What causes green stains on siding?
The green film on siding is usually algae. It grows where siding stays damp and shaded, and it’s especially common on north-facing walls, behind shrubs and under trees in Virginia’s humid summers.
Algae needs moisture and light, and it feeds on the thin layer of dust and pollen that settles on siding. It shows up first near the ground, under decks, behind downspouts and wherever sprinklers hit the house. It’s mostly a surface growth, which is why it usually responds well to a proper soft wash. Our article on how professionals remove algae from siding covers the process in detail.
What causes black stains on siding?
Black stains on siding usually come from one of four sources: mildew, runoff from a stained roof, dirt and soot, or artillery fungus spores from nearby mulch. The pattern of the staining is the best clue to which one you have.
- Mildew
- A surface fungus that shows as gray or black spotting, often in shaded, damp areas and under eaves. It can appear alongside algae.
- Roof runoff
- Long black streaks on shingle roofs are commonly caused by Gloeocapsa magma, a cyanobacterium. Rain can carry that staining down onto gutters and the siding below, leaving vertical streaks.
- Dirt and soot
- Gray or black film that’s heaviest near roads, driveways, grills or chimneys. It often feels gritty and appears evenly across a wall.
- Artillery fungus
- Tiny, hard black dots, like specks of tar, usually on the lower wall near mulch beds. The fungus grows in mulch and shoots sticky spore masses onto siding, cars and windows. These specks are stubborn and often don’t wash off completely.
Siding stain guide: what you see, the likely cause, and the typical fix
Use the color, texture and location of a stain to narrow down its cause. This table pairs common siding stains with their likely source and the approach professionals typically use.
| What you see | Likely cause | Typical approach |
|---|---|---|
| Green film, worst on shaded or north side | Algae | Soft wash with a cleaning solution, then a thorough rinse |
| Gray or black spotting, often under eaves | Mildew | Soft wash; reduce shade and moisture where possible |
| Vertical black streaks below the roofline | Runoff from roof staining (Gloeocapsa magma) | Soft wash siding; the roof itself needs its own low-pressure treatment or streaks return |
| Even gray or brown film, gritty to the touch | Dirt, road dust, soot | Soft wash with a detergent-based cleaner |
| Chalky white residue that rubs off on your hand | Oxidation or chalking of paint, aluminum or older vinyl | Gentle cleaning; heavy chalking may call for a specialty cleaner or repainting |
| Orange or brown streaks low on the wall | Rust from irrigation water or metal fixtures | A cleaner formulated for rust; standard house wash solutions usually won’t lift it |
| Tan or brown blotches near trees | Tannins from leaves, seeds and wood | Appropriate cleaner and rinse; often fades further with sun |
| Gray, fuzzy or greasy patch around a vent | Dryer vent lint and moisture | Clean the vent and wall; organic growth may colonize the area |
| Tiny hard black specks near mulch beds | Artillery fungus | Careful spot cleaning; may not fully remove |
Which siding stains aren’t caused by organic growth?
Oxidation, rust, leaf tannins, dirt and dryer lint are not organic growth, so the solutions used for algae and mildew won’t always remove them. Identifying these before cleaning avoids wasted effort and disappointment.
- Oxidation (chalking): as paint, aluminum siding and older vinyl weather, the surface breaks down into a fine powder. Aggressive washing can leave uneven streaks, so it needs a gentle approach.
- Rust: irrigation systems drawing from wells with iron in the water often leave orange spray patterns. Metal fasteners, railings and fixtures can also bleed rust down the wall.
- Tannin: leaves, acorns and wet wood release natural brown pigments, which also stain concrete. See how to clean concrete for the hardscape side.
- Dryer vent lint: lint mixed with warm, moist air collects on the siding around the vent and can hold moisture that encourages growth.
Why do black and green stains come back after cleaning?
Stains return when the conditions that caused them are still there: shade, trapped moisture, sprinklers hitting the house, an untreated roof, or overhanging trees. Cleaning removes the growth, and changing those conditions slows its return.
- Trim shrubs back so air can reach the siding.
- Adjust sprinklers so they don’t spray the walls.
- Keep gutters and downspouts flowing so water doesn’t spill onto siding.
- Clean lint from dryer vents regularly.
- Keep mulch beds from touching or crowding the lower siding.
How Ultimate Powerwash handles siding stains
Ultimate Powerwash looks at the stain, the siding material and its condition before choosing a method, because the right approach for algae is different from the right approach for rust or chalking. For most organic staining, that means a low-pressure soft wash.
We’re a Stafford-based company that has served homes and businesses since 2005, and our team has 30+ years of combined pressure washing experience. Learn more about our soft washing service, why soft washing is safe for siding when done correctly, or house washing in Stafford County.
The work is satisfaction guaranteed and quotes are free. Request a free siding cleaning quote, and feel free to send photos of the stains that concern you most.
Related questions
Is the green stuff on my siding mold?
Usually not. The green film on siding is most often algae, while mildew tends to show as gray or black spotting.
Why is only the north side of my house green?
Because it gets the least sun. Shaded, north-facing walls stay damp longer, which gives algae and mildew the moisture they need.
Will a house wash remove orange stains from sprinklers?
Not usually. Rust from iron in irrigation water is a mineral stain and typically needs a cleaner made for rust rather than a standard house wash solution.
Can black streaks on siding come from the roof?
Yes. Roof staining commonly caused by Gloeocapsa magma can wash down onto gutters and siding, so the streaks may return if only the siding is cleaned.
What are the tiny black dots on my siding near the flower beds?
They’re often artillery fungus spores launched from mulch. They stick tightly and may not come off completely with washing.