Black streaks come off every common roof the same basic way, a low-pressure treatment followed by a gentle rinse, but who sets the cleaner, how much pressure the surface takes and how the crew moves on the roof all change with the material. The Asphalt Roofing Manufacturers Association (ARMA) notes that “almost all types of roofing systems are susceptible to algae discoloration” (ARMA algae bulletin), so the dark trails on a clay tile or a painted metal panel are often the same organism found on shingles. The roof underneath is what differs. On asphalt shingle the shingle maker’s instructions govern, on tile the risk is breakage underfoot, on metal it is the factory finish, and on a flat roof it is the membrane maker’s approval.
Ventura County Roof Cleaning soft washes shingle, tile and metal roofs across Ventura County. If your roof is asphalt shingle, our guides to what black streaks are and why they form and the four checks before anyone sprays go deeper. This page matches the removal method to the roof you actually have.
Key Takeaways
- Same organism, different roof. ARMA says almost all roofing systems are susceptible to algae discoloration. It simply shows most on light colors.
- The roof’s maker sets the cleaner. ARMA says other chemicals or methods “should not be used without the approval of the shingle manufacturer”, and tile, metal and membrane makers publish their own care rules.
- No power washer on shingles. ARMA says high-pressure washing systems “are likely to damage asphalt roofing.”
- Each material fails differently during cleaning. Shingles lose granules, tile cracks underfoot, metal loses its finish, and membranes leak at the seams.
- Cleaning is temporary on every roof. ARMA says the discoloration “may recur”, and it does not recommend adding zinc or copper strips to an existing shingle roof.
What the Streaks Are, on Any Roof
Gloeocapsa magma is an airborne blue-green algae (a cyanobacterium) that ARMA calls “probably the most prevalent” of the species behind dark roof discoloration. Research at 3M in the 1960s and 1980s concluded that colonization by Gloeocapsa was “by far the most common cause of dark stains” on asphalt shingle roofs, according to Professional Roofing. ARMA’s bulletin adds that the pigment can turn light roofs dark brown or black, either evenly or in streaks, and that it is easily mistaken for soot, dirt, moss or tree droppings.
The stain is mostly a looks problem. ARMA’s moss and algae page says “there is no scientific evidence that algae is damaging to asphalt shingles” (ARMA). That makes diagnosis the first step on any roof, because several other marks look similar and need different work.
| What you see from the ground | Likely cause | What it means for removal |
|---|---|---|
| Flat dark streaks running downslope on intact roofing | Algae | A low-pressure treatment approved for that material |
| Raised, crusty or fibrous growth | Moss or lichen | It dies on the day but releases over the following weeks |
| A dark patch under one tree | Tree droppings or debris | Clear the debris first; it may not need a treatment at all |
| A narrow trail below a vent, chimney or flashing | Rust, sealant or soot | Find and fix the source before cleaning |
| Bare spots, cracked or slipped pieces | Wear or damage | Inspection, and a roofer if needed, before any cleaning |
The Method by Roof Material
The table is the short version. The sections under it explain what changes on each roof.
| Roof | Who sets the cleaner | Pressure | Main risk during removal |
|---|---|---|---|
| Asphalt shingle | The shingle manufacturer | Low only; no power washer, brush or broom | Granule loss from pressure or scrubbing |
| Clay or concrete tile | The tile manufacturer’s care guide | Low; tile makers publish pressure ceilings | Cracked tile from foot traffic |
| Painted or coated metal | The panel or paint maker’s care guide | Low; no abrasive pads | Damage to the factory finish and fasteners |
| Flat membrane | The membrane manufacturer | Low | Water forced into seams, penetrations and drains |
| Roof with solar panels | The panel maker, for the glass | Low | Debris trapped under the array; spotting on glass |
Asphalt shingle
ARMA’s bulletin is the clearest written method for any roof. The solution is “gently dispersed” on the surface, left on “for at least 15 minutes but no more than 20 minutes”, then rinsed “by gently spraying the surface with water.” It says not to use “a power washer or any type of brush or broom”, because scrubbing “may loosen and remove granules.” It also says mixing directions “may vary among shingle manufacturers”, which is why we do not publish a recipe. Our checklist for shingle roofs covers diagnosis, runoff and access in order.
Clay and concrete tile
The tile itself takes a gentle wash well. The risk is the walk. A tile that is sound to look at can crack under a boot placed in the wrong spot, and moss and lichen are more common on tile than on shingles in shaded parts of the county. Before any treatment, we photograph the cracked, chipped or slipped tiles that are already there and give that record to the homeowner, so nobody argues later about what the cleaning caused. Our guide to the best tile roof cleaner shows the tile makers’ pressure ceilings and the questions to put to a crew, and our tile roof cleaning service explains how we work on clay and concrete.
