Drive down your own street and look up. Count the roofs with dark streaks running down from the ridge, worst on the shaded side. Around here it’s most of them.
Almost everyone assumes it’s dirt, or “just staining,” or that the roof is getting old.
It’s none of those. It’s a living thing, it has a name, and it is feeding on your house while you read this.
It has a name
The black on a Florida roof is Gloeocapsa magma — a cyanobacteria. It travels through the air as spores, lands on your roof, and if the conditions suit it, it stays and multiplies. Florida gives it everything it wants: heat, humidity, and surfaces that stay damp for hours after a rain.
The dark color is a protective coating the organism produces to shield itself from UV. So the black you see isn’t a stain sitting on your roof. It’s the colony’s own sunscreen. It’s a sign the growth is established and healthy.
You are not looking at a dirty roof. You are looking at a living colony, and it is feeding.
What it eats
Asphalt shingle manufacturers blend limestone into shingles as a filler. Gloeocapsa magma feeds on that limestone. That’s not a theory — it’s the specific reason algae-resistant shingles exist, and why they’re made with copper granules that inhibit growth.
As the colony feeds, it works into the shingle and loosens the ceramic granules on top. Those granules are the only thing standing between the asphalt and Florida sun. Go look in your gutters or at the bottom of your downspout — if you find what looks like coarse black sand, that’s your roof, and it’s in the wrong place.
Tile roofs don’t get eaten the same way, but they get something arguably worse. The colony holds moisture against the tile and in the valleys. Tile that should dry within an hour of a rain stays damp all day. Constant damp brings moss and lichen, and lichen roots into the surface rather than sitting on it. Water that should be shedding now sits — and sitting water eventually finds the underlayment beneath the tile, which is the only real waterproofing your roof has.
The timeline
Year one
Light streaking, mostly on the north-facing and shaded slopes. This is purely cosmetic and completely reversible. A soft wash at this stage is about as routine as it gets, and the roof looks new again.
Years two through four
The colony spreads across the field of the roof. On shingle, granule loss becomes measurable — you can see the difference in the gutters. On tile, moss establishes in the valleys and along the laps. The roof is holding water it shouldn’t be holding. Still very much fixable, but you’ve now spent a few years of your roof’s life on it.
Year five and beyond
Bare spots where granules are gone entirely. Asphalt exposed directly to UV starts to dry, harden and crack. On tile, moss has rooted and lifted, and the underlayment beneath has been damp on and off for years. Underlayment is not designed to be wet continuously — it’s designed to be a backup for the occasional wind-driven rain.
The part that costs real money
Water gets in. It doesn’t announce itself. It follows the decking, soaks the insulation, and shows up as a stain on a bedroom ceiling months after it started. By then you’re not paying for a roof cleaning. You’re paying for decking, insulation, drywall, paint, and possibly a roof.
A routine cleaning becomes a major structural repair. That’s the actual math, and it’s the part nobody explains at the beginning.
Why you can’t just pressure wash it
You can. It works — for about a year, and it’s a genuinely bad idea.
High pressure blasts the top layer of the colony off and takes granules with it. It drives water up under tile and shingle laps, where it has no business being. On most asphalt shingles, it voids the manufacturer’s warranty outright.
And it doesn’t solve anything. The organism lives down in the surface, not on top of it. Knock the visible layer off and what’s left simply regrows — usually within a year, usually faster than the first time, because now the surface is rougher and holds moisture better.
What actually works
Soft washing. Low pressure — not much more than a garden hose — carrying a properly mixed cleaning solution onto the surface. The chemistry kills the organism where it lives, roots and all. Nothing gets scoured, nothing gets forced under the tile, and because the colony is actually dead rather than displaced, the roof stays clean for years instead of months.
Two things separate a good soft wash from a bad one, and neither is visible on the day:
- Dwell time. The solution needs time to work. A crew in a hurry sprays and rinses. That roof will be black again in six months.
- Landscaping protection. Beds must be saturated with clean water before the first drop of solution goes on, and rinsed continuously throughout. It’s the step that costs time and gets skipped, and skipping it is how hedges die.
What to do about your own roof
Go outside and look at the shaded side. If you see streaking, the colony is established. It’s not an emergency this week — but it is a clock, and it only runs one direction.
If you want a straight answer on your specific roof, record a short video walking around the house and send it to 813-447-0140. I’ll tell you what I see — including if the answer is that yours can wait another year.