Those dark streaks aren't dirt — they're a living organism eating your shingles. Here's the safe, manufacturer-approved way to get rid of them.
If you've noticed dark streaks running down your roof — usually starting near the ridge and working their way down — you're not looking at dirt or water stains. You're looking at a living organism called Gloeocapsa magma, a cyanobacteria that feeds on the limestone filler in your asphalt shingles.
Left untreated, it spreads across your entire roof and shortens shingle life. The good news: it's completely removable — but only with the right method.
Why Black Streaks Are Worse Than They Look
- Gloeocapsa magma is highly contagious — it spreads from roof to roof via wind and birds.
- It holds moisture against shingles, which accelerates granule loss and UV damage.
- It can reduce roof life by 5–10 years if left untreated for multiple seasons.
- Insurance companies and home inspectors increasingly flag roof algae as a deferred maintenance issue.
- It significantly reduces curb appeal and perceived home value.
The ARMA-Recommended Removal Method
The Asphalt Roofing Manufacturers Association (ARMA) has a specific recommendation for removing algae from asphalt shingles: a solution of equal parts water and liquid chlorine bleach (sodium hypochlorite), applied at low pressure and rinsed off thoroughly. This is what the industry calls soft washing.
What ARMA explicitly advises against: power washing or pressure washing. High-pressure water strips the granules from shingles — and those granules are what protect the asphalt from UV degradation. Once they're gone, you can't get them back, and shingle life drops sharply.
If a roof cleaning company arrives with a pressure washer instead of a soft wash system, ask them to leave. The damage they'll do will cost far more than whatever you paid them.
Why This Isn't a Good DIY Project
You can technically mix a bleach solution and apply it to your roof yourself. But there are real reasons most homeowners hire professionals for this:
- Roof access is dangerous — ladders, steep pitches, wet surfaces, and no fall protection.
- Bleach solution needs to be applied at the correct concentration and dwell time.
- Runoff kills grass, plants, and landscaping if not properly neutralized.
- Improper rinsing leaves residue that can damage shingles or cause re-streaking.
- A professional brings commercial-grade equipment, proper PPE, and plant protection.
How Long Does Roof Cleaning Last?
A professional soft wash removes the algae and provides some residual protection from re-growth. In Cookeville's climate — humid, heavily treed — most roofs need cleaning again within 2–3 years. Copper and zinc strips near the ridge can slow re-growth after cleaning.
What About Moss and Lichen?
Black streaks (algae) are the easiest to remove. Moss — the green, fuzzy growth — requires more treatment and sometimes multiple applications. Lichen is the hardest: it forms a chemical bond with the granule surface and can take weeks to fully break down after treatment. Heavy lichen jobs typically come in at the higher end of the pricing range.
What Roof Cleaning Costs in Cookeville
| Up to 1,500 sqft roof area | $300 – $475 |
| 1,500–2,500 sqft roof area | $400 – $625 |
| 2,500–3,500 sqft roof area | $525 – $775 |
| Heavy moss or lichen | Add $75–$150 |