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Almost every roof in Jacksonville eventually develops dark vertical streaks. Homeowners often assume the roof is failing or that the shingles are staining from the inside out. Neither is usually true.
What the black streaks are
The streaking is largely Gloeocapsa magma, a cyanobacteria that feeds on the limestone filler used in asphalt shingles. It is airborne, which is why it tends to spread through a neighbourhood, and it moves down the roof plane as rain carries it — producing the characteristic vertical streaks rather than random patches.
It establishes first where the roof stays wettest: north-facing slopes, sections under overhanging branches, and areas below roof vents. On tile roofs the same growth appears in the valleys and shaded courses. Moss and lichen are separate organisms that sometimes accompany it, and lichen in particular grips much harder.
Why you must not pressure wash a roof
This is the important part. Asphalt shingles are protected by a layer of mineral granules bonded to the surface. Those granules provide UV protection and fire resistance, and shed water. High pressure strips them. You can literally watch them wash into the gutter.
The other failure modes:
- Water forced up under the shingle course, past the intended drainage path
- Water driven through flashing details around vents, valleys and chimneys
- Cracked tiles on concrete and clay roofs, from both pressure and foot traffic
- Loosened or lifted shingle edges
- Manufacturer warranty implications — many are explicit about cleaning method
A roof cleaned with high pressure often looks fine for a season and has had years taken off its life. If a contractor proposes pressure washing your roof, that is a complete answer about the contractor.
The correct method: low-pressure soft washing
Soft washing applies a cleaning solution at roughly garden-hose pressure, allows dwell time for it to kill and release the growth, then rinses gently. No scrubbing, no force, no granule loss. It is the approach the roofing industry points to for algae removal, and it is what a professional roof cleaning service in Florida should be doing.
What a proper roof cleaning looks like
- Assessment. Roof material, age, pitch, condition, flashing and gutter state — before anything is applied. Some roofs are too deteriorated to clean safely, and a good contractor will say so.
- Protection below. Landscaping saturated, downspout outlets managed, screens, pool cages, driveways and walkways covered or pre-wet.
- Application. Low-pressure application across affected slopes, with proper dwell time.
- Rinse and re-rinse. The roof, then everything below it — gutters, walls, walkways, beds.
Roof first, then the house. On any property having both done, the roof is cleaned before the walls — otherwise roof runoff re-dirties freshly washed siding. Any company sequencing it the other way is telling you something.
Tile and metal roofs
Concrete and clay tile — common on San Marco's Mediterranean Revival homes and throughout newer coastal construction — are also low-pressure only. Tile cracks under point loads, so foot traffic is minimised and the walking pattern matters. It is slower and more expensive than shingle work for good reason.
Metal and standing seam clean up well at low pressure. The concern is the finish and the fasteners rather than the panel itself, and aggressive chemicals or pressure can dull or damage a factory coating.
How often, and what to expect
Most Jacksonville roofs need attention every one to three years, driven by shade and tree cover more than by calendar. Roofs under dense canopy regrow faster. Any company promising an exact number of years is guessing — the variables are specific to your roof.
Expect substantial improvement rather than a brand-new roof. Heavy lichen may leave faint shadowing. Very old shingles that have lost granules will look cleaner but still worn. A good contractor sets that expectation before starting.
What about gutter guards, zinc strips and prevention?
Zinc or copper strips installed near the ridge release trace metal ions as rain runs over them, which inhibits growth on the slopes below. They are a partial measure — coverage is uneven and effectiveness varies — but some homeowners find them worthwhile. Trimming overhanging branches to increase sun and airflow is the more reliable intervention.
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