Those black streaks on your roof aren't dirt. They're alive.
It is not dirt, and it did not just land there
The dark stripes running down a Florida roof are an organism. The shingle makers' association, ARMA, names Gloeocapsa magma as probably the most prevalent of the algae species behind the discoloration. It is alive, which is why the stripes grow.
ARMA also says there is no scientific evidence that algae is damaging to asphalt shingles, although it certainly affects the aesthetics of a roof. So the streaks are an appearance problem, and the cleaning must not do the damage the algae did not.
ARMA says these species are transported through the air and tend to collect and grow on roofing structures.
Why the streaks always point at the ground
The stripes are not dirt washing downward. They are the organism travelling downward. Rain carries spores from the upper courses of shingles to the lower ones, so the colony spreads with the runoff — which is why the pattern is always vertical, always downward.
It also explains something homeowners find genuinely confusing: a roof can look perfectly clean from the driveway and be heavily colonised on the back slope. The pattern follows water and shade. It does not follow what you can see from the street.
Why shade decides where it takes hold
ARMA says the algae can form where moisture tends to dwell on roof surfaces. In Winter Park the Florida Forest Service measured tree canopy over 37 percent of the city from 2021 imagery. A roof under that canopy stays damp after an exposed roof has already dried.
Two houses a street apart can look completely different depending on which one sits under a tree. On north-facing planes ARMA names a different growth: moss tends to grow there, where a roof gets less direct sun and stays damp longer.
The same holds wherever a roof sits under mature tree cover.
Why pressure washing a shingle roof is the wrong answer
This part is worth being blunt about, and we are deliberately not the ones you should take it from.
The Asphalt Roofing Manufacturers Association — the trade body representing the companies that actually make the shingles — addresses this directly in its guidance on algae discolouration. Their position is that high-pressure washing should not be used on asphalt roofing, and their moss and algae guidance warns that pressure washing an asphalt shingle roof causes granule loss and can lead to premature failure of the roof system.
The granules are the roof's UV protection. Strip them off and you have not cleaned the roof — you have shortened its service life and quite possibly created a warranty problem. A roof that looks better for a while and fails early is not a good trade.
It is also damage that takes no special equipment to do: a ladder, a consumer pressure washer, and no idea that the black stripes are a living thing rather than dirt.
What actually works: soft washing
What the Asphalt Roofing Manufacturers Association publishes is a method that uses chemistry instead of force: a 50:50 mix of laundry-strength liquid chlorine bleach and water, applied with a sprayer, left on the roof for 15 to 20 minutes, then rinsed thoroughly with low-pressure water. The association calls that mix the most effective method of cleaning algae and moss from a roof.
Because the treatment kills the colony rather than knocking the visible top layer off it, regrowth has to start from scratch rather than from a surviving population. We will not quote you a guaranteed number of years — your shade, your tree cover and your irrigation decide it, not the contractor.
Tile roofs are a related but distinct case. They still cannot take high pressure without risking damage to both the tile and the underlayment beneath it.
What nobody can honestly promise you
It will come back.
The organism travels through the air, so it comes back. What changes is the timeline: a shaded slope that stays damp goes again sooner than one in full sun. Shade, humidity and irrigation overspray are the three levers, and you meaningfully control exactly one of them — point the sprinklers away from the house.
New growth appearing months later is nature rather than a defect, and we would rather say that plainly now than argue about it with you later.
Common questions
- Will the cleaning solution damage my roof or my plants?
- The method the Asphalt Roofing Manufacturers Association publishes for an asphalt shingle roof is a 50:50 mix of laundry-strength liquid chlorine bleach and water, left to dwell and then rinsed thoroughly with low-pressure water. For plants, the same guidance says to take proper precautions to protect landscaping and surrounding areas from the solution; it does not say what they are. So ask any contractor how they handle plant protection — and be wary of one who has no specific answer.
- How long before the streaks come back?
- It depends almost entirely on shade, tree cover and irrigation, so we do not quote an interval.
- Does soft washing void my roof warranty?
- A low-pressure rinse is what the shingle makers' association, ARMA, publishes for algae. High-pressure washing is what it warns against. Check your own warranty documents, since terms vary by manufacturer.
- Can I just do this myself?
- The chemistry is not exotic, but the roof is. Walking a wet Florida roof is genuinely dangerous, and a pressure washer is what the Asphalt Roofing Manufacturers Association says never to use on an asphalt shingle roof. If you do try it, do it from the ground.
- Is it the same problem on a tile roof?
- Related, but not identical. Tile still cannot take high pressure without risking the tile and the underlayment.