Homeowner Guide
Black Streaks on Your Roof?
It's algae — and in Mississippi humidity, nearly every asphalt roof gets it eventually. Here's what's cosmetic, what's causing real damage, and how to deal with both without wrecking your shingles.
By Shurden's Roofing · Updated July 25, 2026 · 8 min read
Drive through any North Mississippi neighborhood more than a decade old and you'll see them: dark streaks running down shingle roofs like water stains that never dry. That's algae, not dirt — a living colony feeding on your shingles, fueled by the same humidity that makes our summers feel the way they do. Most of it starts cosmetic. Some of it turns destructive. This guide covers how to tell the difference, how to clean it without destroying the roof, and how to keep it from coming back.
What's Actually Growing Up There
Black Streaks: Blue-Green Algae
Those dark streaks running down shingles are Gloeocapsa magma, a blue-green algae that feeds on the limestone filler inside asphalt shingles. It thrives in humid Southern air, spreads by airborne spores from roof to roof, and shows up worst on north-facing slopes and shaded areas that stay damp longest. Early on it's mostly a cosmetic problem — but it doesn't stay that way forever.
Moss: The One That Does Real Damage
Moss grows where shade and moisture persist — under overhanging trees, on slopes that never get sun. Unlike algae, moss holds water against the shingle surface like a wet sponge and works its way under shingle edges, lifting them and breaking their wind seal. A mossy roof section is actively deteriorating, not just discolored.
Lichen: Stubborn and Destructive
Lichen — crusty, coin-like growths — bonds into the granule surface itself. Scraping or aggressive cleaning takes granules with it, permanently damaging the shingle. Lichen on an older roof is usually a sign the surface has been holding moisture for years.
Why Mississippi Roofs Get It Worse
Algae needs moisture, warmth, and a limestone food source — North Mississippi supplies all three generously. High humidity, heavy dew, long warm seasons, and tree cover mean roofs here streak years earlier than the same shingles would in a drier climate. If your neighbors have black streaks, spores are already in the neighborhood air.
Cleaning It: The Right Way
- Have it soft-washed: a low-pressure application of a roof-safe cleaning solution (the shingle industry standard is a 50/50 mix of laundry-strength bleach and water), rinsed after it has time to work
- Protect landscaping — plants and grass below the roofline get soaked with fresh water before and after cleaning
- Work from a ladder or hire it out — wet shingles are dangerously slick, and algae makes them slicker
- Expect streaks to fade over weeks as rain rinses the dead algae away, not vanish the same afternoon
- Pair cleaning with prevention, or the streaks return within a couple of seasons
What Not to Do
- Never pressure wash asphalt shingles — it strips the protective granules and takes years off the roof in one afternoon
- Don't scrape or wire-brush moss and lichen off dry shingles; you'll remove granules with them
- Don't use harsh chemicals or degreasers not made for roofing — they can damage shingles and void warranties
- Don't walk a wet, algae-covered roof to save a service call — the fall risk isn't worth it
When Streaks Mean Something Bigger
Algae itself rarely causes leaks — but the conditions that grow it do. Slopes that stay damp enough to streak heavily are the same slopes where moss lifts shingle edges, where granule loss accelerates, and where decking stays wet after every rain. If a streaked roof is also showing curling shingles, granules in the gutters, or interior ceiling stains, the streaks are the least of it. That's when it's worth reading our repair vs. replacement guide and our breakdown of how long roofs actually last in Mississippi — and when a free inspection tells you exactly where the roof stands. If growth has already caused lifted shingles or leaks, our repair crew handles it every week.
Keeping It From Coming Back
- 1
Choose Algae-Resistant Shingles at Replacement
Modern GAF Timberline shingles come with StainGuard® algae protection — copper-infused granules that suppress algae growth for years, backed by an algae-discoloration limited warranty. If your roof is due for replacement anyway, this is the permanent fix: the next roof simply doesn't streak the way the old one did.
- 2
Install Zinc or Copper Strips
A strip of zinc or copper near the ridge releases trace metal ions every time it rains, washing down the slope and suppressing algae growth below it. It's a proven retrofit for roofs with years of life left that you'd rather not clean repeatedly.
- 3
Trim Back Overhanging Trees
Shade is half the equation. Opening slopes up to sunlight lets them dry after rain and dew, which starves algae and moss of the persistent moisture they need. It also cuts down the debris that feeds moss growth in valleys and behind chimneys.
- 4
Keep Gutters and Valleys Clear
Leaves and debris hold moisture against shingles and give moss a foothold. Clean gutters and clear valleys keep water moving off the roof instead of soaking into organic buildup on top of it.
- 5
Fix Ventilation and Moisture Problems
A poorly ventilated attic keeps the roof deck warmer and damper — friendlier to growth on the surface above it. Balanced soffit-and-ridge ventilation helps the whole system dry out, which is why it's part of every complete GAF system we install.
Frequently Asked Questions
What are the black streaks on my roof shingles?
Blue-green algae (Gloeocapsa magma) feeding on the limestone filler in your shingles. It spreads by airborne spores, thrives in Mississippi humidity, and shows up worst on north-facing and shaded slopes. It's not mold, and it's not dirt that rain will rinse off.
Do black streaks damage the roof or just look bad?
Mostly cosmetic at first — but long-term colonies hold moisture against the shingle, accelerate granule loss, and in shaded damp areas open the door to moss, which does real damage by lifting shingle edges. Streaks also drag down curb appeal and can raise questions during a home sale.
Can I pressure wash my shingle roof?
No. Pressure washing strips the protective granules that shield shingles from UV, aging the roof years in a single cleaning and often voiding manufacturer warranties. The safe method is soft washing — low pressure with a roof-safe cleaning solution given time to work.
How much does roof algae cleaning cost compared to ignoring it?
A professional soft wash is a modest maintenance expense. Ignoring growth costs more slowly: moss lifting shingle edges leads to leak repairs, and heavy long-term staining can mean replacing shingles that still had structural life left. If the roof is near end-of-life anyway, put the money toward algae-resistant replacement instead of cleaning.
Do algae-resistant shingles actually work?
Yes. Shingles with copper-infused granules — like GAF's StainGuard® lines — release trace copper ions that suppress algae growth for years, and they're backed by algae-discoloration limited warranties. They're the reason newer roofs in a neighborhood stay clean while older ones streak.
My roof has streaks and it's 20 years old. Clean it or replace it?
At 20 years, a Mississippi architectural roof is near the end of its realistic lifespan, and cleaning won't add structural life. Get it inspected first — if the shingles are brittle or losing granules, the money is better spent on a replacement with algae-resistant shingles than on washing a roof that's nearly done.
Streaked Roof? Find Out What's Underneath.
A free inspection tells you whether you're looking at a cosmetic cleaning, a moss problem eating shingle edges, or a roof near the end of its life. Photos, straight answers, and a written quote — across Starkville, Columbus, West Point, Tupelo, Oxford, and the rest of North Mississippi.
Free Roof Inspection
Don't Let a Damp Slope Become a Leak.
Free inspection with photo documentation — we'll tell you if those streaks are cosmetic or the first sign of something that needs fixing.