Problem
Black streaks on the roof are usually algae
Dark streaks on the north or shaded side are usually algae that feeds on the limestone in asphalt shingles. It is not indoor mold, and it is not automatically a leak. It becomes a roofing problem when it comes with missing granules, lifted shingles, or a roof that is already worn out.
Signs we look for
- Black or dark-green streaks on shaded slopes
- Worse staining under trees or on the north face
- Granules in gutters along with the staining
- Soft or bald shingles where the algae is heaviest
Why it happens
Humidity and shade keep the surface wet. Charlotte, Matthews, and Fort Mill see this most of the year. St. Louis and Naperville see it on tree-covered lots. Zinc or copper strips can slow new growth. They do not put granules back.
What we do next
An inspection separates cosmetic algae from a roof that needs repair or replacement. Pressure washing shingles is usually the wrong tool—it strips granules. We will say so in writing if a cleaner or a new roof is the honest next step.
What not to do
Do not pressure-wash asphalt shingles from a ladder. You can void a warranty and shorten the roof. Ask for an inspection if you also have leaks, missing shingles, or a buyer’s deadline.
Services that address this
Markets where we see this often
Related: All roof problems · Request an estimate.
People also ask
Does algae mean I need a new roof?
- Not by itself. Algae is often just staining. We look at missing granules, remaining life, and leaks before anyone writes a replacement estimate.
Topic cluster
More on roofing materials
Architectural asphalt is the usual steep-slope covering in our markets. Metal and tile can fit some houses. The right product depends on structure, climate, budget, and rules—not a single “best roof.” The written estimate names the line and the warranty that apply to that job. Full topic.
Next step
Request an inspection for this problem
Send the address, the storm date if you have it, and what you have noticed. We inspect, then give you a replacement or repair price. Your insurance company decides what they pay.