If you've noticed black or dark gray streaks running down your tile or shingle roof, you're not imagining it - and it isn't dirt. It's a living organism called Gloeocapsa magma, and South Florida is close to a perfect environment for it.
You'll see it running vertically, worst on the north-facing slope, and heavier under any section shaded by trees. That pattern is the giveaway. Dirt doesn't organise itself into streaks.
What It Actually Is
Gloeocapsa magma is a cyanobacterium - a bacteria that photosynthesises like a plant. It feeds on the limestone filler used in asphalt shingles and settles happily into the porous surface of concrete and clay tile.
The black color is a defence mechanism. The organism produces a dark protective pigment to shield itself from UV. So the streaks you're looking at aren't staining left behind by something - they're the colony itself.
Why It Spreads Down the Roof
It reproduces by spores, which travel by wind and rain. That's why it moves between neighboring houses, and why one property on a street often signals several.
On your own roof it spreads downward, because rain carries spores down the slope. The vertical streaking is the trail of a colony that started higher up and washed down. Left alone it consolidates until the whole slope is uniformly dark, at which point most owners assume the roof has simply aged.
What It Does Beyond Looking Bad
Three things, none of them cosmetic.
It holds moisture. The colony retains water against the roof surface long after rain has stopped, which is exactly what roofing materials are designed to shed.
It degrades shingles. Feeding on the limestone filler breaks down the shingle itself, and the retained moisture accelerates granule loss. Granules are the roof's UV protection.
It raises cooling costs. A dark roof absorbs more heat than a light one. A fully streaked roof runs measurably hotter than a clean one, and in Miami you pay for that difference every month.
Why You Cannot Pressure Wash It
This is the part that costs people money.
High pressure will remove the streaks. It will also strip the granules off asphalt shingles, and on barrel tile it can crack, chip, or lift tiles and drive water underneath them. Both outcomes are considerably more expensive than the algae.
It also doesn't work for long. Pressure removes the visible colony from the surface but leaves rooted material in the pores, so regrowth is typically faster than the original spread.
And it's a warranty problem. Most shingle manufacturers void the warranty if the roof has been pressure washed, and the Asphalt Roofing Manufacturers Association's published guidance is low-pressure cleaning only.
What Actually Works
Soft washing. Low pressure - roughly garden-hose force - with a cleaning solution that kills the organism at the root rather than knocking the top off it.
The solution does the work; the water only carries and rinses. Nothing is scoured, nothing is displaced, no granules come off, and because the colony is killed rather than removed, it stays clean far longer than a pressure-washed roof.
You'll often see continued lightening for a couple of weeks after the visit as rain rinses the dead material away. That's normal and expected.
How Often, and What It Costs to Wait
Every 1 to 2 years for most South Florida roofs. Sooner if you're under heavy tree cover or near the water.
Waiting is the expensive option. A roof treated on schedule is a routine maintenance visit. A roof left for five or six years has usually lost granules, and no cleaning restores those - at that point you're looking at reduced remaining roof life, which is a different order of cost entirely.
It's also the single most common item cited in HOA violation letters across Miami-Dade and Broward. Our HOA guide covers that side of it.
Our roof soft wash is included in The Show-Off recurring plan, which is the simplest way to keep it on a schedule rather than remembering it.