Green, black, red or white staining on render and brick: what it is and how to treat it

Updated 6 November 2026 · By the Soft Wash team

Short answer: Most green, black and red or orange staining on render and brick is living growth, usually algae, mould or lichen, which soft washing removes. White powdery marks are usually efflorescence, rust-brown streaks come from metal fixings, and dark vertical streaks under sills are usually water run-off. These are not organic, so a biocide alone will not remove them.

Green, black, red or white staining on render and brick: what it is and how to treat it

Why staining matters

Knowing what a stain is tells you whether soft washing will remove it, whether something else needs fixing, and how quickly it will come back. Most exterior staining in the UK is biological, encouraged by damp, shade and the UK climate. Some is mineral or chemical, and those stains need a different approach.

Quick identification guide

What you see	Likely cause	Does soft washing remove it?
Green film or patches	Green algae	Yes, usually on the day
Black spots or patches	Black mould or black algae	Yes, usually on the day or within days
Small crusty grey, black or yellow circles	Lichen	Yes, but fades over weeks
Red, orange or rusty-looking patches on render	Red algae such as Trentepohlia	Yes, often fades over days to weeks
White powder or crust	Efflorescence from salts in the masonry	No, it is mineral
Brown or orange streaks from fixings	Rust from metal brackets, nails or bolts	No, needs a rust treatment
Dark vertical streaks under sills and copings	Water run-off carrying dirt and growth	Partly, but fix the drip detail too
Grey or black crust on old stone	Carbon from historic pollution	Usually not, needs specialist methods

Algae and mould

Green algae is the most common staining on UK render, brick and fascias. It thrives where surfaces stay damp: north-facing walls, areas shaded by trees or neighbouring buildings, and walls near leaking gutters or overflow pipes. Black mould and black algae often appear alongside it, particularly on lower walls and around sills.

Both are killed by soft washing. The biocide kills the cells and spores in the surface, which is why growth takes much longer to return than after a pressure wash.

Red and orange staining

Orange or rusty-red patches on render are often mistaken for rust, but they are usually red algae, commonly Trentepohlia, which is green inside but has orange pigments. It is common on render and painted masonry in damp, clean-air areas. A simple test is to wet a small area: red algae often looks brighter when wet. It responds to a biocide, though it can take a little longer to fade than green algae.

True rust staining runs in streaks from metal items such as satellite brackets, gate hinges, nails and wall ties, and needs a rust remover rather than a biocide.

Lichen

Lichen forms small, crusty circles, often grey, black, white or yellow, and is very common on roofs, walls and copings. It is a partnership between a fungus and an alga, and it bonds tightly to the surface. A biocide kills it, but the dead lichen often takes several weeks to fade or wash away. Scraping it off by hand risks damaging the surface underneath. On roofs and copings in particular, the faint pale circles left after the lichen dies can remain visible for a while before weathering evens them out. Lichen grows very slowly, so once it has been killed it usually takes several years to become noticeable again.

Efflorescence and run-off marks

Efflorescence is a white, powdery or crusty deposit left when water moving out of masonry evaporates and leaves salts behind. It is common on new brickwork and on walls that are getting wet. It is not organic, so soft washing does not remove it. It can usually be brushed off when dry and should fade as the wall dries out; persistent efflorescence suggests a moisture problem.

Dark streaks running down from window sills, copings and render beads happen where water runs off and down the wall instead of dripping clear. Cleaning removes the growth within the streaks, but they will return unless the water path is improved, for example with a sill that projects properly and has a drip groove.

What to do next

Take close-up photos of each type of staining
Note which walls are affected and whether they are shaded or near gutters
Check for leaking gutters, downpipes and overflows
Send the photos when asking for a quote so the right treatment can be planned

Fixing sources of damp, trimming back overhanging trees and clearing gutters will all help a soft wash last longer.

If you are unsure what a stain is, a small test area is the safest way to find out. A good contractor can treat a discreet patch and show you how it responds before cleaning the whole elevation, which also confirms the right solution strength for your surface.