The black streaks running down your shingles are Gloeocapsa magma — a cyanobacteria, not dirt — which is why rain never washes them away and why roof cleaning is a chemistry job rather than a scrubbing job.
A living thing, eating your roof
Gloeocapsa magma feeds on the crushed limestone that shingle manufacturers use as filler. It arrives as airborne spores, establishes on the slopes that hold moisture longest — north-facing and shaded first — and spreads downhill as rain carries new spores with the runoff. That is why the streaks run vertically and widen over time.
The dark colour is the organism’s UV shield, a protective pigment it produces as the colony matures. By the time a roof looks streaked from the street, the colonisation is well advanced.
The sequence explains the pattern. Spores land on the whole roof, not just the streaked part, and only survive where the surface stays damp long enough for them to establish in the texture between granules. Every rain then washes living material further down the slope, which is why streaks start narrow near the ridge and fan out as they travel.
Why the north slope goes first
A north slope in Missouri gets the least direct sun, holds dew until mid-morning and in winter may not dry properly for days. That extra dwell time is the whole story. The south slope of the same roof takes the same spore load and the same rain, dries soon after sunrise, and stays clear for years longer.
If two slopes look like different ages, they are not — they are the same shingles with different drying times, and the north slope is your early warning for everything that eventually reaches the rest of the roof. The same rule governs walls, which is why the green film on siding also starts on the shaded side.
Why it matters beyond appearance
The colony holds moisture against the shingle surface, and it consumes the limestone filler as it feeds. Both accelerate granule loss — and granules are the sacrificial layer protecting the asphalt underneath from sunlight. A colonised roof ages measurably faster than a clean one.
There is also a heat effect: darkened shingles absorb more summer sun, which works against your cooling bills on the worst months of the year.
It helps to know what a shingle is: a fibreglass mat saturated in asphalt, with ceramic-coated granules pressed into the top face. The granules exist to stop ultraviolet light reaching the asphalt, and nothing else in the assembly does that job. As the layer thins, the asphalt hardens, loses flexibility and cracks along the courses. Anything that speeds up granule loss is taking years off the roof, whether or not it looks dramatic from the drive.
What the granules in your gutters are telling you
Scoop a handful of debris out of your gutter and look at it. This is the cheapest roof inspection available to you, and it decides whether cleaning is the right conversation at all.
A scatter of coloured granules mixed with grit and leaf litter is normal — shingles shed loose granules heavily in their first year and at a trickle for life. What is not normal is a sludge of granules lying in the bottom of the gutter like coarse sand, or a splash pad under the downspout gone the colour of your shingles. That is the roof surrendering its protective layer faster than it can afford, and no cleaning reverses it.
Judge it against what you can see from the ground. Shingles that lie flat and look uniform, with modest granule loss, are a candidate for treatment; curled edges, shiny black patches where the asphalt shows through and thin corners point elsewhere, and the honest comparison is laid out in our piece on whether cleaning or replacing is the better spend. If the gutter is so packed you cannot see the bottom, that is its own problem — a full gutter keeps the roof edge permanently wet and gives the colony a foothold to work back up from, which is why gutter clearing often goes on the same visit.
The fix, and the method that makes it worse
A low-pressure chemical treatment kills the organism outright — you can watch the staining lift within minutes on most roofs — and because the roots are dead rather than trimmed, the roof stays clean for years.
A pressure washer also removes the streaks, along with the granule layer, several years of roof life, and quite possibly your shingle warranty. The correct method is also the gentler one, which makes choosing it easy.
This is not a contractor’s opinion. The Asphalt Roofing Manufacturers Association — the trade body for the shingle makers themselves — advises against high-pressure washing of asphalt roofs and points instead to gentle, low-pressure cleaning with a dilute chlorine solution and a thorough rinse. That is what a soft wash is: garden-hose pressure at the surface, with chemistry and dwell time doing the work.
