Those dark streaks running down your shingles are alive. They come from a wind-borne cyanobacterium called Gloeocapsa magma, and this guide explains why it settles on asphalt roofs, what harm it does and how a low-pressure soft wash removes it without scrubbing the shingles.
In This Article
Meet the organism
Gloeocapsa magma is a cyanobacterium, often called blue-green algae. It grows as a gel-like colony wrapped in a dark sheath that shields it from sunlight, which is why the stains look black or dark brown, not green. Its cells travel on the wind, so no roof is immune and nothing you did caused it to arrive.
Once a colony lands somewhere damp, it spreads outward. What you see from the street is a stain made of millions of these cells and their sheaths.
From the ground, with binoculars, you can sort the usual suspects. Algae streaks are smooth and even. Moss stands up in soft green tufts. Lichen forms rough, flat rosettes that look glued on.
Why shingles, and why in streaks
- A food source. Many asphalt shingles contain limestone filler, which algae can draw nutrients from, and moisture from dew and rain does the rest.
- Water carries it downhill. Every rain washes cells and nutrients down the slope, so the stain grows in vertical streaks that follow the flow.
- Shade and slow drying. North-facing slopes, valleys and areas under overhanging branches stay damp longest, so they darken first. Leaf litter and needles in valleys or behind chimneys hold water there too.
- Little natural resistance. Shingles made with copper-containing granules resist algae, but not every roof has them. Check your roofing paperwork or ask your roofer.
Black streaks creeping across your siding?
Our low-pressure soft wash removes algae, mould and black streaks without damaging your siding. Free, no-obligation quotes across Kitchener, Waterloo, Cambridge & Guelph, ready within 1 business day.
Is it only cosmetic?
Mostly, for the dark streaks themselves. They spoil the look of the roof and can make a home seem neglected, but the stain is not moss and it is not rot.
Two cautions keep that from becoming an excuse to ignore it. First, moss and lichen often arrive alongside the streaks. They hold moisture against the roof and can lift shingle edges, which is a real durability problem. Second, the wrong cleaning method does more harm than the algae. Scrubbing and pressure loosen the mineral granules that protect the asphalt, and most shingle makers advise against pressure washing, so read your warranty paperwork before anyone touches the roof.
How a soft wash removes it, step by step
- Plan the day. The crew picks a calm, dry, frost-free day, because damp or frosty shingles are dangerously slippery, and sets up fall protection and safe access.
- Protect the surroundings. Plants, windows, air intakes and painted surfaces below the roof are pre-wet or covered.
- Apply at low pressure. The cleaning mix goes onto the shingles with a light spray, from the roof or from the ground where it reaches.
- Let it dwell. The mix sits for the time the product label allows, without scrubbing.
- Rinse gently. Low-pressure water carries the loosened material off, and the plants below are rinsed again.
- Let the rain finish. Faint shadows can take a rain or two to fade as dead cells weather away.
Roofs are the highest-risk part of any exterior job. Wet algae is slippery, and a fall is serious. D&D's crew members are Working at Heights certified, which is one reason roof work should be left to a trained team.
What a soft wash cannot fix
It cannot replace lost granules, curling tabs or cracked shingles. If your eavestrough is full of gritty sand, or you can see bare black asphalt through the granules, the roof has aged, and no cleaning will reverse that.
It also will not stop future spores from landing. Trimming overhanging branches, keeping the roof edge and eavestroughs clear and choosing algae-resistant shingles at replacement all slow the return. If your roof is near the end of its life, get a roofer's opinion before you spend on cleaning. Washing a roof that needs replacing puts the money in the wrong place. For a fuller comparison of prevention products, see roof soft washing versus roof cleaning tablets.
Getting a roof quote
There is no published starting price for roof cleaning because pitch, size, access and how much growth is present all change the job. A free written quote is confirmed before any work begins. If the streaks are on your walls as well, ask about soft washing in the same visit.
Key Takeaways
- Black roof streaks are a wind-borne cyanobacterium, Gloeocapsa magma, whose dark sheath shields it from sunlight.
- It spreads down the slope with every rain and darkens shaded, slow-drying areas first.
- The streaks are mostly cosmetic, but moss and lichen are not, and scrubbing or pressure washing strips protective granules.
- A soft wash treats the growth at low pressure and cannot restore lost granules or stop new spores.
Request a home-service quote
Select the service you need and describe the work area and access.