What Causes Black Streaks on Your Roof (and How to Remove Them)

Those dark streaks running down your roof are not dirt, soot or weathering. They are alive, they are spreading, and they came from a neighbour.

The streaks have a name

The staining is a blue green algae called gloeocapsa magma. It feeds on the limestone filler that manufacturers blend into asphalt shingles, and it thrives anywhere warm and humid. Northeast Florida is close to ideal for it.

It spreads by airborne spores, which is why roof staining moves through a neighbourhood. Once one roof on the street is colonised, the houses downwind usually follow within a season or two.

Why the streaks always run downward

Algae colonises the ridge first, because that is where spores land and where moisture lingers longest. Rain then carries the colony down the slope, which produces the vertical streaking. It is gravity plus rainfall, drawing a map of how the growth is spreading.

You will usually see it worst on the north facing slope. That side gets the least direct sun, stays damp longest, and gives the algae the conditions it wants.

It is not only cosmetic

Algae holds moisture against the shingle surface. Shingles are designed to shed water and dry out, so a permanently damp surface works against them. Over time that means:

• Granule loss, which exposes the asphalt underneath to ultraviolet light
• Higher attic temperatures, because a dark stained roof reflects less heat
• A foothold for lichen and moss, which root into the shingle and are far harder to remove
• Visible ageing that knocks value off the house when it is time to sell

None of this happens quickly. It happens quietly, over years, which is exactly why so many roofs get replaced earlier than they needed to be.

What removes it, and what destroys your roof

Pressure will take the streaks off. It will also take your granules off, and those granules are the only thing protecting the asphalt from the sun. The tell is in the gutters afterwards: a gritty black sediment is shingle, not dirt.

The correct method is soft washing. A cleaning solution is applied at roughly garden hose pressure, given time to kill the colony at the root, then rinsed gently. Nothing is scrubbed and nothing is blasted.

Because soft washing kills the organism rather than knocking off the visible layer, the result also lasts far longer. A pressure cleaned roof often looks stained again within a year. A properly soft washed roof commonly holds for several.

Tile and metal are different again

Concrete and clay tile, common around St. Augustine and Ponte Vedra, crack under foot traffic and under pressure. Metal roofs have coatings that the wrong chemical will dull permanently. Neither should be treated like an asphalt roof, and neither should be walked on casually.

Keeping it from coming back

• Trim branches that shade the roof and drop organic debris onto it
• Keep gutters clear so water leaves the roof instead of sitting at the edge
• Have the roof checked every two to four years rather than waiting for visible streaks
• Treat the whole roof, not just the visible slope, since spores do not respect sightlines

We clean roofs across St. Johns, Fleming Island, Mandarin, Orange Park and the surrounding communities.

Frequently asked questions