Maintenance & Repairs

Black Streaks on Your Roof: What Knoxville Homeowners Should Know

Updated October 2026 · 7 min read · By the Knoxville Roofing Quotes team

Drive through almost any Knoxville neighborhood and you'll see roofs with dark streaks running down the shingles, usually worst on light gray or white roofs. It's rarely dirt, soot or mold. In most cases it's a common airborne algae, and while it looks bad, it isn't usually what wears a roof out. This guide explains what the streaks are, why East Tennessee's weather keeps bringing them back, how to clean them without hurting the shingles, and when dark marks point to a real problem.

What the dark streaks actually are

People often call the brown-to-black stains "fungus" or "mold," but ARMA identifies them as algae, primarily Gloeocapsa magma. The algae travels through the air, and ARMA notes that nearly every type of roofing can be affected. As the colonies grow, their dark pigment gradually turns a light roof brown or black. The color shows most on white, pastel and light gray shingles and is much harder to spot on darker colors, which is why one house on a street can look streaked while the darker roof next door looks clean.

Moss is something else. It grows in green, raised clumps rather than flat stains, and ARMA notes that it tends to show up on north-facing slopes and under overhanging trees, where leaves and debris hold moisture. Moss carries more risk than algae, which we'll cover below.

Why East Tennessee roofs are so prone to it

ARMA says this algae grows to some degree in every part of the country but does best in warm, humid conditions, and its maintenance guidance notes that algae forms where moisture lingers. Knoxville checks every box:

• Humidity: average relative humidity is about 72%, with mornings averaging around 85% and July mornings close to 90%.
• Rain: the airport averages about 52 inches of precipitation a year, according to the National Weather Service in Morristown.
• Warmth: NOAA's 1991-2020 normals put the average July high at about 88 degrees.

Shade makes it worse. Roof sections shaded by trees or by a taller part of the house stay damp longer after rain and morning dew, which gives algae more of the moisture it needs.

Are the streaks hurting your roof?

Usually not. ARMA's current guidance states that there's no scientific evidence algae damages asphalt shingles, and its older technical bulletin treats the discoloration as an appearance issue. There are two exceptions worth knowing about:

• Reflective "cool" shingles: ARMA says algae can reduce the long-term effectiveness of highly reflective roofs, so it's worth dealing with early on those products.
• Selling your home: if you're getting ready to list, cleaning is a cosmetic choice worth weighing, as long as it's done gently.

Often the bigger threat is the cleaning itself. ARMA warns that scrubbing loosens the protective granules on the shingle surface and can shorten the roof's life, and that a pressure washer can cause granule loss and premature roof failure.

How to clean algae streaks safely

If the look bothers you, the stains can be lightened. Recommended mixes have changed over the years: a 2003 ARMA bulletin described blends of chlorine bleach, trisodium phosphate and water, while ARMA's current maintenance page suggests a 50:50 mix of laundry-strength liquid chlorine bleach and water. ARMA also says each shingle maker's own instructions take priority, so check your manufacturer's guidance before you start. The general process looks like this:

1. Protect what's below. Cover or shield shrubs, flower beds and anything else the runoff will reach, and wear eye and skin protection.
2. Spray gently. Apply the solution with a garden-style sprayer, ideally from a ladder or walkboards rather than walking on the shingles.
3. Let it work. ARMA suggests letting the mix sit for 15 to 20 minutes, without letting it dry completely.
4. Rinse at low pressure. Use ordinary hose pressure. ARMA says any remaining algae typically washes away with later rains.

Set expectations, too. ARMA's bulletin says cleaning results are temporary and the discoloration will likely return, especially in a climate as humid as ours.

Preventing streaks on your next roof

Algae-resistant shingles

ARMA recommends algae-resistant shingles as the main way to prevent discoloration. These products mix copper-containing granules into the surface to inhibit growth for an extended period, and most major manufacturers offer them. Warranty coverage for staining varies. Owens Corning, for example, expanded the algae-resistance limited warranty on most of its laminate shingles from 10 to 25 years in 2023, with the full term tied to using its approved hip and ridge shingles. Ask each roofer which algae warranty comes with the shingle they're quoting and get it in writing. If you're choosing materials now, our guide to roofing materials for Tennessee weather walks through the options.

Zinc or copper strips

ARMA lists zinc or copper strips as another option for preventing algae and moss, but only when the roof is being replaced. It advises against adding them to an existing roof, because doing so means either exposed nails that can leak over time or breaking the shingle sealant bond, which can lead to wind damage later.

Simple upkeep

• Trim back branches so the roof gets more sun and less debris.
• Clear leaves with a leaf blower or another non-abrasive method, blowing down the slope so debris doesn't get pushed under shingle edges.
• Keep gutters flowing, and have upper-roof downspouts drain into the lower gutter instead of onto a lower roof. Our gutter maintenance guide has more on this.

Cosmetic streaks vs. signs of trouble

Flat, even streaks on shingles that still lie flat, still have their granules and aren't leaking are almost always cosmetic. These are different and worth a closer look:

• Moss clumps: ARMA says moss can lift or curl shingle edges, which raises the risk of blow-off in high winds, and heavy growth can let water travel sideways under shingles and cause deck moisture damage or leaks.
• Dark patches that look bare or smooth: missing granules can look like a stain from the ground. A roofer can tell the difference up close.
• Curled, cracked or missing shingles anywhere on the roof.
• Water stains on ceilings or the underside of the roof deck in the attic.
• An older roof: streaks often get noticed on roofs that are also nearing the end of their life. See our list of signs you need a new roof.

If you're unsure, a local roofer can tell you in one visit whether you're looking at harmless algae or a repair that shouldn't wait.

Frequently Asked Questions

Sources

• ARMA: Algae Discoloration of Roofs (technical bulletin excerpt, via IIBEC) (cause, appearance, cleaning results, copper-granule shingles)
• ARMA: algae and moss prevention and cleaning (no evidence of shingle damage, 50:50 mix, no pressure washing, moss risks, zinc and copper strips)
• Current Results: Tennessee average humidity
• National Weather Service Morristown: Knoxville climate
• NOAA NCEI: 1991-2020 monthly climate normals, Knoxville McGhee Tyson Airport
• Roofing Contractor: Owens Corning expands algae resistance warranty coverage (January 2023)