Black Streaks on Your Roof: Algae, Moss, or Just Dirt? A Delta & Surrey Fall Guide
You notice it from the end of the driveway on the first clear afternoon after a week of rain. Long dark stripes run down the north slope of the roof, darkest near the bottom, almost like someone spilled coffee off the ridge. The south side looks fine. The neighbour's roof has the same stripes in the same spot.
Most Delta and Surrey homeowners assume it is dirt, soot, or the shingles "just getting old." Sometimes that is true. More often on a shaded Lower Mainland roof, those streaks are alive. Knowing which one you are looking at decides whether you should ignore it, clean it gently, or deal with it before it turns into moss.
What those dark streaks usually are
On asphalt shingle roofs in wet coastal climates, the most common cause of black or dark-grey streaking is roof algae, a type of airborne blue-green algae (often called Gloeocapsa magma) that spreads easily from roof to roof. It settles on the shingle surface, grows where the roof stays damp longest, and darkens as it builds a protective outer layer against UV. That is why it looks black rather than green.
It tends to show up in a predictable pattern:
- North- and east-facing slopes, where the sun dries the surface last
- Below tree cover, where branches drip and shade the roof long after rain stops
- Streaks that start high and widen as they run down, following the path water takes to the eave
- The same pattern on nearby houses, because the spores travel on wind and runoff
If you have ever seen a clean, light band right under a metal flashing or vent while the rest of the slope is streaked, that is a strong clue too. Runoff from some metals makes it harder for algae to settle directly below them.
Algae vs moss vs dirt vs worn shingles
These four look similar from the street and need very different responses. Here is a quick way to tell them apart:
| What you see | Likely cause | What it means |
|---|---|---|
| Flat black or dark-grey vertical streaks, no texture | Roof algae | Mostly cosmetic early on, but it holds moisture and spreads |
| Raised green or brown cushions, especially at shingle edges and seams | Moss | Holds water, lifts tabs, sheds debris into gutters |
| Pale grey-green crusty spots that look painted on | Lichen | Grips tightly; needs patient, gentle treatment |
| Even grey haze that wipes or rinses off, worse near trees or roads | Surface dirt and pollen | Usually not a growth problem |
| Bare, shiny, or patchy spots with grit piling in the gutters | Granule wear | Shingle aging; worth having a roofer look |
Two things are worth saying plainly. First, algae and moss often show up together. Algae is frequently the early stage on a slope that later grows moss, because both love the same damp, shaded conditions. Second, granule loss is not a cleaning problem. If you see a lot of shingle grit in your gutters along with bald patches, cleaning will not reverse that, and a roofer should take a look.
For the moss side of the story, including why fall is the window to clear it, see our guide to fall roof moss removal.
Why the streaks get worse in fall
Algae grows year-round on the coast, but fall stacks the odds in its favour:
- Shorter, cooler days. Slopes that used to dry by noon in July now stay damp most of the day.
- Leaf and needle litter. Debris sitting on the roof and in valleys holds moisture right against the shingles.
- Overflowing gutters keep the bottom courses wet. The lowest rows of shingles are often the darkest, and a slow eave is a big reason why. If your gutters have screens, they still need attention; see do gutter guards still need cleaning.
- The long wet season is next. Whatever is growing in October has November through March to spread.
That is also why roof streaks rarely show up alone. The same damp, shaded side of the house usually has a green band on the fascia and soffits too, which we cover in fall fascia, soffit, and eave cleaning.
Why a pressure washer is the wrong tool for shingles
It is tempting to grab a pressure washer, climb up, and blast the stripes off. Please do not. On asphalt shingles, high pressure can:
- Strip the protective granules that shield the shingle from sun and weather
- Force water up under the courses, where it is not meant to go
- Leave clean "wand stripes" that look worse than the algae did
- Put you on a wet, slippery pitch with a hose pulling at your balance
Roof algae is a growth problem, not a dirt problem, so the fix is treatment rather than force. That is where soft washing comes in: low pressure, cleaning solutions designed for organic growth, and controlled rinsing so runoff does not hammer your garden beds or siding. Hard surfaces like concrete driveways can handle more pressure. Roofs and painted surfaces need the gentle approach. Our pressure washing vs soft washing guide explains the difference in more detail, and you can see both methods on our pressure washing page.
What a proper roof algae clean looks like
A good roof clean should leave the shingles intact and the slope evenly lighter, not patchy. On a typical Delta or Surrey home that means:
- A ground-level and roof-edge look first to separate algae, moss, lichen, and wear before anything gets sprayed
- Protecting or pre-wetting plants and landscaping below the eaves
- Soft-wash treatment on the affected slopes, with time for the solution to work rather than scrubbing or blasting
- Moss and debris removal from edges and valleys where it has taken hold
- Clearing the gutters and downspouts afterward so the loosened growth and grit do not just sit in the trough. Pairing roof work with gutter cleaning keeps it to one visit.
- Flagging anything that looks like a repair issue, such as lifted tabs, damaged flashing, or heavy granule loss, so you know before winter
Expect the streaks to fade as the treatment finishes working rather than vanishing the moment the crew leaves. Heavier growth can take a little time after the visit to fully lighten.
Our roof cleaning work, which covers moss removal, algae treatment, and stain removal, sits under our residential exterior services. Strata buildings and commercial roofs get the same careful approach through our commercial services.
Slowing the streaks down once the roof is clean
Algae spores are in the air everywhere on the coast, so no one can honestly promise a roof that stays streak-free forever. You can make it a lot harder for growth to come back:
- Trim branches that shade or drip on the roof. More sun and airflow is the single biggest help.
- Keep the roof and valleys free of leaf and needle piles. Debris is a sponge.
- Keep the gutters flowing so the lower courses dry out between storms.
- Check the shaded slopes once a year, ideally in early fall, and treat early growth while it is still light.
Ready to find out what is really on your roof?
If the north side of your Delta or Surrey roof has started to look striped, fall is the right time to sort out what it is and deal with it calmly, before the long wet season gives algae and moss months to settle in.
Pricing depends on roof size, pitch, access, and how much growth is up there. Send a couple of photos of the streaked slopes when you get a free quote, and we will tell you whether you are looking at algae, moss, dirt, or something a roofer should see.