What are the black streaks on my roof, and do I need to do anything about them?
Dark streaks running down asphalt shingles are almost always algae, not damage. Here's how to tell it from moss, who may pay to clean it, and why it's never a reason to re-roof.
The short answer
Dark streaks running down asphalt shingles are usually blue-green algae. GAF calls it harmless to the roof's function, so it's a cosmetic problem, not a leak risk. Before you pay for cleaning, check whether your shingles carry an algae-resistance warranty. Those run 10 to 25 years depending on the brand and line, and some manufacturers pay for cleaning under them.
Estimate, not a final quote
This range is for planning. Final pricing depends on an on-site inspection, materials, permits, access, and contractor scope.
Compare written bids
Ask each contractor for the same scope, warranty terms, permit handling, cleanup details, and exclusions before choosing.
Source-backed guidance
Guides use public references and practical cost modeling, then point you to official sources where rules or rebates can change.
Owens Corning, GAF, and CertainTeed all sell shingles with a written warranty against one specific problem: blue-green algae. Owens Corning's runs 25 years on its main lines. CertainTeed's runs 10 or 15. If the dark streaks on your roof started inside that window, the manufacturer's warranty may cover the fix. So check the warranty before you call anyone with a sprayer.
The streaks themselves are rarely an emergency. GAF describes roof algae as harmless to the roof's function. It makes a roof look older than it is, and that's about all it does. The real decisions here are who pays to clean it, how it gets cleaned, and whether what you're looking at is actually algae.
First, make sure it's algae
Three things grow on asphalt shingles, and only one of them is purely cosmetic. Here's how they look from the ground, and what each one means.
| What you see | What it usually is | Does it harm the shingles? | Your move |
|---|---|---|---|
| Flat brown-to-black streaks running down the slope, no texture | Blue-green algae | No, per GAF; cosmetic only | Check the warranty, then clean only if the look bothers you |
| Green, fuzzy, raised clumps, often at shingle edges and shaded spots | Moss | Yes. GAF says it holds moisture, swells, and can lift shingles | Get it inspected and removed by a pro |
| Flat, crusty, gray-green rings or rosettes, tightly attached | Lichen | Can loosen granules where it grips | Get it inspected before anyone scrapes it |
| Bare black patches with no streaking, granules in the gutters | Granule loss, not growth | Yes, the asphalt is exposed to UV | See our granule loss guide |
The texture tells you most of it. Algae is a stain: it lies flat and follows the path the water takes down the roof. Moss has height and holds water against the shingle. GAF says that trapped moisture and swelling can lift shingle edges and open gaps for water. It also wears off the granules that protect the asphalt from sunlight. Algae doesn't do either of those things.
Use binoculars or your phone's zoom from the yard. Don't climb up for a closer look. A roof with algae or moss on it is slippery even when it's dry, and walking on it scrubs off granules either way.
Find out what shingles you have
Algae resistance comes from copper. CertainTeed's StreakFighter line mixes copper-containing granules into the surface blend, and the copper releases slowly over time. GAF's StainGuard Plus works on the same idea, with time-release copper granules. The copper keeps algae from establishing for a period, and the warranty covers that period.
The terms vary a lot by brand and product line:
| Manufacturer and line | Algae warranty term | Condition that changes it |
|---|---|---|
| GAF, StainGuard Plus (e.g., Timberline HDZ) | 25 years | Only on packages bearing the StainGuard Plus logo |
| GAF, StainGuard Plus PRO | 30 years | Offered only through GAF-certified contractors |
| Owens Corning, Oakridge, Duration, Woodcrest, Woodmoor | 25 years | Drops to 10 years if a qualifying Owens Corning ridge product wasn't used |
| Owens Corning, Supreme | 10 years | — |
| CertainTeed Landmark | 10 years | — |
| CertainTeed Landmark PRO, Premium, TL | 15 years | — |
That Owens Corning ridge condition matters more than it looks. If your roofer capped the ridge with a cheaper generic hip-and-ridge product, the 25-year algae term may already be a 10-year one. CertainTeed says it will pay, without proration, the reasonable cost to clean or replace shingles affected by blue-green algae during the warranty period.
Your closing paperwork, the roofer's invoice, or a leftover bundle wrapper in the garage should name the shingle line. If you can't find any of those, the roofer who installed it usually can. Here's the flat rule: if the roof is younger than its algae warranty term, file the claim with the manufacturer before you pay a cleaning company. Once a contractor has cleaned it, you've lost the evidence a claim depends on, and you may have paid for work the manufacturer would have covered.
