If your roof looks like something has been dripping down it in dark vertical stripes, you are looking at a living organism, not a stain. That distinction matters, because it changes how the problem has to be solved.
What Gloeocapsa magma actually is
It is an airborne algae that lands on the roof, takes hold, and feeds on the limestone filler used in asphalt shingles. It protects itself with a dark UV-resistant sheath — which is the black you can see. It spreads downward with rainwater, which is why the streaks always run with the slope and typically start below ridge lines and around chimneys where moisture lingers.
Why Tampa Bay roofs get it so badly
- Humidity and daily summer rain keep roof surfaces damp for long stretches, which is exactly what the algae needs.
- Warm nights mean the roof rarely dries fully, unlike drier climates where growth stalls.
- Tree cover and shade. North-facing slopes and anything shaded by oaks stay damp longest and are always the worst-affected.
- It spreads between houses. Spores travel on the wind, which is why you often see it move down a street over a few seasons.
Is it actually damaging the roof?
Yes, though slowly. The algae holds moisture against the shingle surface, and as it feeds on the limestone filler it loosens the granules that protect the asphalt underneath. Those granules are the shingle’s UV protection — once they go, ageing accelerates. The dark colouring also absorbs more heat, which does your attic temperature and cooling bill no favours through a Florida summer. It is not an emergency, but it is not cosmetic either.
Why you should never pressure wash it off
High pressure will absolutely remove the streaks. It will also remove a meaningful share of the granules along with them, drive water up under the shingle courses, and take years off the roof. On tile it can crack and dislodge pieces and force water into the underlayment. This is the single most expensive mistake we get called out to look at, and it is usually done with good intentions.
How soft washing removes it properly
Soft washing applies a cleaning solution at roughly garden-hose pressure and lets the chemistry do the work rather than the force. The solution kills the algae at the root — including the growth you cannot see yet — and rinsing carries it away. The roof usually looks dramatically better immediately, and because the organism is dead rather than merely knocked off the surface, it stays clean far longer.
How long before it comes back?
On a properly soft-washed Tampa Bay roof, expect several years before regrowth becomes visible again, depending on shade and tree cover. Roofs that were pressure washed instead tend to re-streak much faster, because the living algae was never killed — only the visible layer was blasted away. Trimming back overhanging branches to get more sun and airflow on the roof is the single most useful thing a homeowner can do between cleanings.