Not every dark mark on a Kitchener house is the same thing, and the cause decides the cure. This guide shows how to tell algae, mould, drip stains and rust apart by where they sit, why the streaks run the way they do and what a soft wash will and will not remove.
In This Article
Read the streak before you blame the siding
Four different problems can look like "black streaks" from the driveway: living growth such as algae and mould, dirt that washes off ledges, rust from metal, and oxidation of the surface itself. Only the first is what soft washing treats. Location is your best clue, because each cause shows up in its own places.
A rough field guide: growth looks slimy or fuzzy and smears when touched, dirt is gritty and wipes away, rust is orange or brown, and oxidation is chalky and rubs off as a fine powder.
Where the dark marks sit, and what they usually are
| Where you see it | What it usually is | What deals with it |
|---|---|---|
| Vertical dark streaks down a shingle roof slope | A cyanobacterium often called roof algae | Low-pressure treatment of the roof, never scrubbing or pressure |
| Green film on shaded or north-facing siding | Green algae | Soft washing |
| Dark specks under eaves, on soffits and around trim | Mould or mildew | Soft washing |
| Black stripes down the face of an eavestrough | Dirty roof runoff carrying growth, sometimes surface oxidation | Cleaning the gutter face, plus treating the source |
| Grey tracks below window sills and ledges | Dirt washing off, not growth | Ordinary cleaning |
| Orange or brown trails from a nail head or metal part | Rust | A rust-specific treatment, not a soft wash |
Many houses have more than one cause at work, such as rust under algae or an overflowing eavestrough above a mouldy soffit. Work out each one separately.
Black streaks on the eavestrough are a common look-alike. Our note on black streaks on gutters explains that case.
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.
Why the streaks run the way they do
Gravity draws the pattern. Growth starts on a damp, shaded patch, and every rain then carries spores and nutrients down the surface. That is why a roof streak follows the slope, why siding stains hang below a drip edge and why the lower corner of a wall is often the worst.
Water sources add their own signature. A downspout that discharges near the wall leaves a fan of staining at splash height. A leaking or overflowing eavestrough leaves a stripe from the joint down. Read the direction of a streak and you can often guess where the moisture is coming from.
Why Kitchener homes see so much of it
- Humid summers. Warm, muggy weather and heavy dew keep surfaces damp for hours.
- Shade. Mature maples and oaks line many older streets, and shaded roof slopes and walls dry slowly.
- Close spacing. Where houses sit near each other, walls between them get little sun or breeze.
- Roof materials. Asphalt shingles are common here, and not all of them are algae-resistant. Check your roofing paperwork, or ask your roofer.
- Nearby water. Homes backing onto ravines, creeks or ponds tend to sit in damper air.
What a soft wash does, and does not do, about it
A soft wash applies a cleaning mix at low pressure, lets it work for the label's dwell time and rinses it away. It kills algae and mould on siding, trim and, with the right access and equipment, roofs. Stains that are only surface growth usually clear, sometimes with a day or two of rain to finish the job.
Roofs deserve a warning. Wet algae is slippery and a fall is serious, so roof work belongs with a trained crew. D&D's crew members are Working at Heights certified.
It will not remove rust, repair a pitted or eroded surface, or replace granules that a roof has already lost. Painting over growth traps it under the paint film, so clean first. A soft wash also cannot stop the next round of spores. For that, you fix the moisture: clear eavestroughs, extend downspouts and open up the shade.
Before you book: three checks
- Rub a small spot with a damp cloth. Dirt wipes off, growth smears, and rust leaves an orange mark.
- Trace the streak upward to its source: a roof edge, a joint, a downspout or a shady corner.
- Photograph it in daylight and note which sides of the house are affected.
Send those photos with your request for soft washing in Kitchener, or ask about roof cleaning if the streaks are on the shingles. D&D quotes are free and confirmed in writing before work begins.
Key Takeaways
- Dark marks on a house can be algae, mould, dirt, rust or oxidation, and only the living growth is what a soft wash treats.
- Location is the strongest clue: roof slopes, shaded walls, soffits, gutter faces, sills and nail heads each point to a different cause.
- Streaks follow the flow of water, so their direction often reveals the moisture source that needs fixing.
- A soft wash removes surface growth but not rust or lost shingle granules, and it will not stop new spores without drier conditions.
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