What’s the green and black stuff on my house?

If the shady side of your house has turned green or developed black spots, you’re not alone. It’s one of the most common things we’re asked about in Christchurch.

Green film: algae

A green, sometimes powdery film is usually algae. It grows where surfaces stay damp and don’t get much sun, like south-facing walls, under eaves and behind shrubs.

Black spots and streaks: mould and mildew

Black or dark grey spots are usually mould or mildew. You’ll often see streaks running down from windowsills and gutters, where water runs and dirt collects.

Crusty grey, white or yellow patches: lichen

Lichen is a combination of fungus and algae that grips tightly to surfaces. It’s most common on roofs, fences and concrete, and is the hardest growth to shift.

Is it harmful?

Growth holds moisture against paint and cladding, which shortens the life of your paintwork. On paths and decks it makes surfaces slippery, which is a real safety risk in winter.

How to remove it properly

Scrubbing or blasting removes the surface but often leaves the roots behind, so it grows straight back. A soft wash kills the growth with a cleaning solution, then rinses it away at low pressure, which is gentler on your home and lasts longer.

Frequently asked questions