Those dark streaks running down the north side of a roof are the question we get more than any other. They're not dirt. They're a living organism called Gloeocapsa magma, and they're all over Central Kentucky.
Here's what they actually are, what cleaning them off involves, and what we do and don't know about how much they hurt the roof underneath.
What Gloeocapsa Magma Is
Gloeocapsa magma is a cyanobacteria. It's airborne, which is why the streaks almost always run downward from the peak of the roof. The wind drops the spores on the shingles, they take hold on the side that stays wettest the longest (usually the north slope), and once a colony starts it spreads.
The dark color isn't dirt and it isn't damage. It's a UV-protective sheath the bacteria builds around itself. The longer the colony lives there, the thicker that sheath gets and the darker the streak.
One reason this hits harder in Central Kentucky than in drier states is the limestone. The Bluegrass region sits on top of one of the largest limestone deposits in the country, and limestone is also the standard filler material in asphalt shingles. The bacteria can pull calcium and minerals from either source. Humid air, mature tree canopies, limestone in the soil, limestone in the shingles. It's a buffet.
How Much Does It Actually Hurt the Roof?
We want to be honest about what's settled and what isn't.
The Asphalt Roofing Manufacturers Association is the industry body whose members include GAF, Owens Corning, CertainTeed, and most other major shingle producers. Their official position is that algae discoloration is primarily an aesthetic issue and that there's no scientific evidence that algae damages asphalt shingles. You can read the full position on ARMA's site.
What we see in our work backs the aesthetic part of that. People call us because the roof looks bad. After a soft wash it doesn't. That's the main reason this service exists.
The thing we'd add from 17 years of doing this in Lexington is that a heavy bacterial colony holds moisture against the shingle long after the rain has stopped. Wet shingles don't last as long as dry ones. We can't prove cleaning extends roof life and we won't tell you it does. What we can tell you is that the math on a $599 to $1,299 roof wash compared to a $12,000 to $18,000 replacement is straightforward enough that most homeowners don't wait for the science.
What Pressure Washing Does to a Roof
If the bacteria question is open, the pressure washing question isn't. Both ARMA and every major shingle manufacturer say the same thing. Don't pressure wash an asphalt shingle roof. From ARMA's position document:
"Never use a pressure washer to clean an asphalt shingle roof as this will cause granule loss and very likely premature failure of the roof system."
The protective granules embedded in the surface of an asphalt shingle are what give it UV resistance and its rated lifespan. High-pressure water blasts those granules off. They end up in your gutters and on the ground for weeks afterward. That damage is permanent.
If a contractor offers to pressure wash your roof, hire someone else. We'd say the same thing if you came to us asking for it.
What a Soft Wash Actually Looks Like
Soft washing is the only method ARMA and the shingle manufacturers approve. The published recommendation is a sodium hypochlorite solution applied at low pressure, allowed to dwell on the roof, then rinsed off. That's what we do.
Our process on a typical job:
- Pre-soak every plant and bed within about 15 feet of the foundation with plain water before any chemistry hits the ground.
- Mix a stabilized sodium hypochlorite solution with a surfactant at the concentration appropriate for the staining and the roof age.
- Apply from a pump system at the ground using a soft-wash tip. No high pressure. No walking on the roof when we can avoid it.
- Let the solution dwell 15 to 20 minutes. The bacteria dies down to the root.
- Rinse with low-pressure water. Rinse the landscaping again on the way out.
The roof looks dramatically cleaner immediately. Whatever stain shadow remains continues to fade for two to four weeks as rain washes the dead bacteria off.
How Long the Wash Lasts
Three to five years on most Central Kentucky roofs. The variables are sun exposure and tree cover. A south-facing slope on an open lot in Hamburg or Andover can hold up for five plus years. A north slope under a Norway maple in Chevy Chase can be regrowing visible streaks inside 18 months.
If you have heavy tree canopy and want to push the cleaning interval longer, trimming branches that overhang the roof helps more than anything. Less shade means less moisture means slower regrowth.
What This Costs
Most residential roof cleanings in the Lexington area run between $599 and $1,299. The variables are roof size, pitch, accessibility, and how heavy the staining is. We quote every job individually and the price we give you is the price you pay. If we look at the roof and tell you it's not worth cleaning because it's near end of life, we'll tell you that too.
When to Call
If you can see the streaks from the street, the colony has been there at least a year. The earlier we treat it, the easier the job and the cleaner the result.
Call us at 859-312-0704 and we can usually quote the same day. If you can't see the roof clearly from the ground, send a few photos and we'll work from those.
