s guide to algae and moss prevention points to shingles that “incorporate algae resistant copper granules,” and its technical bulletin says several types of algae-resistant shingles are commercially available. The copper in those granules is released gradually and discourages growth on the slope.

At replacement, ask for these in writing: the exact shingle product and color, whether it is sold as algae resistant, and the warranty wording on algae, including what it covers and for how long. Algae terms differ by product, so compare the documents rather than the brochure. For the broader choice between shingle types, see asphalt shingles compared.

ARMA’s guidance on coatings or rejuvenators applied after installation, dated July 24, 2026, warns that a coating or rejuvenator applied on the roof may affect algae resistance, and the fire classification and impact resistance as well. A coating sold to “renew” the roof can undo the feature you paid for.

Zinc and Copper Strips: How They Work and Where They Fall Short

Metal strips near the ridge are designed on the same idea as copper granules: rain washes a small amount of metal down the slope below the strip to discourage growth there. Treat any claim about how well they work as the strip maker’s until it shows you the test behind it, and follow its instructions for placement, spacing and coverage.

The practical limit is timing. ARMA states that “adding zinc or copper strips to an existing roof is not recommended,” because installing them on a finished roof can leave exposed nails or break the shingle seal. Consider strips only as part of a re-roof, where the roofer can place and fasten them within the shingle courses.

The second limit is the metals on your house. OSU Extension warns that zinc sulfate products should not be used on roofs with copper gutters, downspouts or flashing, because zinc corrodes copper. Check what your flashing and gutters are made of before adding any zinc to the roof.

What Copper and Zinc Runoff Does Downstream

Whatever the strips or granules release ends up in your gutters, your yard and, through storm drains, local streams. Washington State’s Department of Ecology, in its explainer on copper and zinc in runoff, dated October 2017, says copper can harm fish and other aquatic organisms and reduces salmon’s ability to detect predators, and that for zinc, “moss control is the biggest slice of the pie.” The same agency’s 2014 study found elevated metals in runoff from copper roofs, treated wood shakes, Zincalume and EPDM.

Those findings come from one state agency, and rules on roof runoff, where they exist, differ by state and locality. If your downspouts drain toward a creek, pond or garden, ask your local stormwater or environmental agency whether it has guidance before adding metal-based prevention.

Cut the Shade, Debris and Moisture That Feed Growth

Growth needs moisture, and shade and debris keep a roof damp longer. Three habits reduce all three growths without products:

• Trim branches that shade the roof or drop leaves on it, so the surface dries faster after rain. Limbs that hang over the roof or near the service wires are work for a tree crew or the utility, not a homeowner on a ladder.
• Clear debris from the roof surface and gutters. A wildfire fact sheet from IBHS and its federal partners, Reduce Your Roof’s Vulnerability to Wildfire, gives the same advice for a different reason: “Regularly remove vegetative debris from your roof and gutters.” Do it from the ground with a tool designed for the job, or hire it out.
• Keep gutters flowing, so water does not back up at the eaves; see how to clean gutters safely.

Add these to the seasonal routine in the roof maintenance checklist.

Prevent, Remove, or Leave It Alone?

If you see only algae streaks, leave them or clean them gently by the methods in roof cleaning, and choose algae-resistant shingles when the roof is replaced.

If moss is thick enough to lift shingle edges or fill the gaps between tabs, have it removed without pressure washing, trim the shade, and keep debris off the roof. If lichen is the only growth and the shingles are sound, monitor it rather than scraping. In every case, photograph the growth close up first, and if lifted or damaged shingles show under the moss, ask for a repair estimate; for what a full inspection covers, see what a roofer’s inspection should check and photograph.

This guide is general information for U.S. homeowners, not legal, insurance, tax, or engineering advice. Codes, laws, and policy terms vary by location and change over time; confirm details with your local building department, your insurer, or a licensed professional.

Sources

• Algae Discoloration of Roofs (Technical Bulletin), Asphalt Roofing Manufacturers Association
• How to control roof moss and prevent long-term damage, Oregon State University Extension Service
• Algae and Moss Prevention and Cleaning for Asphalt Roofing Systems, Asphalt Roofing Manufacturers Association
• Application of Coatings or Rejuvenators on Asphalt Shingles After Installation, Asphalt Roofing Manufacturers Association
• Cu L8r, copper and zinc, Washington State Department of Ecology
• Reduce Your Roof’s Vulnerability to Wildfire, IBHS with USDA Forest Service and partners, hosted by Texas A&M Forest Service