Roof washing in North Alabama

Roofs with dark streaks on north-facing slopes, moss in shaded valleys, or spots of lichen. This is common across the region because the climate suits the organism. It is not a sign that you have neglected your roof.

The dark vertical stains are a colony of cyanobacteria, a tiny organism related to algae, called Gloeocapsa magma. It feeds on the limestone filler in asphalt shingles and protects itself with a dark coating, which is what you see from the street.

It is not dirt, and rain will not wash it off. Unless the organism is killed, it grows back from whatever is left behind.

North-facing slopes get the least direct sun, so they stay damp longest. Add heavy tree cover, which describes much of Harvest, Hazel Green and large parts of Jackson and Marshall counties, and the roof stays wet for hours after every rain.

Warm, humid and shaded is close to ideal for this growth. That is why it is a regional pattern rather than a sign of neglect.

Shingles are protected by their granules, the gritty layer on top. Granules are the wear surface and the shield against the sun, and they are held in asphalt that was never designed to resist a pressure washer. Pressure removes the streaks and strips granules along with them. The roof looks clean, and it has been aged.

Pressure washing a shingle roof can also void the manufacturer's warranty. If a contractor proposes putting real pressure on your shingles, that alone is a reason to decline.

• Wet down and protect the plants and beds below the roof line
• Apply the cleaning solution at low pressure, about like a garden hose
• Let it dwell long enough to kill the growth
• Rinse the roof, then the walls, windows and plants below

Not entirely. The colony feeds on filler in the shingle, and moss and lichen, which often arrive with it, root into the surface and lift granules as they grow.

It is not an emergency, and it will not cause a leak next month. But on a roof already ten or twelve years old, it is worth dealing with rather than living with.