Green or Black Growth on Exterior Walls?

Short answer

Green or black growth on an exterior wall is algae, mildew or moss living on a surface that stays damp. Green film and green streaks are usually algae. Black, grey or speckled spots are usually mildew. Moss grows at the base of walls and in joints. It shows up on shaded walls (often north-facing), walls close to trees, fences or plants, and anywhere a gutter, downspout, sprinkler or splashback keeps the surface wet. The growth itself is mostly cosmetic, and it can be cleaned off with a suitable cleaner and a gentle rinse, never a pressure washer on stucco or EIFS. It comes back unless the wall is allowed to dry: cut back plants, fix the water source, and use a coating with mildew resistance when you repaint.

A shaded stucco wall and patio behind a house in Scarborough before FAIME refinished it, with growth in the damp paver joints — A shaded wall and patio in Scarborough before FAIME refinished it. Shade, plants and paving that stays wet are what let growth take hold.

Algae, mildew, moss or just dirt?

What you are probably looking at

Green film or green streaks, often below windows or along the base: usually algae
Black, grey or brown speckles in a spotty pattern: usually mildew
Green, cushion-like growth in joints, at the base of the wall or on paving: moss
Crusty grey-green or orange patches on masonry: lichen

When it means more than cleaning

Growth concentrated below a window, a sill or a roof line, pointing at a water source
Growth together with cracking, efflorescence or peeling paint
Stucco or EIFS that feels soft, sounds hollow or bulges where the growth is
Musty smells, mould or damp patches inside on the same wall

The bleach drop test

Not sure if it is mildew or dirt? Put a drop or two of diluted household bleach on a dark spot in an out-of-the-way place and wait a couple of minutes. Mildew lightens or disappears. Dirt and soot stay the same. Wear gloves and eye protection, keep it off plants, and never mix bleach with other cleaners.

Why this wall and not the others

Algae, mildew and moss need moisture, shade and time. They grow where a wall stays wet long after rain has stopped:

North-facing and shaded walls that get little direct sun to dry them out.
Walls close to trees, hedges, fences or a neighbouring building, which block sun and air movement.
Plants, soil and mulch against the wall, which hold moisture right at the surface.
Water from gutters, downspouts and sprinklers soaking the same section again and again.
Splashback at the base from hard paving, and poor drainage away from the wall.
Textured finishes such as stucco, which hold moisture and dirt in their texture longer than a smooth surface.

If one elevation is green and the others are not, the difference is almost always sun and airflow. See why paint and stucco fail on one side first.

How to clean it without damaging the wall

Use a cleaner made for exterior algae and mildew, suited to the surface (stucco, EIFS, brick, siding or paint), and follow the label.
Wet plants nearby first and protect them, and keep the cleaner off windows and metal where the label says so.
Apply at low pressure and give it time to work. A garden sprayer and a soft brush do most of the work.
Rinse gently. A garden hose, not a pressure washer.

Why not pressure-wash it?

High pressure erodes stucco and EIFS finishes, strips texture, drives water into cracks and joints, and can force water behind the finish, which is exactly the moisture the growth was feeding on. On brick it can damage mortar and softer units. On painted surfaces it can lift sound paint. Low-pressure washing with the right cleaner removes growth without those risks.

Stopping it from coming back

Let the wall dry

Cut plants and branches back from the wall, pull soil and mulch away from the base, and open up airflow where you can.

Fix the water source

Clear and re-pitch gutters, extend downspouts away from the wall, redirect sprinklers, and deal with splashback and grading at the base.

Repair what the damp has damaged

Cracks, failed joints and caulking, and hollow or soft areas get repaired before the wall is refinished, or they keep feeding moisture into it.

Repaint with the right coating

When the wall is repainted, a coating with mildew and algae resistance, suited to the surface and breathable on masonry and stucco, applied over a properly cleaned and dried wall.

Is exterior mould a health risk?

Growth on the outside of a wall is mainly a maintenance and appearance problem. The concern is when the same wall shows signs of moisture inside: musty smells, stains, peeling paint or mould on interior surfaces. That means water may be getting into the wall assembly itself, and it should be looked at from both sides. For concerns about mould inside your home, speak to a qualified mould or indoor air professional.

Where FAIME fits

Stucco and EIFS repair, caulking and sealant work, and exterior painting are self-performed FAIME scopes across Toronto, North York, Scarborough, Vaughan, Richmond Hill, Markham, Mississauga, Brampton and the wider GTA. When we refinish a wall that has been growing algae or mildew, we deal with why it stayed wet before anything new goes on. Our Scarborough stucco project shows a shaded, weathered wall ground back, meshed and refinished.

$2M general liability · Active WSIB (Class G5) · 2-year workmanship warranty
Durabond and Sto for stucco and EIFS; Sherwin-Williams, Benjamin Moore and Dulux coatings

Frequently Asked Questions

What is the green stuff growing on my stucco?

Green film or streaks on stucco are usually algae, which grows on surfaces that stay damp, especially shaded, north-facing walls and walls close to plants or trees. It is mostly cosmetic and can be cleaned with a suitable cleaner and a gentle rinse, but it comes back unless the wall is allowed to dry.

How do I tell mildew from dirt on my exterior wall?

Put a drop or two of diluted household bleach on a dark spot in an out-of-the-way place. Mildew lightens or disappears within a couple of minutes; dirt does not change. Wear gloves and eye protection, keep it off plants, and never mix bleach with other cleaning products.

Can I pressure-wash algae off stucco?

It is not recommended. High pressure erodes stucco and EIFS finishes, strips texture and drives water into cracks and behind the finish. A cleaner made for exterior algae and mildew, applied at low pressure and rinsed gently with a garden hose, removes growth without the damage.

Why does the north side of my house turn green?

The north side usually gets the least direct sun, so it dries slowest after rain, and algae, mildew and moss need exactly that: damp surfaces and little UV. Trees, fences, plants and gutter or sprinkler water on that side make it worse.

How do I stop algae from coming back on my walls?

Let the wall dry: cut back plants and branches, keep soil and mulch off the base, and fix gutters, downspouts, sprinklers and splashback. When you repaint, use a coating with mildew and algae resistance over a properly cleaned and dried wall.

Does FAIME clean and refinish walls with algae or mildew in the GTA?

FAIME repairs and refinishes the stucco, EIFS and painted surfaces underneath, and fixes the caulking, sealant and water problems that keep a wall damp, as self-performed scopes across Toronto, Vaughan, Richmond Hill, Markham, Mississauga, Brampton and the wider GTA. FAIME is WSIB Active under Class G5, carries $2M general liability and backs its workmanship with a 2-year warranty. Site visits are free.

Green or Black Growth on Your Walls? Why That Wall Stays Damp