Those black streaks on your Fort Lauderdale roof are alive
It is not dirt, and it is not staining from the shingles. Here is what it actually is.
The dark streaks running down the north-facing slope of an asphalt roof are one of the most common things we get called about in Fort Lauderdale — and one of the most commonly misunderstood.
It is algae, not dirt
The organism is Gloeocapsa magma, a hardy blue-green algae that travels by air and settles on roofs. It feeds partly on the limestone filler used in many asphalt shingles, which is why it colonizes the roof and not the wall beside it. The black color is a protective outer sheath the algae develops against UV — which is also why the streaks darken over years rather than washing away in rain.
Why it is always the shaded side
Algae needs moisture to survive. The slope that gets least sun stays damp longest after dew or rain, so that is where it establishes first. If your roof is streaked on one side only, that is the explanation — not a defect in the shingles.
The streaks run downward because rain carries spores down the slope, seeding as it goes. That is also why the pattern tends to start below ridges and around anything that channels water.
It shortens the roof's life
An algae mat holds moisture against the shingle surface instead of letting it dry. Over time that accelerates granule loss and, in colder parts of the season, gives freeze-thaw damage somewhere to happen. The staining is cosmetic at first. What is underneath it is not.
Never let anyone pressure wash it
This is the single most important thing on this page. High pressure strips the granules that protect the shingle from UV, and nearly every major manufacturer voids the warranty if the roof has been pressure washed. The correct method is a soft wash: a cleaning solution applied at very low pressure, given time to kill the organism, then rinsed gently. The streaks lift without anything being scoured off.
Keeping it away
Trimming back branches that shade the roof helps, because it lets the surface dry faster. Keeping gutters clear stops water backing up under the lower courses. And zinc or copper strips near the ridge release trace metal ions when it rains, which suppresses regrowth on the slope below.
If your roof has streaks, Fort Lauderdale Pressure Washing will tell you honestly whether it needs treating now or can wait. Call (754) 654-3991.