Exterior Surfaces
What Are the Black Streaks on My Roof?
By Solpath Window Detailing · Published · Updated
The black streaks on South Florida roofs are algae growing on the shingles or tile, usually on shaded and north-facing slopes first. They are not dirt. Pressure washing removes the surface layer, damages the roof and lets the growth return quickly. Soft washing with a solution that kills the algae removes the streaks and keeps them away for longer.
What it actually is
The streaks are colonies of a blue-green algae, most often a species called Gloeocapsa magma. It travels on the wind, settles where a roof stays damp longest, and darkens as it grows, because it forms a dark outer coating that shields it from the sun.
It runs in streaks because rain carries it down the slope. It shows first on north-facing and shaded slopes and under overhanging trees.
Does it damage the roof?
At first it is mostly cosmetic. Over time the growth holds moisture against the roof, and the lichen and moss that can follow it are harder to remove and do more harm.
Associations often ask owners to clean a streaked roof long before it becomes a problem for the roof itself. If a letter has a deadline on it, that decides the timing more than the roof does.
Why pressure washing is the wrong answer
- On shingles it strips the granules that protect them from the sun
- On tile it cracks pieces and forces water underneath
- It takes off what you can see and leaves the rest to grow back
- The damage usually shows months later, as a leak or a stain inside
How soft washing removes it
A cleaning solution is applied at low pressure, left to work on the growth, and rinsed. It kills the algae rather than scraping it off, which is why the roof stays clean for longer than it would after a rinse.
Plants below the roofline are wetted before the work and rinsed after it. Anything unusually delicate is worth pointing out beforehand.
How long it stays clean
It depends on shade, trees and which way the roof faces. A roof in full sun with no trees nearby stays clean far longer than a shaded one under oaks. The shaded slopes come back first, and they are the ones that tell you when it is time again.
When a roof should not be done
Roof work depends on pitch and access. If a roof cannot be done safely, we say so at the quote rather than on the day.
