Mould and algae do not need a tropical climate. They need moisture, food, mild warmth and time, and a Southern Ontario summer delivers all four. Here is why growth can look sudden, where it shows up first on your house and what slows it down before a wash is needed.
In This Article
The four things exterior mould needs
Spores are in the air around your house all year, so the question is never whether they land. It is whether the surface gives them what they need to stay.
- Moisture that lingers. Not one shower, but a surface that stays damp for hours or days.
- Food. Pollen, dust, insect debris and leaf residue form a thin film that feeds colonies.
- Mild warmth. Growth picks up as temperatures climb out of the cold months and stays active through the warm season.
- Time. Colonies compound. A few cells become a film, and the film becomes a stain.
Our climate supplies each of these in turn, which is how a home here can go from clean to speckled in one season.
Why a humid July matters more than a hot one
Heat dries a wall. Humidity stops it drying. When the air is already loaded with moisture, damp siding and roofing give up their water slowly, sometimes not before the next evening arrives.
Night adds dew. On clear, calm nights the outside of a house loses heat and cools below the air temperature, so moisture condenses on it. Shaded walls, roof slopes that face open sky and the undersides of eaves collect it first. Add a thunderstorm or a run of drizzly days and a wall can stay wet for a week.
Rain also spreads growth. Runoff carries spores and nutrients down a roof or a wall, which is why streaks run in the direction the water flows.
That is a long stretch by mould standards. It is also why the worst weeks are often the muggy ones, not the hottest.
Black streaks creeping across your siding?
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Where it starts on your house
- North and east walls. Least direct sun, longest drying times.
- Behind shrubs, hedges and fences. Air barely moves, and leaves keep the wall damp.
- Below downspouts and dripping eaves. Splashback keeps the lower wall wet.
- Soffits, fascia and porch ceilings. Rain misses them, but dew and humidity do not.
- Caulk lines, window sills and corners. Water sits there and dust collects.
- Deck boards and fence rails. Flat surfaces hold water, and fallen leaves make it worse.
Check these spots first each summer. Our guide to black mould on siding shows what the dark specks look like up close.
Why it seems to appear overnight
Growth is invisible until it is dense. Colonies build for weeks as a thin, clear film, and then a warm, damp spell thickens them enough to show colour. To you it looks sudden. To the mould it was steady work.
Green tints are usually algae. Dark speckles are more often mould or mildew, and dark streaks on roofs come from a cyanobacterium, which we cover in our piece on black streaks on asphalt shingles. Homeowners lump them together as mould, and for cleaning purposes that is close enough. All of them are living growths, and a soft wash treats them the same way.
What to do when it is already on the wall
For a small patch on smooth vinyl, a soft brush, mild soap and water may lift a light film. Rinse well and keep the runoff off plants. Skip stiff brushes and abrasive pads, which scratch siding and leave rough spots where new growth grabs hold.
Do not reach for a pressure washer. Force strips the surface film but does not treat growth lodged in pores, and it can push water behind laps and into trim. Widespread or stubborn growth is what a low-pressure treatment is for.
Slowing it down before you need a wash
- Trim branches and shrubs so morning sun and moving air can reach the shady walls.
- Keep eavestroughs clear so water goes down the downspout, not down the siding.
- Add downspout extensions so splashback stays off the wall.
- Aim sprinklers at the lawn instead of the house.
- Keep mulch, soil and leaf piles from touching the siding.
These habits cut the moisture growth depends on. They do not remove growth that is already there. Once it is spreading across a wall or into the trim, soft washing treats it at the source, and D&D quotes are free and confirmed in writing before work begins. To time your habits and any booking around real conditions, check the local forecast from Environment and Climate Change Canada.
Key Takeaways
- Exterior growth needs moisture, food, mild warmth and time, and a Southern Ontario summer supplies all four.
- Humidity and overnight dew keep shaded walls, soffits and roof slopes wet long after a hot day has ended.
- Growth starts on north and east walls, behind shrubs, below downspouts and along caulk lines, then looks sudden once colonies are dense.
- Trimming for light and airflow, clearing eavestroughs and aiming sprinklers away cut moisture but will not remove existing growth.
Sources & References
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