What the treatment does and does not touch
A soft wash kills organic growth on the shingle surface. It does not lift flashing, drive water under the courses or disturb ridge caps, because there is no force behind it to do any of those things. Skylights are rinsed like everything else and left clean; the failure mode there is not the solution but a seal that was already leaking, which a wet roof will find for you. Flashing at chimneys and valleys is metal and unaffected by the chemistry, though a competent operator will notice if it is lifting and say so.
Landscaping is a matter of preparation rather than luck. Beds under the eaves are pre-wetted, kept wet through the work and rinsed afterwards, so anything reaching them is heavily diluted. Downspouts get watched, because that is where runoff concentrates. The plants that get hurt on roof jobs are the ones nobody wet down first.
Algae-resistant shingles, zinc and copper
Copper is what suppresses this organism, and two products in common use exploit that. Algae-resistant shingles have copper-bearing granules blended through the surface layer. Rain dissolves a trace of copper as it passes, and that trace makes the shingle a hostile place to settle. It works, and it is worth having.
What it is not is permanent. The copper is a finite supply held in the granules, it leaches out over years of weather, and the protection fades with it — which is why algae-resistant roofs often start streaking later in life, on the north slope, like any other roof. Manufacturers warrant that performance for a limited period, shorter than the warranty on the shingle itself. If you were told your roof could not streak and it has, the copper has simply run its course.
Zinc and copper strips fitted just below the ridge cap work on the same principle from outside the shingle. Runoff picks up metal ions at the ridge and carries them down the slope. They help, with two limits: they only protect what the runoff flows over, so you still get streaking below dormers, chimneys and anywhere the water is diverted, and they do nothing to an already colonised roof. A strip is a way of holding a clean roof clean, not a way of cleaning a dirty one.
How this plays out in Southeast Missouri
This region is close to ideal habitat for the organism. Humid summers keep surface moisture available for months; river-valley fog around Cape Girardeau and Jackson sits on rooftops well past sunrise; and mature oak and pine cover over most of the older neighbourhoods means large parts of many roofs never see direct sun at all. Spring drop — pollen, catkins, needles — settles into the granule texture and holds damp against the shingle exactly where the colony wants it.
The pattern we see driving the radius out of Patton is that heavily wooded properties in the hollows around Marble Hill and Fredericktown streak years earlier than open-lot houses in the same postcode, and often only on one or two slopes. Freeze-thaw winters add the rest: water that gets into a moss cushion or a debris dam at the eave expands overnight and works at the shingle edge every time the temperature crosses zero.
The upshot is that the interval here runs short for shaded properties and long for open ones. Your neighbour’s roof tells you little about yours — the canopy over the house matters more than the town it stands in.
Common questions
Why is only one side of my roof streaked?
Because that side dries slowest. North and north-east slopes hold dew and shade longest, so spores that land everywhere only establish there. It is not a defect and it does not mean that slope was built differently.
My shingles are algae-resistant, so why are they streaked?
The copper granules that provide that resistance leach out over years of rainfall, and the protection fades with them. Algae-resistant shingles delay colonisation rather than prevent it permanently, and most of them start showing streaks once they are well into their life.
Do zinc strips at the ridge actually work?
Yes, within their reach. They release metal ions into the runoff, which suppresses growth on the slope directly below them. They do not help where water is diverted around dormers or chimneys, and they will not remove staining that is already there.
Will the streaks come back?
Eventually, because the spores are airborne and your roof stays where it is. But recolonisation has to start from nothing, which takes years rather than months. Cutting back overhanging branches and keeping the gutters clear stretches that interval further, as covered in our guide to how often a roof here actually needs treating.
Does the weather matter on the day?
It does. The solution needs dwell time on a dry surface, so we do not treat in rain or with rain imminent, and hard wind risks drift onto neighbouring property. Mild, still, overcast days are close to ideal.
Send a photo of your worst slope and we will tell you what stage it is at and what treating it costs. Read more about roof cleaning →