Black streaks, green cushions and grey-white crust are three different organisms with three different implications for your shingles. All three are treatable, but the right expectations for roof cleaning treatment depend on which one you have.
Algae: the streaks
Gloeocapsa magma shows as dark vertical streaking, usually starting near the ridge and widening downward. It is the most common, the most cosmetic in the short term, and the most responsive to treatment — most of the discolouration lifts as the solution works.
Left alone it degrades the shingle slowly, holding moisture and consuming limestone filler. Treat it and the roof typically stays clear for years.
Identifying it is mostly a matter of texture. Algae is flat. Run your eye along the slope from the side and the streaked area sits at exactly the same height as the clean shingle beside it — there is nothing to catch a shadow. It is also directional: it follows the path water takes down the roof, so it starts under ridge caps and around anything that concentrates runoff. If what you have is dark, flat and running downhill, it is algae, and the mechanism behind it is covered in more detail in our piece on what those black streaks actually are.
Moss: the cushions
Moss grows as green mounds along shingle edges and in the shaded gaps between courses. It is the most structurally urgent of the three: the root-like rhizoids work under shingle edges and lift them, and the cushions hold water against the roof like a sponge.
Moss dies quickly once treated, but it releases gradually — expect the cushions to brown, shrink and shed over following weeks of weather. Nobody should be scraping or blasting moss off shingles; that removes granules and lifts edges.
Rhizoids are not true roots — they do not feed the plant, they anchor it. That distinction matters because it means moss will happily anchor into anything with texture, including the granule surface and the gap under a shingle butt. Once anchored, the cushion works mechanically: it holds water, the water wicks under the course, and in winter it freezes. Ice expands, the shingle edge lifts a fraction of a millimetre, and next season the cushion has a slightly bigger gap to grow into. That cycle, repeated over several winters, is how moss turns a cosmetic problem into a lifted-edge problem and eventually a wind problem.
A lifted edge is also where wind gets a purchase. Shingles are designed to seal down to the course below; anything holding that seal open is a liability in a spring storm.
Lichen: the crust
Lichen — grey-white or pale green discs bonded tightly to the surface — is the stubborn one. It anchors into the granule layer itself, and mechanically removing it takes granules with it.
Treatment kills it, but dead lichen releases slowly, over weeks to months, and heavy colonisation can leave shadowing. An honest contractor tells you that before starting rather than promising an instant spotless roof.
The reason lichen behaves differently from the other two is that it is not one organism. It is a partnership: a fungus providing the structure and the grip, and an alga living inside it doing the photosynthesis. The fungal half grows into the surface it sits on, which on a shingle means growing between and around individual granules. That is why a lichen disc feels bonded rather than stuck, and why prising one off brings a coin-sized patch of granules away with it.
It is also why the timeline is long. Killing the organism ends the damage immediately, but the physical crust remains until weather breaks it down and rain carries it off, and where a disc has sat for years the shingle beneath may have faded differently from the surrounding surface. That shadowing is permanent. It is the honest ceiling on what any treatment can do, and worth knowing before you set your expectations against a photograph in an advert.
How to tell which one you have from the ground
You do not need to get on the roof, and you should not. Stand back far enough to see a whole slope, take the clearest photograph your phone will manage, and then zoom in on the image rather than on the roof.
Three tests settle it. First, height: algae is flat, lichen is slightly raised and crusty, moss stands proud enough to break the line of the slope. Second, shape: algae runs in streaks, lichen appears as scattered circular patches, moss forms irregular mounds concentrated along the horizontal lines where courses overlap. Third, colour: algae is charcoal to black, lichen is grey-white or sage, moss is a true green that turns rust-brown in a dry spell and greens up again after rain.
Most roofs that have gone a few years without attention have all three, in that order of arrival. That is normal, and it does not change the treatment — it changes the timeline for what the roof will look like afterwards.
What treatment does, and what it cannot do
All three respond to the same thing: a low-pressure application of a dilute chlorine solution with a surfactant, left to dwell, then rinsed at low pressure. The Asphalt Roofing Manufacturers Association, the trade body for the shingle makers, recommends exactly this approach and advises against pressure washing asphalt roofs — the method that works is also the one the manufacturers endorse.
What it does not do is remove anything mechanically. The organism dies; the weather takes the corpse away. Algae is gone visually within minutes to days, moss browns and sheds over weeks, and lichen can take months. Nothing legitimate speeds that up. Anyone offering an instantly spotless heavily lichened roof is planning to use force, and force on shingles costs granules.
Around the roof, a soft wash is deliberately undramatic. There is no pressure to drive water under flashing or up beneath the courses, skylights are simply rinsed, and beds and shrubs under the eaves are wetted before, during and after so that anything reaching them arrives heavily diluted. The parts of the job that need care are preparation and rinsing, not the cleaning itself.
Metal and tile roofs behave differently
On standing-seam metal, algae and lichen have almost nothing to grip; growth concentrates in the seams and low-slope panels where debris and water sit, and there are no granules to lose. On concrete tile and slate the surface is porous and heavily textured, which suits moss and lichen very well, and the fragility problem shifts from the coating to the tile itself — walking a tile roof breaks tiles. In both cases the chemistry is the same and the handling is not.
Moss and lichen in Southeast Missouri
Moss and lichen need more sustained damp than algae does, so around here they track tree cover almost exactly. The wooded hill country out towards Piedmont and Ironton produces the heaviest moss we see — mature canopy, north slopes that stay shaded most of the day, and a long humid season. Roofs on open ground a few miles away in the same weather may have streaking and no moss at all.
Two regional details make it worse. Heavy spring drop from oak and pine settles into the gaps between courses and rots down into exactly the sort of moisture-holding debris moss establishes in. And freeze-thaw winters give the cushions repeated opportunities to work at shingle edges — a moss problem in this climate is on a clock in a way it would not be further south.
The other thing worth checking on a moss roof is the eaves. Where gutters are blocked with the same debris, the roof edge never dries and the moss line thickens along the bottom course, which is why properties we treat around Marble Hill and Patton often need the gutters cleared in the same visit. Otherwise the treated edge simply starts again from a permanently wet foundation, a point covered further in our guide to how often gutters need clearing here.
Common questions
How do I tell moss from lichen without getting on the roof?
Photograph the slope from the ground and zoom in on the image. Moss is a true green, stands proud of the surface and gathers along the horizontal course lines. Lichen is grey-white or sage, sits flat and crusty, and appears as scattered discs.
Do I need to remove the dead moss myself?
No, and it is better if you do not. Dead cushions brown, shrink and shed with wind and rain over the following weeks. Scraping them off takes granules with them and can lift the shingle edges you are trying to protect.
Does moss on the roof mean it is leaking?
Not by itself. Moss is a sign the surface stays wet, not evidence that water is getting inside. But it lifts shingle edges over successive winters, and a lifted edge is where a leak eventually starts, which is why moss is worth dealing with sooner than algae.
Why does lichen leave a mark after it is dead?
Because it grew into the granule layer and shielded the shingle beneath it from sun for years. When the crust finally breaks down, that patch has weathered differently from everything around it. The shadowing fades but may not disappear.
Can you treat a roof that has all three?
Yes, and most roofs past a certain age do. One treatment addresses all three; what differs is how quickly each stops being visible. Judge the result at the six-week mark rather than the same afternoon, and see our guide to how often a Missouri roof needs treating for what to expect afterwards.
Photograph what is growing up there and we will identify it, tell you how urgent it is, and quote the treatment. Read more about roof cleaning treatment →