Black algae streaks
The dark stains running down half the roofs in Broward are not dirt and not age. They are colonies of an airborne algae that feeds on the limestone filler in shingles and the surface of tile, and this county’s combination of heat, humidity and shade is close to its ideal habitat.
- 1 Ridge caps. The ridge is first to bake, first to lift.
- 2 The field. Barrel tile or shingle, over an underlayment quietly doing the work.
- 3 Valleys. One line carrying what two slopes shed, at the pace this coast rains.
- 4 Flashing. Where salt reaches the metal years ahead of the covering it protects.
- 5 The vent boot. A rubber collar that hardens, cracks, and lets the rain straight in.
- 6 Eaves. Where the screen enclosure meets the roof, and where fasteners give up.
What it does, and where it starts
The organism arrives airborne, anchors on the covering, and spreads downslope with the water path, which is why it reads as vertical streaks. It starts where moisture lingers: northern exposures, slopes shaded by Plantation’s oaks or Davie’s canopy, the strip below a valley. For a long while the damage is cosmetic. With years, the colonies hold moisture against the surface, granule loss accelerates along the streaks, and the shaded half of a roof ages measurably faster than its sunny twin: same covering, same year, different remaining life.
It also tells on the market: streaked roofs read as neglected at sale time, whatever their actual condition. On a street of clean rooflines, the streaked one gets the questions.
What works and what damages
Prevention is a materials decision: algae-resistant shingle lines carry copper-bearing granules that suppress growth for years, and on shaded lots they are worth their small premium every time. Copper or zinc strips near the ridge help more modestly. Removal is a chemistry job, a soft wash that kills the colonies, and explicitly not a pressure-washing job: high pressure strips granules and takes years off a shingle roof in one afternoon. On tile it can void surface treatments. Anyone proposing pressure on a covering is proposing damage.