What Causes Black Streaks on Roofs in Louisiana?

If you've driven around Baton Rouge, Prairieville, or Denham Springs and noticed dark streaks running down otherwise decent-looking roofs, you've seen the most common roof problem in the Gulf South. It isn't dirt, it isn't "just age," and it isn't a shadow. It's algae.

The streaks are a living organism

The blue-green algae behind roof discoloration is usually Gloeocapsa magma. The Asphalt Roofing Manufacturers Association (ARMA) — the trade body whose members are the shingle manufacturers — describes it as the most prevalent of several algae species that discolor roofs, and notes that it's carried through the air and collects and grows on roofing surfaces.

Two things follow from that:

• It's airborne. Spores travel on the wind from roof to roof, which is why one stained roof in a neighborhood is often followed by several more.
• It grows. What starts as a faint gray haze becomes defined black streaks over a few seasons as the colony spreads downslope with rainwater.

Why Louisiana roofs get it worse

ARMA points out that algae growth happens in all regions of the U.S. but is worst in areas with warm, humid conditions. That's a fair description of a Baton Rouge summer. Add our tree canopy — live oaks and pines keeping roofs shaded and damp — and you get close to ideal conditions for algae to establish and stay.

You'll usually see it first on:

• North-facing slopes, which stay shaded and hold moisture longest
• Sections under overhanging branches
• The bottom third of a slope, where water (and spores) collect

Is it actually hurting the roof?

ARMA treats the discoloration itself as an appearance issue. The bigger risk to the roof, in the guidance, is how people try to remove it — which is the subject of a separate post on whether you can pressure wash asphalt shingles (you can't, and here's why). Blasting a roof with high pressure strips the granules that protect the shingle.

What actually removes it

ARMA's method is a cleaning solution applied at low pressure, given time to work, then rinsed off gently — never a pressure washer, never a brush or broom on the shingles. That low-pressure approach is exactly what professional roof soft washing is. If you want the step-by-step, see how professional roof soft washing works.

One honest caveat: because the algae is airborne and our climate keeps feeding it, cleaning results are temporary. ARMA says so plainly. Most Louisiana homeowners plan on re-cleaning their roof on a schedule rather than treating it as one-and-done.

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Seeing streaks on your roof? Get a free quote — send a couple of photos and we'll tell you what we're looking at.