Black Streaks on Your Roof: Algae in Middle Tennessee and What to Do

By Don Hane, Navy Combat Veteran

Those dark streaks running down your shingles are almost always algae, a blue-green kind that feeds on the limestone filler in many asphalt shingles. In Middle Tennessee’s humid summers it spreads fast, especially on north-facing and shaded slopes. On its own it’s mostly cosmetic, but it can be a sign the roof stays damp, and it’s not the same as moss.

If you want someone to look at it, we’ll do it as part of a free roof inspection. If the roof is near the end of its life anyway, see roof replacement.

What causes algae on a roof here

Algae needs moisture, shade and something to eat. Our summers bring the humidity, trees bring the shade, and the shingles bring the food. That’s why you’ll see streaks on one side of a house and not the other, and why they start at the top and run down with the rain. Spores also travel on the wind, so once a few roofs on a street have it, the rest often follow.

Is algae bad for my roof?

Algae itself mostly affects how the roof looks. It can make a roof look older than it is, and some HOAs send letters about it. A few things are worth checking, though:

• Moss is different. Moss is green and thick, grows in clumps, holds water against the shingles and lifts their edges. That one does cause damage.
• Heavy streaking can point to a damp roof under constant shade, where gutters overflow, or in an attic without enough ventilation.
• Dark patches aren’t always algae. Bare spots where granules are gone can look dark too, and those mean worn shingles. A close look tells you which.

How to clean it without wrecking the shingles

Never pressure wash a shingle roof. It strips the granules that protect the asphalt from the sun and can void the shingle warranty. Shingle makers generally suggest a gentle cleaner made for roofs, applied at low pressure and rinsed off, and following the product label. If you’re not comfortable on a roof, don’t go up there. A wet roof with cleaner on it is slick.

Cleaning is temporary. If the conditions stay the same, the streaks come back.

How to keep it from coming back

• Trim back branches so more sun reaches the roof.
• Keep gutters clean so water doesn’t back up at the roof edge. See gutter cleaning.
• Fix attic ventilation if the deck stays damp.
• When you replace the roof, pick an algae-resistant shingle.

The shingle our crews prefer for algae

We’re certified with GAF, CertainTeed, Owens Corning, TAMKO and Malarkey, and most makers have algae-resistant lines. For Middle Tennessee our crews prefer Malarkey Vista AR. Vista is a UL 2218 Class 4 impact-resistant shingle made with NEX rubberized asphalt that includes upcycled rubber and plastic. Vista AR adds 3M copper granules that help prevent the black streaks caused by blue-green algae. That gives you hail resistance and algae resistance in one shingle. Our post on Class 4 impact-resistant shingles explains what the Class 4 rating does and doesn’t mean.

A new roof takes about 1 day on average; larger, steep, metal or tile jobs can take longer. You can check payment options through Acorn Finance.