What Are the Black Streaks on My Roof?

By the EverGlow Exteriors Team

Those black streaks running down your shingles are algae — specifically a cyanobacteria called Gloeocapsa magna — not dirt, soot, or a failing roof. It feeds on the limestone filler in modern asphalt shingles and builds a dark protective coating against sunlight. That coating is the streak. It spreads a little every humid Iowa summer, and it will not wash off in the rain.

Good news: it’s removable, safely, without replacing anything. Here’s what’s happening up there and how to fix it without wrecking your shingles.

Why does my roof have it and my neighbor’s doesn’t?

• North-facing slopes get it first. Same reason the north side of your siding goes green — less sun, slower drying, happier algae.
• Shade. Tree cover keeps sections damp. Mature-canopy neighborhoods in Urbandale and Clive see it heavily.
• Humidity. Central Iowa summers give the organism months of ideal growing weather. Morning dew alone is enough moisture.
• Shingle age and type. More limestone filler, more food. Once one roof on the street has it, spores travel — which is why streaks move through a neighborhood over a few seasons.

Is it damaging my roof or just ugly?

Both, slowly. The algae is eating the limestone in the shingle and holding moisture against the surface — through every freeze-thaw cycle of an Iowa winter. Established colonies also pave the way for lichen and moss, which physically pry up shingle granules as they grow. None of this fails a roof this year. Over many years it shortens shingle life and keeps the roof damper and hotter than it should be.

The faster cost is the way it looks. Black streaks read as “old roof” from the curb — home shoppers and even insurance inspectors flag them — when the roof underneath may be perfectly sound. Cleaning is a fraction of the cost of the replacement the streaks make people assume you need.

The one thing you must not do: pressure wash it

This is the most important line in this post. The Asphalt Roofing Manufacturers Association (ARMA) says asphalt shingles should never be pressure washed. Pressure strips the ceramic granules that protect the asphalt from UV — the granules are the roof’s lifespan — and it can void your shingle warranty. A pressure-washed roof looks clean for a season and then ages years ahead of schedule.

If a company quotes you a roof “pressure washing,” that’s your cue to end the call. Method matters more here than on any other surface. (Full comparison: soft washing vs. pressure washing.)

The right fix: soft washing

A roof soft wash applies a cleaning solution — the approach ARMA itself points to is a sodium hypochlorite blend — at low pressure, lets it kill the algae, and rinses gently or lets rain finish the job. No granule loss, no blasting. The streaks fade as the dead organism releases and washes away; heavy staining sometimes takes a few weeks of rain to fully clear.

What it costs: professional roof cleaning generally runs $250 to $600 depending on roof size, pitch, and how established the growth is. Compare that against premature replacement of a $12,000-$20,000 roof, and the math isn’t close.

How long it lasts: typically 2-4 years in Iowa before streaks begin returning, longer on sunny exposures. Zinc or copper strips at the ridge can slow regrowth on some roofs.

What about moss and lichen?

Streaks are stage one. Left alone in a shaded, damp spot, the colony invites lichen (crusty gray-green patches that bond to the shingle surface) and moss (the green carpet, usually on heavily shaded slopes and along the north edge). These are worse than streaks: their root structures physically grip and lift granules, and moss holds a wet sponge against your shingles through every freeze-thaw cycle. Soft washing kills both, but dead lichen releases slowly — sometimes over months of weathering — so the visual fix isn’t instant. The practical takeaway: cleaning at the streak stage is cheaper and gentler than waiting for the moss stage.

Why this is not a DIY job

Even setting aside the $90/day rental machine being the wrong tool entirely — walking a pitched roof with a sprayer and wet chemicals is how homeowners end up in the ER. Roof algae solution also kills landscaping if it’s not diluted, applied, and rinsed correctly; pros pre-wet and protect everything below the roofline. This is one of the few jobs where we’ll flatly say: don’t do it yourself.

How to vet a roof cleaning company

1. Ask their method. The only right answer is soft wash, low pressure, no exceptions.
2. Ask for a certificate of insurance. People on your roof with chemicals — this is the job where insurance matters most.
3. Ask how they protect landscaping — pre-wetting and rinse-down should be part of the answer, not an upsell.
4. Check reviews for roof jobs specifically. Plenty of good driveway outfits have no business on shingles.

The short version

Black streaks = algae eating your shingles, worst on shaded north slopes, normal for Iowa, and spreading slowly. Never let anyone pressure wash them. A soft wash removes them for $250-$600 and buys years of clean roof. We soft wash roofs across the metro — West Des Moines, Ankeny, Grimes, Urbandale — and a photo from your driveway is enough for us to quote it.