The black streaks on your roof are most likely algae. They grow where moisture sticks around, which is why they favor shaded and north-facing slopes, and they show up on both tile and shingle. Washing them off at low pressure with a cleaning solution is the gentler approach. Scraping and pressure washing can do more harm than the streaks ever did.
Here’s what’s going on up there and what to do about it.
What are those black streaks?
Roof streaking is usually a living growth. The most common culprit is a dark algae that spreads down the roof in long vertical lines, following the path rain water takes. Mildew and moss can join it, especially in damp corners and under trees.
You’ll often see it start as a faint gray shadow, then darken over a season or two. From the street it can look like dirt or an oil stain. Up close it’s growth sitting on the surface of the roofing.
Why do some slopes get streaks and others stay clean?
Algae needs moisture and doesn’t get much of it in direct sun. So the pattern on a roof tells you a lot.
| Where it shows up | Why |
|---|---|
| North-facing slopes | Little direct sun, so the surface stays damp longer after rain, dew or sprinklers |
| Under trees and overhangs | Shade plus leaf litter that holds moisture against the roof |
| Valleys and low spots | Water collects and runs there more often |
| Near the eaves | Damp air and overspray from sprinklers or irrigation |
| Behind chimneys and vents | Small shaded pockets that dry slowly |
That’s why one side of a house can look nearly black while the other looks fine. In the Valley’s dry summers a sunny slope dries out fast. A shaded one in a Thousand Oaks oak neighborhood or on a tree-lined street in Valencia holds on to moisture much longer.
Do tile and shingle roofs both get it?
Yes. On concrete tile the growth settles into the textured surface and the low edges where water runs. On composition shingle it can cling to the granule surface. Many homes in Porter Ranch and Calabasas have tile roofs, and homes in Granada Hills and Woodland Hills have a mix of tile and shingle. Both show streaks, and the cleaning approach differs only in how carefully we work around the material.
Is roof algae damaging my roof?
We can’t tell you it’s destroying your roof, and we won’t scare you into it. What we can say is that growth holds moisture against the surface, and over many years that can be hard on some roofing materials. On shingles, a heavy coating can sit on the same granule layer that protects the shingle. Streaks also make a roof look older and dirtier than it is.
If you’re worried about the actual condition of your roof, ask a roofer. We clean roofs. We don’t do structural or roof repairs.
Why not scrape it or pressure wash it?
This is the part we feel strongest about.
Scraping with a brush or tool pulls at the surface. On shingles, the granules are the protective layer, and they come loose easily. On tile, scraping can chip edges and scratch the finish.
High pressure is worse. The force can strip granules from shingles, crack or chip tile, and drive water up under the roofing where it doesn’t belong. It also tends to leave clean, bright stripes where the nozzle passed, so the roof ends up looking blotchy.
And the roof itself is a risk. A wet, algae-covered slope is slick. Tile can break underfoot. Homeowners with ladders and pressure washers get hurt every year, and we’d rather you didn’t join them.
What does a softwash do instead?
Softwashing uses low pressure and a cleaning solution. We apply the solution, let it break down the algae and mildew, and rinse it away. The solution does the cleaning, so the water doesn’t have to hit hard.
That’s the same approach we use on stucco and house exteriors. It’s gentler on tile and shingle, and it handles the long streaks that a brush would only smear. You can read more on our house washing and softwash page.
A few practical notes:
- We work on the roof, and we’re Licensed & Insured.
- We protect and rinse plants and landscaping around the house before and after.
- If you have solar panels, we clean those separately by hand with purified water. We don’t use the softwash solution on panels.
- We clean the roof. We don’t patch, seal or replace it.
Do I need to worry about solar panels on a streaky roof?
Panels and algae often share a roof, and the shade pattern that helps algae also shows up around arrays. Dark streaks right below the lower edge of a panel are common because water runs off the glass and across the roofing. Clean the roof and the panels in the same visit and the whole thing looks even. Our solar panel cleaning is a separate hand wash, and we can schedule both together.
How do you keep it from coming back?
We won’t promise how long a roof stays clean, because it depends on shade and moisture. But you can slow regrowth.
- Trim branches back so the roof gets more sun and less leaf litter.
- Keep gutters and valleys clear so water drains instead of pooling. Our gutter cleaning covers that.
- Adjust sprinklers that spray the roof edge or fascia.
- Sweep or have someone clear leaves and needles off the roof surface.
- Check the shaded slopes each fall, before the rainy season, when moisture returns.
How often should a roof be cleaned?
There’s no single number. A sunny, dry roof in an open Simi Valley tract might go years between cleanings. A tile roof under oaks or on the north side of a hill can start streaking again sooner. We’d rather look at your roof and tell you what we see. It’s also worth checking in the fall, as part of our before-the-rains checklist.
If birds are roosting on the ridge or nesting at the tile ends, bird proofing is a separate fix, and cleaning comes first.
Get a roof look and a free estimate
Send us a photo of the shaded side of your roof, or have us take a look. Estimates are free. Call (818) 913-1626 Monday through Saturday, 7am to 7pm, see our house washing and softwash page, or find your neighborhood on the cities we serve page.