Those Black Streaks On Your Roof? It's Algae Eating Your Shingles.

Quick Answer: The black streaks on your roof are Gloeocapsa magma — a cyanobacteria (not mold) that feeds on the limestone filler in asphalt shingles. It spreads by wind and rain. Left untreated it shortens shingle life by 5–10 years. A professional soft wash kills it at the root; results last 2–4 years. Pressure washing does not kill it and voids your roof warranty.

You know those ugly black streaks running down the north or west side of your Tampa Bay roof? The ones that look like dirt smudges no amount of rain washes away?

It’s not dirt. It’s not soot. It’s not your imagination.

It’s Gloeocapsa magma — a cyanobacteria (blue-green algae) that’s literally eating your shingles for breakfast. And in Tampa Bay’s humidity, it’s growing on every untreated roof in the bay area.

Here’s what it actually is, why it matters, and how to kill it without spending $15,000 on a new roof.

What is Gloeocapsa magma?

Gloeocapsa magma is a single-celled cyanobacteria — a primitive organism that’s somewhere between bacteria and algae. It thrives in:

Warm climates (Tampa Bay: ✅ year-round)
Humid air (Tampa Bay: ✅ year-round)
North/west-facing roof slopes (less sun = more moisture retention)
Asphalt shingles (the limestone filler is literally food for it)

When you see black streaks on a roof, you’re looking at colonies of millions of Gloeocapsa magma cells, accumulated over months or years, secreting a dark protective sheath that gives them their characteristic black color.

Why is it specifically on asphalt shingles?

Modern asphalt shingles contain crushed limestone as a filler. Limestone is calcium carbonate. Gloeocapsa magma feeds on calcium carbonate.

Your shingles are, from the algae’s perspective, a buffet.

As the algae digests the limestone, it weakens the shingle structure. Heat reflection drops. The roof gets hotter. The shingles age faster. After 5-10 years of unchecked growth, the algae can shorten a 25-year shingle’s lifespan to 15.

Tile and metal roofs aren’t safe either

While Gloeocapsa magma prefers asphalt shingles, it also colonizes:

Clay tile (especially in the grout lines and porous tile surface)
Concrete tile (concrete has calcium carbonate too)
Metal roofs (any rough surface, especially galvanized steel)

On tile and metal it’s less destructive but equally ugly.

Why is it always on the same side?

You’ll notice the streaks form heaviest on:

North-facing slopes (Northern Hemisphere)
West-facing slopes (afternoon shade)
Sections under tree cover

That’s because Gloeocapsa needs moisture to survive. Sunny south-facing slopes dry out fast after rain. Shaded north/west slopes stay damp for hours. The algae loves it.

In Tampa Bay specifically, the prevailing storm pattern + tree coverage + humidity makes north + west slopes the worst.

How does it spread between houses?

Wind. Gloeocapsa magma releases spores that travel on the wind. Once one house in a neighborhood has a colony, every shaded north-facing slope nearby gets seeded.

That’s why “all the houses in my neighborhood have streaks” is so common. They share the same wind, the same humidity, and the same algae spore cloud.

Will it go away on its own?

No. Algae doesn’t die from rain. Rain feeds it. Tampa Bay’s afternoon thunderstorms are essentially Gloeocapsa fertilizer.

Without treatment, the streaks get longer and darker year over year until the roof is fully colonized.

Can pressure washing kill it?

No, and you should never try.

Pressure washing a roof:

Voids your shingle manufacturer warranty (GAF, Owens Corning, Eagle Tile — all of them)
Blasts off protective granules that protect shingles from UV
Doesn’t kill the algae — just removes the surface colonies, leaving the underlying cells intact to regrow within months

Pressure washed roofs look clean for ~6 months, then come back twice as bad.

How does soft wash kill it?

A professional soft wash uses biodegradable cleaning solutions that kill Gloeocapsa magma at the molecular level. The active ingredient (a sodium hypochlorite blend with specific surfactants) does three things:

Disrupts the protective sheath the algae secretes
Kills the individual cells by damaging cell membranes
Leaves a slight residual protection so regrowth takes 3–5 years instead of 6 months

After soft wash, the dead algae washes away in the next rain. Your roof goes from “streaky” to “looks like new” in 1–2 weeks.

Why does soft wash last 3+ years?

Because soft wash kills the algae completely, not just the surface colonies. The same way killing a weed at the root prevents it from coming back.

A pressure wash that “looks the same” only removes the top layer. Soft wash kills everything down to the bottom.

Sandbar Soft Wash backs every roof cleaning with a 48-Hour Satisfaction Guarantee — if you spot a missed spot within 48 hours, we come back and make it right, free.

Can I DIY it?

You could buy strong soft wash chemicals online and try, yes. But the warning labels are real:

The solution is mildly caustic — eye protection mandatory
Plants need pre-rinsing and post-rinsing or they die
Wrong dilution either doesn’t kill the algae or damages roof granules
Two-story+ roofs are dangerous to walk on (especially wet)
Most DIYers end up needing a professional cleanup of plant damage anyway

We charge $600–$1,800 for a complete roof cleaning. DIY costs ~$200 in chemicals + 8 hours of work + risk of dead landscaping. Math doesn’t work for most homeowners.

What does professional roof cleaning cost in Tampa Bay?

Typical Tampa Bay range: $600–$1,800

Factors that affect the final number:

Roof size
Roof material (tile +25%, metal -10% vs shingle baseline)
Algae severity (light = -10%, heavy/moss = +35%)
Years since last cleaning
Stories (2-story +15%, 3+ +35%)

Get an instant quote → or use the cost calculator → for an exact number for your house.

How often should you clean your roof in Tampa Bay?

For Tampa Bay specifically:

Asphalt shingle: every 18–36 months
Tile: every 24–36 months
Metal: every 36–48 months

Full Florida frequency guide →

TL;DR

Those streaks are Gloeocapsa magma — a cyanobacteria
It’s literally eating the limestone filler in your asphalt shingles
Tampa Bay’s humidity + warmth makes it thrive year-round
North + west slopes get hit hardest
Pressure washing doesn’t kill it (and voids your warranty)
Soft wash kills it at the root, lasts 3+ years
Sandbar Soft Wash is licensed, insured, and backs every job with a 48-Hour Satisfaction Guarantee

Got streaks? Get an instant quote or call (727) 712-6281. We’ll be at your inspection within 48 hours.