Black Streaks on Your Roof: What Causes Algae and How to Remove It

By the Orlando Roof Inspection team · Updated October 2026 · 8 min read

Those ugly black streaks running down your roof are not dirt, soot, or a sign that your shingles are rotting. They are a living blue-green algae called Gloeocapsa magma, and it is the most common cause of roof discoloration, according to the Asphalt Roofing Manufacturers Association. In its early stages the black streaks are mostly a cosmetic problem, but the algae holds moisture against your shingles, so it is still worth removing the right way before it spreads.

1. What the black streaks actually are

The streaks are a colony of algae, not stains from pollution. The species is almost always Gloeocapsa magma, a hardy blue-green algae that the ARMA calls the most prevalent organism behind roof discoloration. This Old House identifies the same blue-green algae as the cause of the dark streaks seen on light asphalt shingles.

Airborne algae spores land on your roof and settle in. As the colony grows it builds a dark, protective pigment that turns a light roof brown or black over time, which is why the ARMA notes that these pigments can make a white or light-colored roof gradually darken. The reason the stains run downward in streaks is simple: rain and gravity wash spores and the growing colony down the slope, following the path water takes from the ridge to the eaves. You often see the heaviest streaking below roof features like chimneys and valleys where water concentrates.

2. Why humid and shaded roofs get it worst

Algae needs three things to spread: warmth, moisture, and a surface it can feed on. That is why hot, humid climates like Florida see black streaks so often, and why roofing contractors point to heat, humidity, and shade as the main drivers of roof algae.

Shade is the part most homeowners miss. A roof slope that stays damp longer grows algae faster, so north-facing slopes and any area shaded by overhanging trees tend to streak first and worst. A south-facing slope that dries quickly in the sun usually stays cleaner than the shaded side of the same house. Trees make it worse twice over: they block drying sunlight and they drop leaves and debris that hold moisture against the shingles.

3. Is roof algae actually damaging?

In the short term, black streaks are mainly a curb-appeal problem, not a structural one. Over a longer period, though, algae can shorten the life of asphalt shingles, which is why it is worth dealing with rather than ignoring.

Here is the mechanism. Many asphalt shingles use crushed limestone as a filler, and that limestone is food for the algae. As Ask the Builder explains, the powdered limestone filler is a food source the algae feeds on. A thick algae colony also holds moisture against the roof surface and can trap heat, both of which work against your shingles. None of this destroys a roof overnight, but it is a good reason to clean streaks and to check how the shingles underneath are holding up. If you are unsure how much life your roof has left, our guide on how long a roof lasts puts the numbers in context.

4. How to remove roof algae safely

The safe method is a low-pressure soft wash, not scrubbing or blasting. The ARMA says algae discoloration may be lightened by applying a solution of liquid household bleach (sodium hypochlorite) and water, then rinsing. Many manufacturers and contractors use roughly a 50/50 mix of chlorine bleach and water, though the ARMA stresses that mixing directions vary by shingle maker, so check your manufacturer's instructions first.

A sensible soft-wash sequence looks like this:

• Pick a calm, overcast day. Direct sun dries the solution too fast. This Old House recommends a calm, overcast day for these methods.
• Protect your landscaping. Bleach harms plants, so wet down shrubs and grass first and cover what you can. The ARMA warns to apply the solution carefully to avoid damage to the surrounding landscape.
• Apply gently and wait. Spray the solution on with a low-pressure sprayer and let it sit about 15 to 20 minutes so the chemistry does the work.
• Rinse with low pressure. Rinse the roof by gently spraying it with a garden hose, never a high-pressure nozzle.
• Do not scrub. The ARMA says not to use any brush or broom, because friction knocks off the protective granules.

One important safety note: the ARMA points out that the cleaning process makes a roof slippery and hazardous, which is why it encourages homeowners to use professionals. If you cannot do the work safely from a ladder, hire a soft-wash pro rather than walking a wet, chemical-coated roof. The ARMA also warns that any other cleaning chemical or method should not be used without the approval of your shingle manufacturer, so a product promising a faster result may quietly void your warranty.

5. Why you should not pressure wash shingles

Pressure washing is the single biggest mistake homeowners make with roof algae. A pressure washer can strip the mineral granules off asphalt shingles, and those granules are what protect the shingle from the sun and the weather.

The ARMA is blunt about it: do not use a power washer to clean algae from the roof surface, because high-pressure washing is likely to damage asphalt roofing. Once the granules are blasted away, the asphalt underneath ages faster and the shingle's useful life drops. The high pressure can also force water up under the shingles and into the roof deck. Pressure washing often looks like it worked on day one, then leaves you with bald, prematurely worn shingles and sometimes a leak. Soft washing with the chemistry doing the work is the method the manufacturers endorse. For a sense of what granule loss and other wear look like, see the roof inspection checklist.

6. How to prevent black streaks coming back

The most proven prevention is metal. When rain runs over zinc or copper, it carries trace amounts of the metal down the roof, and that metal is toxic to algae. Ask the Builder describes exactly this: where copper or zinc from flashing washed down onto a roof, it poisoned the algae and it would not grow.

You can put that to work in two ways:

• Zinc or copper strips near the ridge. Strips installed just below the ridge shingles, with a couple of inches exposed to the weather, release a little metal every time it rains. They slow regrowth rather than killing an existing colony, so install them after a cleaning. Because the install involves lifting shingles, it is best done by a roofer.
• Algae-resistant shingles at your next reroof. The ARMA recommends algae-resistant shingles when it is time to reroof. These shingles mix copper-containing granules into the surface, so they release algae-fighting metal on their own. Ask the Builder notes you can buy shingles with copper built into the colored granules.
• Cut back shade and keep the roof clean. Trim overhanging branches and keep gutters clear so the roof dries faster and holds less debris. Less shade and moisture means slower algae growth.

If you are already planning a new roof, it is worth comparing surfaces while you are at it. Our metal roof vs. shingles breakdown covers how each handles Florida's climate.

7. When black streaks signal a bigger problem

Most of the time, black streaks are a cosmetic issue on an otherwise sound roof. Sometimes, though, they sit on top of real wear, and that is the case you do not want to miss.

Treat the streaks as a prompt to look closer if the shingles underneath are curling, cupping, or shedding granules, if the roof is already 15 years or older, or if you see sagging, soft spots, or interior staining. In those cases the algae is cosmetic damage riding on an aging roof that may be near the end of its life, and cleaning it will not fix the underlying problem. If any of those signs are present, read our guide on the signs you need a new roof and get the roof looked at before you spend money on a cleaning. An inspection is also the honest way to know whether you are dealing with harmless algae or the start of a failure.