Black Streaks on Siding: Algae, or Gutter Runoff?
Black streaks on siding have two common causes: algae and mildew growing on the surface, or 'tiger striping' — oxidized residue washing down off the gutters. Algae spreads in patches on shaded walls; tiger stripes run in clean vertical lines directly below the gutter line. Both come off with soft washing, never pressure.
What causes black streaks on siding?
Two different things, and telling them apart tells you what the fix is. The first is biological: algae and mildew feeding on the film of pollen, dust, and organic residue that settles on siding. In the Shoals this is relentless — humidity off the Tennessee River keeps walls damp, and shaded lots never dry out fully. Growth like this shows up as blotches and patches, usually worst near the ground and under the eaves.
The second is mechanical. Gutters oxidize, and rain carries that residue down the outside face of the gutter and onto the wall below in narrow vertical lines. Those are 'tiger stripes,' and they're a dead giveaway: perfectly straight, evenly spaced, always directly under the gutter run, and unaffected by how much sun that wall gets.
Why is the north side always the worst?
Because it dries last. The north wall gets the least direct sun, so after rain or a heavy dew it stays damp for hours longer than the south side — and damp is the one thing algae actually needs. On most homes in the Shoals you can walk the perimeter and watch the growth intensify as you come around to the north and into whatever the trees shade. Same siding, same age, same everything, radically different amount of growth.
That's also why scrubbing one wall never fixes the problem for long. If the conditions on that wall haven't changed, the growth is coming back on the same schedule it did before.
Why soft washing and not pressure washing?
Vinyl and fiber-cement siding are lapped, not sealed — every course overlaps the one below it, and there are open seams by design. Aim a pressure washer up under those laps and you drive water behind the siding and into the wall cavity, where it has no fast way out. That's how a cosmetic problem turns into a moisture problem. High pressure also chalks and cracks aging vinyl, and it will strip paint off wood or fiber-cement.
Soft washing solves it a different way: a plant-safe cleaning solution applied at low pressure, given time to break the growth down at the root, then rinsed. Because it kills the growth rather than just blasting off what's visible, the wall stays clean for years instead of greening up again in a few months. The tiger stripes need the same treatment plus direct agitation — the oxidized residue is bonded to the surface and a rinse alone won't move it.
How long before the streaks come back?
After a proper soft wash, most homes here go two to three years before the growth is noticeable again — less on a heavily shaded north wall, longer on an open sunny lot. Nothing is permanent, because the spores are airborne and the humidity isn't going anywhere. What changes is the starting point: a wall cleaned at the root starts from zero, while a wall that was pressure-washed still has living growth in the surface texture and comes back within a season.
House soft washing starts from $400 in the Shoals, priced by the measured wall area. If gutters are the actual source of the stripes, cleaning them at the same visit is the part that stops it recurring — and bundling both into one trip is cheaper than two separate visits.
Questions we hear about this
What are the black vertical lines under my gutters?
Tiger striping — oxidized residue from the gutter face washing down the wall in rain. It is not algae, it will not respond to a plain rinse, and it needs a cleaning solution with direct agitation to release it. Straight, evenly spaced lines directly below the gutter run are the tell.
Can I pressure wash my own siding?
We would talk you out of it. Siding is lapped, so upward pressure forces water behind it and into the wall cavity, and high pressure chalks and cracks aging vinyl. Low-pressure soft washing is the method siding manufacturers actually call for.
How much does it cost to remove black streaks from siding?
Soft washing starts from $400 in the Shoals, priced by measured wall area rather than a flat guess. Our instant quote gives you an exact figure for your house in about a minute.
Will the streaks come back after cleaning?
Eventually — airborne spores and Alabama humidity guarantee it. But soft washing treats the growth at the root, so most homes here stay clean two to three years. Pressure washing leaves living growth in the surface and typically greens up again within a single season.