Cost estimator
Roofing cost estimator
A budgeting range, not a quote. Move the inputs to see what changes the number.
Estimated national cost
Typical project lands near $11,750.
If a written quote comes in above the high end, that's not automatically wrong — ask what scope is driving it.
What this assumes
- Asphalt shingle material selected.
- 1,500 sq ft house size selected.
- Standard tear-off and replacement scope assumed.
What moves this price
- 01Roof pitch, height, access, and number of layers.
- 02Decking repairs, ventilation, flashing, and underlayment.
- 03Permit requirements, disposal fees, and local labor rates.
Walk into your quotes ready
This range is your anchor. Bring these to every contractor conversation so you can tell a fair quote from a padded one.
- Is this a full tear-off or a layover, and how many layers are on there now?
- What underlayment, flashing, and ventilation work is included?
- Who pulls the permit, and is disposal of old material in the price?
- What's the workmanship warranty versus the shingle manufacturer's warranty?
- A bid far below every other, usually meaning a layover or skipped flashing.
- Asking for a large deposit before any material is delivered.
- No written scope listing tear-off, underlayment, and ventilation.
If you're paying for cleaning yourself
Out of warranty, cleaning is optional. Algae won't shorten the roof's life, so this is a question of how much the look bothers you, and maybe of curb appeal before a sale.
If you do hire it out, the method matters more than the price. GAF's technical bulletin on algae staining calls for a diluted bleach solution applied at low pressure and rinsed off with low-pressure water. It also says to protect the plants below the eaves from runoff. That's the standard to hold a contractor to. Four things on a cleaning quote should make you stop:
A pressure washer. The granules on a shingle are what shield the asphalt from UV. A pressure washer strips them off. You'd be trading a cosmetic stain for real wear to the roof.
Scrubbing or brushing. Same problem, more slowly. GAF's bulletin relies on the solution's dwell time, not on abrasion.
A chemical the manufacturer hasn't approved. Ask what's in the tank. If your shingles are still under any warranty, check the manufacturer's cleaning guidance first.
A promise that it won't come back. It will. Cleaning removes the stain, not the conditions that let algae grow. On a shaded, humid slope, expect the streaks to return within a few seasons.
Professional low-pressure roof washing often starts around $300 for a small, single-story roof and can reach $1,200 or more. The price depends on the roof's square footage, its pitch, how many stories up it is, and how heavy the growth is. Moss removal usually costs more than algae, because it takes more time and care. Get two quotes priced per square foot so you can compare them.
Don't do this yourself. Roof work with a sprayer and a slick, bleach-wet surface is a fall hazard. GAF's own bulletin warns that wet shingles get slippery. A cleaning contractor with fall protection and liability insurance is worth the price here.
Keeping it from coming back
Two things slow regrowth, and you can price both at your next roofing visit.
Trim the shade. GAF names overhanging branches as a contributor, because algae favors damp, shaded surfaces. Cutting back limbs that shade the roof all day is often the cheapest prevention there is. Keeping gutters clear helps too.
Zinc or copper strips at the ridge. The idea is that rain washes metal ions down the slope, which does the same job as the copper in algae-resistant granules. A roofer installs them near the ridge, tucked under the shingles with a narrow edge exposed. They work best right after a cleaning, on a roof with years of life left. They won't remove existing streaks.
Don't re-roof for algae
This is where homeowners lose money. A roofer who looks at a streaked roof and quotes a full replacement is selling you something the algae didn't cause. Algae on its own is never a reason to replace a roof.
Replacement makes sense only when something else is wrong: bare asphalt from granule loss, lifted or curled shingles, moss damage, active leaks, or a roof near the end of its rated life. If one of those is present, the streaks are beside the point. Get the roof inspected for the real problem, and see our guide to reading a roofing estimate before you sign. If you are replacing anyway, specifying algae-resistant shingles with the matching ridge cap costs little extra up front and buys you 10 to 25 years without the streaks.
Research sources
- GAF: Technical Bulletin R-102, Algae Staining on Shingled Roof Surfaces
- GAF: What's the Best Way to Prevent Blue-Green Roof Algae Stains?
- GAF: Roof Moss Removal, How to Remove Moss on Roof Shingles
- GAF: StainGuard Plus Algae Protection Technology brochure
- GAF: Timberline HDZ Time-Release Algae-Fighting Technology announcement
- Owens Corning: Roofing Warranties
- Owens Corning: StreakGuard Algae Resistance
- CertainTeed: StreakFighter Algae-Resistant Technology
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