Black streaking is algae, a flat stain that follows water flow. Green fuzz that sits proud of the shingle is moss, which has root structure and holds water. Flat crusty patches in gray, green, or orange are lichen, which bonds into the surface. All four are identifiable from the ground with binoculars and a phone camera, and telling them apart determines whether the roof needs cleaning, a different method, or a roofer.
Key Takeaways
- Black streaks that run downslope are algae, the most common and most treatable
- Moss stands above the shingle surface and holds water; it is the one that causes damage
- Lichen bonds into the granule layer and is the hardest to remove without harming the shingle
- Identify from the ground with binoculars, not from a ladder or the roof
- Photograph the north elevation, the valleys, and anything near flashing before requesting service
What Each Roof Growth Pattern Looks Like
Four things commonly grow on a roof in this region, and they look different enough to tell apart without getting close.
Black streaking (algae). Dark gray or black staining that runs vertically down the slope, widest at the top and tapering as it descends. It is flat, level with the shingle surface, and it follows the path water takes. In this region it is almost always *Gloeocapsa magma*, a cyanobacteria that feeds on the limestone filler in asphalt shingles. It is a staining problem rather than a structural one, and it is the most responsive to cleaning.
Moss. Green, sometimes brownish, and clearly three-dimensional. It stands above the shingle surface and looks like small cushions or a carpet. It starts at the shingle laps, in valleys, and along the north-facing lower courses, because those hold moisture longest. Moss has root-like structures that grip into the granule surface and it holds water against the shingle, which is what makes it the growth that actually causes damage.
Lichen. Flat, crusty, irregular patches that look almost painted onto the surface. Colors range from pale gray-green to orange or yellow. Lichen is a symbiosis of fungus and algae and it bonds chemically into the granule layer rather than sitting on top of it. It is the hardest of the four to remove, and aggressive removal takes granules with it.
Surface debris and organic film. Leaf litter, pine needles, seed pods, and the dark organic film they leave behind. Not growth, but frequently mistaken for it, and it feeds the other three.
Where to Inspect From the Ground
You can assess almost any residential roof accurately from the ground with binoculars. Walk the full perimeter and look at these areas specifically.
- North-facing slopes. Least sun, slowest drying, first to show every kind of growth. If a house has a growth problem, this is where it starts.
- Under overhanging trees. Canopy drops organic material and blocks drying air simultaneously.
- Valleys. Where two roof planes meet, water concentrates and debris collects. Moss almost always appears here first.
- Lower courses above the gutter line. These stay damp longest after rain and take splash from a full gutter.
- Around chimneys, dormers, and skylights. Flashing joints, and areas where water is diverted and slowed.
- Below any upper roof that drains onto a lower one. Concentrated flow paths grow their own stripe.
- North and east dormer cheeks, which are frequently the shadiest square footage on the whole roof.
Do this on an overcast day if you can. Direct sun flattens the contrast and hides exactly the texture difference you are trying to see.
How to Distinguish Staining From Attached Growth
The practical distinction is whether the thing you are looking at sits on the surface or stands above it, because that determines the method and the risk.
Look at the edge of the patch. Staining has soft, diffuse edges that fade into the shingle. Attached growth has a defined boundary you can see a shadow line at.
Look at it from an angle rather than straight on. From directly below, moss and algae look similar. From an oblique angle along the slope, moss casts a shadow and algae does not.
Look at whether it follows water or ignores it. Algae streaks follow the flow path down the slope. Moss and lichen colonize where moisture sits regardless of flow direction, so they appear in patches rather than stripes.
Look at the gutter. Green material accumulating in the gutter below a slope usually means moss is present and shedding, even if the roof reads as stained from the ground.
Look at color under magnification. Lichen's crusty, often orange or pale gray-green appearance is distinctive once you have seen it once.
The reason this matters: algae staining responds to a low-pressure soft wash and appropriate dwell time. Moss needs the same treatment plus time for the dead material to release and wash off, often over subsequent rains. Lichen frequently does not fully release without taking granules with it, and an honest assessment says so rather than promising a uniform result.
What Roof Details and Photos to Record
A short set of photographs turns a vague service call into a scoped one.
- One wide shot per elevation from far enough back to show the full slope, taken on an overcast day
- A zoomed shot of the worst area, ideally at an oblique angle so texture is visible
- The valleys, individually
- Anything near flashing: chimney, dormers, skylights, plumbing vents
- The gutter line and the ground below it, showing whether green material is shedding
- Any area where the shingle surface looks bare or a different shade, which can indicate granule loss
- The trees, so whoever quotes it can see the canopy situation
Alongside the photos, note the roof material (asphalt architectural, three-tab, slate, cedar shake, metal, synthetic), its approximate age, whether it has been cleaned before and by what method, and whether there has been any leak history.
If the material is slate or cedar, that changes the method substantially. See why slate roof cleaning needs a different approach.
When Not to Climb or Disturb the Roof
The identification above is deliberately designed to be done from the ground, because almost nothing about a roof is worth a fall.
Do not get onto the roof to inspect growth. The information gained is marginal and the risk is not. Do not pull moss off by hand or with a rake or brush, which takes granules with it and can lift shingle edges. Do not attempt to pressure wash a roof under any circumstances, which strips granules, drives water under the courses, and voids most shingle warranties. Do not walk a roof that is wet, mossy, steep, or made of slate or tile, all of which are unsafe or damage-prone or both.
If a slope genuinely cannot be assessed from the ground, that is a reason to have someone look at it properly, with an aerial survey or appropriate access, rather than a reason to climb.
What to Include When Requesting Service
When you call, the useful information is:
- Roof material and approximate age
- Which elevations are affected and how badly
- Which of the four categories you believe you are seeing, and that it is your read rather than a certainty
- Whether green material is coming off into the gutters
- Any leak history, past repairs, or replaced sections
- Whether the roof has been cleaned before, when, and how
- Tree canopy situation and whether trimming is possible
- Your photos
A provider should come back with a method for the growth type present, an access plan, a plan for where the runoff goes since roof solution lands on the walls and plantings below, and a realistic statement of what the roof will look like afterward. Lichen in particular does not always fully clear, and being told that in advance is a sign of a straight assessment.
Request an Exterior Cleaning Consultation
Correct identification changes the method, the price, and the realistic outcome. WashLabs Exteriors assesses roof growth before proposing anything and uses low-pressure soft washing rather than pressure on any roof surface.
Call (610) 665-3995 or request a free consultation. See our roof cleaning, soft washing, and gutter cleaning services.