Painted or coated metal
On metal the thing to protect is the factory finish. Use only a cleaner the panel or paint maker approves, keep abrasive pads off the panels, and do not swap an approved application for a long bleach soak. Orange dots and rust at fasteners are not algae. Our metal roof cleaning guide covers safe methods by panel type, and our guide to reading a metal roof cleaning quote explains the orange dots other trades leave behind.
Flat membranes and roofs with solar
Ask the membrane maker before anyone treats a flat roof, and keep the spray and rinse away from seams, penetrations and drains. Low slopes also hold water longer, so the rinse has to have somewhere to go. On a roof with solar panels, the glass follows the panel maker’s cleaning instructions, and the roof under the array is where leaves and debris collect. We finish glass with deionized water so hard tap water does not dry to spots. Our solar panel cleaning service covers the panels themselves.
Runoff and Access Apply to Every Roof
Whatever the material, the rinse water lands somewhere. ARMA’s bulletin says to apply the solution “carefully to avoid damage to other parts of the building and the surrounding landscape.” In practice that means soaking the beds below the eaves before anything is applied, keeping water running through them while the solution works, and keeping wash water out of the street, where it reaches a storm drain untreated. Ventura County gets almost no rain from late spring to fall, so the roof and the plants are rinsed on the day, not left for weather. Our soft wash roof cleaning page explains why plant damage shows up days after a wash, not on the day.
Access is the other constant. ARMA warns that the treatment and rinse “will make the roof slippery and potentially hazardous to walk on”, suggests working “from a ladder and/or walkboards”, and recommends that only trained professionals work on a roof. The U.S. Consumer Product Safety Commission estimates that 273,000 people were treated in emergency departments in 2024 for injuries involving ladders and stools (CPSC). Steep roofs, two-story roofs, tile and anything with panels on it are crew jobs.
Why the Streaks Come Back, Whatever the Roof
ARMA’s bulletin says the effect of cleaning “is only temporary, and the discoloration may recur.” The slopes that darken first are the shaded ones that stay damp longest, so the useful changes are to the conditions:
- Trim overhanging branches so the slope gets sun and drops less debris.
- Keep gutters and valleys clear so wet leaves do not sit against the roof edge.
- Redirect upper-roof downspouts. ARMA’s moss and algae page says not to let gutters from an upper roof drain directly onto a lower roof, and to extend the downspout into the lower gutter.
- Skip the retrofit strips. ARMA says “adding zinc or copper strips to an existing roof is not recommended”, because it means either exposed nails, which can leak over time, or breaking the shingle sealant bond. Algae-resistant shingles are the option when the roof is due for replacement.
Our preventative roof maintenance plan puts these checks into a year-round schedule.
Frequently Asked Questions
Can black streaks grow on tile or metal roofs, not just shingles?
Yes. ARMA’s algae bulletin says almost all types of roofing systems are susceptible to algae discoloration, and that it is most visible on light-colored roofs. The organism is the same; the safe removal method depends on the roof material.
Can I pressure wash black streaks off my roof?
Not on asphalt shingles. ARMA says high-pressure washing systems are likely to damage asphalt roofing and should not be used to remove algae. Tile and metal makers publish their own pressure and cleaner limits, and we clean every roof at low pressure.
Who decides which cleaner goes on my roof?
The roof’s manufacturer. ARMA says other cleaning chemicals or methods should not be used on shingles without the shingle manufacturer’s approval, and tile, metal and membrane makers publish their own care guides. The cleaning product’s label then sets its handling rules.
Will the black streaks come back after cleaning?
Usually, in time. ARMA says the effectiveness of cleaning is only temporary and the discoloration may recur. Shade, overhanging branches, clogged gutters and upper-roof downspouts draining onto a lower roof all speed up the return.
Should I add zinc or copper strips after cleaning?
ARMA says adding zinc or copper strips to an existing roof is not recommended, because the strips need either exposed nails, which can leak over time, or a broken shingle sealant bond. Algae-resistant shingles are the longer-term option at replacement.
Get a Method Matched to Your Roof
Ventura County Roof Cleaning removes black streaks from shingle, tile and metal roofs across Ventura County at low pressure, with the beds pre-wet, the runoff planned and the roof and landscaping rinsed on the same visit. On a free estimate visit we confirm what the marks are, note the roof material and its condition, and tell you what the job involves. Request your free estimate.