Black streaks on asphalt shingle roofs are most often caused by Gloeocapsa magna, an airborne cyanobacterium commonly called roof algae. It spreads on damp, shaded slopes, and its dark outer pigment creates the streaks that run downhill. The streaks are mainly a cosmetic problem, although heavy growth holds moisture. Options include gentle cleaning with a low-pressure method, algae-resistant shingles when reroofing, and tree trimming.
The short version
- The usual culprit is Gloeocapsa magna, a cyanobacterium. It is airborne, so a clean roof can still pick it up.
- The dark color is a protective pigment, and gravity and rain turn colonies into streaks.
- Industry sources describe the streaks as discoloration. Whether they shorten roof life is debated, so avoid both panic and dismissal.
- Never scrub or pressure wash shingles. Low-pressure treatment is the approach roofing manufacturers generally accept.
- North-facing, shaded and damp slopes streak first. Trimming trees and keeping roofs clear of debris slows the return.
What the streaks are
The dark stripes that run from ridge to eave on shingle roofs look like soot or soil, and many homeowners assume they are dirt or roof damage. In most cases they are a living film. The organism most often responsible is Gloeocapsa magna, which is a cyanobacterium, a photosynthesizing bacterium that looks and behaves much like algae. The trade casually calls it roof algae, and so will this guide.
The organism makes a dark outer sheath, which protects it from sunlight. When colonies thicken, that pigment shows as a brown to black color. Because water running down the roof carries cells and nutrients along, the stains grow in vertical streaks instead of round patches.
The asphalt shingle industry's own technical bulletin on the subject says algae are carried through the air, that Gloeocapsa magna is probably the most common cause, and that the pigments slowly darken the roof, often in streaks. Moss and tree droppings, by contrast, cause localized staining, and it helps to tell them apart before choosing a fix.
Why it shows up where it does
Roof algae needs moisture and a food source, and it spreads on the air. Which slope streaks first tells you a lot. North-facing slopes get less sun and hold moisture, so they darken ahead of the south side. Slopes under tree cover stay wet and collect organic debris, and low-pitch roofs hold water for longer after a rain.
Northern Illinois has the right conditions in many neighborhoods: humid summers, mature trees, and plenty of roofs shaded by oaks and maples. Subdivisions built over the past few decades often have fairly uniform roofs, and one neighbor's growth can seed the next house on the block. The cells arrive on wind, so no amount of cleaning in one spot protects a roof forever.
Some sources note that fiberglass-based shingles with mineral filler, including limestone, may offer more to feed on than older shingle types. Treat that as a plausible contributor rather than a settled rule, since sources differ on how much the roof material itself matters.
Is it harming the roof, or just ugly?
This is the question where you will find the most conflicting claims online. Some cleaning companies say the growth eats shingles and will ruin the roof in a few seasons. Some roofing sources describe it as a discoloration issue only. Both camps overstate what can be proven for the average roof.
What can reasonably be said: the streaks reduce curb appeal, and dark patches absorb more heat than a light roof surface. A dense film that stays wet keeps moisture against the shingle surface, and moss, which is a different organism, can lift shingle edges and hold water. Some references assert that the organism consumes the limestone in shingle granules, but the asphalt industry's bulletin does not make a damage claim, so the effect on service life is uncertain.
A sensible position is to treat roof algae as an appearance problem that can become a maintenance problem if growth is heavy or moss is present. Cleaning it is a reasonable choice, but it is not an emergency, and a roof that is simply stained does not need to be replaced.
Cleaning options and what to avoid
The asphalt roofing industry's technical guidance describes a diluted bleach-based solution laid on gently, left for a short dwell, and rinsed off with plain water. Its cautions are just as clear: friction can loosen granules, so no scrubbing, and power washers, brushes and brooms are ruled out because heavy-handed washing tends to harm asphalt roofing. The same bulletin points out that a treated roof gets slippery and nudges owners toward professionals.
That guidance lines up with how soft washing works: the solution does the cleaning, the water stays gentle, and the plants below get protected. Rob offers roof washing using that approach, starting from $250 after a photo of the house.
Be cautious about any promise of permanent results. Treatment removes the growth that is present, and the guidance above notes that treatments can be temporary and staining can return, since spores arrive again on the wind. How fast varies with how much shade, damp and nearby vegetation the roof has.
- No pressure washers, wire brushes, brooms or scraping on shingles
- Low-pressure solution application, a short dwell, then a gentle rinse
- Protect plants, and rinse the landscaping before and after
- Be cautious on a wet or treated roof; it is slippery
Decision rules: clean now, wait, or call a roofer
Use these rules to decide. If the roof has streaks but the shingles are intact, flat and well-granulated, cleaning is a reasonable choice, especially before listing the house or when curb appeal matters. If the roof is within a few years of replacement anyway, many owners reasonably let it go.
If you see curling or cracked shingles, bare patches where granules are gone, soft spots, lifted flashing or daylight in the attic, call a roofer first. Cleaning will not fix any of that, and a worn roof can be more vulnerable to damage during any kind of treatment. Heavy moss is also a flag, since it may need removal and an inspection of the shingles beneath.
If the roof is old, brittle and heavily granule-shed, a good contractor will tell you honestly that cleaning is not the answer. Rob will say so if a photo shows a roof past cleaning, and send you toward a roofer.
Keeping it from coming back
You cannot stop airborne cells, but you can make the roof less welcoming. Trim branches that shade or touch the roof so slopes dry faster in the sun. Clear leaves, needles and seed debris from valleys and from behind chimneys, since organic buildup holds water and feeds growth. Keep gutters flowing so water does not stand at the eave; see How Often Should You Clean Gutters in Northern Illinois?.
When it is time to reroof, ask about algae-resistant shingles. The industry notes that they are commercially available and designed to inhibit algae growth for extended periods, and the usual approach involves copper-containing granules. The protection is not absolute, and a heavily shaded roof may still streak over time.
Metal strips of zinc or copper at the ridge are sometimes proposed to release metal ions down the roof. Their effectiveness varies with roof shape and rainfall, and they work best on the section below the strip. Treat claims about them with some caution and ask a roofer what they have seen in practice.
Streaks that are not roof algae
Not every dark mark is Gloeocapsa. Moss grows in thick green or brown cushions on shaded, damp roofs and is often more of a physical problem than a cosmetic one. Lichen forms crusty gray or orange patches that bond tightly to the surface. Soot, tree-sap residue and rust from metal fittings can also leave dark or orange streaks.
Rust streaks below a chimney cap or metal vent are a different fix, and tar-like drips could be roof cement or failing sealant. Dark staining near the eaves in winter might be tied to ice dams and damp insulation, which is a heat-loss problem; see What Causes Ice Dams in McHenry County, and What Actually Fixes Them?.
If you are not sure what you have, send a photo. Rob would rather identify the cause before quoting the work, because the method depends on what is actually on the roof. Text a picture to (815) 451-0106.