The Roof Algae Streaks Guide
Those dark streaks on your Northeast Florida roof aren’t dirt. They aren’t tree sap. They aren’t pollution. They’re a specific blue-green algae — Gloeocapsa magma — feeding on the limestone filler baked into modern asphalt shingles. The Asphalt Roofing Manufacturers Association (ARMA Tech Bulletin) identified the cause decades ago. Florida’s humidity, dew-heavy mornings, and long warm season make our roofs a near-perfect host.
What the streaks actually are
Gloeocapsa magma is a cyanobacterium — blue-green algae technically, but it behaves more like a slow bacterial colony. It lands on the roof via airborne spores, settles into the tile or shingle granule texture, and starts consuming the limestone (calcium carbonate) that shingle manufacturers use as a filler.
The colony secretes a dark protective sheath to block UV damage. That dark sheath is what you see — the streak. The algae itself is green; the streak appears black because of the sheath.
The streaks nearly always run top-to-bottom, not side-to-side, because the algae gets flushed downhill by every rain. They start under tree branches, at north-facing slopes (less UV kill-off), and along any wall where moisture collects.
Why pressure washing is the wrong answer
This is the single most important point in this article. Do not let anyone pressure wash your roof.
Every major shingle manufacturer — GAF, Owens Corning, CertainTeed, IKO — explicitly voids warranty coverage when a roof is pressure washed. The reasons:
- Pressure strips the granule layer. Granules are what protect the shingle asphalt from UV. Stripped shingles fail years early.
- Pressure drives water under the shingle laps, into the underlayment, into the sheathing.
- Pressure damages the sealant bead that holds shingles flat.
- Pressure doesn’t kill the algae spores. The colony re-establishes in 6-12 months — usually worse.
If a contractor quotes you a “pressure wash” roof cleaning, they are either untrained or deliberately understating what they’ll actually do. Either way, find someone else.
What ARMA officially recommends — softwashing
The ARMA Tech Bulletin on Algae Discoloration specifies a chemical cleaning method using a diluted sodium hypochlorite solution applied at low pressure, allowed to dwell, then rinsed with low-volume clear water. No scrubbing. No high-pressure wand.
This is what softwashing is. It’s not a marketing term — it’s the industry-standard protocol. FCPE’s softwash complies with ARMA and with every major shingle manufacturer’s warranty terms.
The chemistry:
- Sodium hypochlorite (the same active ingredient in pool chlorine) kills the algae colony, the spore layer, and the lichen attachment points
- A surfactant holds the chemistry on the surface long enough to work, then breaks apart and rinses clean
- A final rinse clears the dead algae and any residual chemistry
Done right, you see results in 24-48 hours as the dead algae breaks down under subsequent rain. Full clearing continues for 2-4 weeks.
When cleaning saves the roof — and when it can’t
Softwashing a roof kills the algae that’s there. It doesn’t regenerate damaged shingles. Here’s the honest guidance we give clients on a site visit:
| Roof condition | Recommendation |
|---|---|
| Streaking visible, shingles still sound (no curling, no missing granules) | Softwash. Will significantly restore appearance. |
| Streaking plus some isolated granule loss | Softwash + spot repair by a roofing contractor. Extends roof life. |
| Heavy streaking + widespread shingle curling / brittleness | Softwash may improve appearance but the roof is at end-of-life. Plan for replacement in 1-2 years. |
| Active leaks or failed flashing | Repair first, softwash after. Don’t add water to a compromised roof. |
We’ll tell you honestly which category your roof is in after a site visit. We do not take softwash jobs on roofs that should be replaced — it’s not honest work.
The post-clean protection cycle
After a full softwash, expect 12-18 months of clean appearance before the algae colony starts to re-establish in Northeast Florida’s climate. North-facing slopes and shaded areas re-algae fastest.
Recommended cycle:
- Heavy shade, wooded lots: softwash every 12 months
- Typical suburban lot with partial sun: every 15-18 months
- Full-sun lot, wind exposure (coastal, water-facing): every 18-24 months
The Annual Exterior Plan sets a once-every-18-months roof cycle by default. We adjust for your specific property on the site visit.
Tile, metal, and flat roofs
Tile roofs (common in St. Augustine and older St. Johns County neighborhoods): softwashing works, but the chemistry ratio changes and pressure is dialed even lower. Metal roofs (standing seam, corrugated): softwashing works, no granule risk, but sealed fastener washers benefit from a slower rinse. Flat roofs: case by case — membrane type drives method. Every roof type gets a pre-visit assessment. We never assume material.
Frequently asked questions
Is softwashing safe for my landscaping?
Yes when done correctly. We pre-wet landscaping, isolate sensitive plants, and continuously rinse during the wash. Sodium hypochlorite at the dilution we use breaks down into salt and water quickly. UF/IFAS guidance considers this safe for healthy landscaping when applied by trained operators.
Will it stain my siding or gutters?
No — but a full house softwash is often done the same day as the roof because drip-down is rinsed off during the house wash. We coordinate that automatically.
Do copper strips or zinc strips really work?
They slow algae regrowth on the few rows of shingles directly below the strip. They do nothing for shingles above the strip (no water flows uphill) and they don’t help with the majority of the roof. Treat them as a supplementary measure, not a replacement for softwashing.
Do algae-resistant shingles solve this?
Newer shingles with copper-based granule coatings (GAF StainGuard Plus, CertainTeed StreakFighter, etc.) resist algae for 10-25 years. They’re worth it on a replacement. If you have a roof installed before ~2015, it’s probably not algae-resistant, and softwashing is your maintenance path.
Can I softwash my own roof?
You can. We don’t recommend it. Roof work falls are one of the leading homeowner injury categories. Softwash chemistry also needs correct dilution — too weak and it doesn’t work, too strong and you’ll burn the shingles. Professional softwashing is insured, rated, and warrantied. DIY is none of those things.
Northeast Florida service area
FCPE softwashes roofs across St. Johns County, Duval County, and Nassau County. Every roof gets a site visit before work starts — we scope the material, the condition, and the surrounding landscaping, then give you a written quote.
First Coast Property Experts — The Gold Standard, Every Time. Serving St. Johns, Duval, and Nassau Counties with more than a century of combined expertise. BBB rated A.