Common brick stains and how they are removed

One of the most important principles in brick cleaning is that identification of the stain type must come before any treatment is chosen. The stains that commonly appear on London brickwork are not all the same in their chemistry, their depth of penetration or the approach needed to remove them. What removes efflorescence will have no effect on carbon staining. What removes algae will not touch black spot lichen. And using an acid cleaner on the wrong type of contamination can permanently worsen the appearance of the brickwork. The following guide covers the six most common brick stain types found on London properties and explains how each is correctly treated by a professional.

Carbon and traffic pollution staining

Appearance: Uniform darkening of the brick surface from grey to near-black. Most visible on properties fronting main roads or with unrestored Victorian brickwork.

Cause: Diesel exhaust particles, tyre rubber dust and airborne pollution particles that bond chemically to the porous surface of the brick over months and years. Unlike surface dirt, this contamination cannot be removed by water alone because the bond between the pollution particle and the brick silica is not water-soluble.

How it is removed: Chemical-assisted cleaning using an alkaline degreasant followed in some cases by a specialist cleaning agent. The wall is thoroughly pre-wetted, the product applied and allowed to dwell, then rinsed thoroughly. For deeper historical carbon build-up on Victorian brickwork, DOFF steam cleaning combined with a specialist chemical pre-treatment is the most effective professional approach.

Efflorescence (white salt deposits)

Appearance: White powdery or crystalline bloom on the brick face or mortar joints. Can appear as patches, dribbles or a uniform coating across sections of the wall.

Cause: Soluble salts within the brick or mortar dissolved by moisture and carried to the surface. As the water evaporates, the salts are deposited. Very common on new brickwork in the first 12 to 24 months as the wall dries out, and on older brickwork following water ingress. Externally, it is usually a cosmetic issue. Persistent or internally appearing efflorescence may indicate an underlying moisture problem requiring investigation.

How it is removed: Light efflorescence on dry brickwork can often be removed by dry brushing with a stiff plastic or natural-bristle brush. Do not use wire brushes. For persistent deposits, a proprietary efflorescence cleaner or dilute acidic solution can be used on dry brickwork. Pre-wetting is critical and the wall must be completely dry before treatment. Never paint over efflorescence as this traps moisture and causes rapid paint failure.

Algae, moss and lichen (biological growth)

Appearance: Green film (algae), thick fuzzy growth in joints (moss) or flat grey-black spots (lichen). Each type requires a different treatment strategy.

Cause: Organic material establishing on the porous brick surface in conditions of moisture, shade and low airflow. All three are common on north and east-facing London properties. Lichen is a symbiotic organism of fungus and algae that anchors itself with microscopic filaments called hyphae, making it far more resistant to removal than surface algae or moss.

How it is removed: Algae and moss respond well to biocidal cleaners and DOFF steam. Lichen requires a specialist sodium hypochlorite-based treatment applied neat and left to dwell for 2 to 4 hours until the black spots turn white, indicating the organism has been killed. DOFF steam is then used to clean the residue. For period brickwork, DOFF steam is preferred over chemical methods wherever possible.

Mortar staining and construction residue

Appearance: Grey smears, splashes or hazing across the brick face, particularly common on recently repointed sections or after construction work near the property.

Cause: Mortar splashes from bricklaying or repointing, construction dust and cement contamination that has bonded to the brick surface. Fresh mortar that has not fully cured is easier to remove than hardened deposits.

How it is removed: Fresh mortar contamination can sometimes be removed with careful brushing once dry. Hardened mortar and cement staining requires a dilute acid cleaner specifically formulated for masonry. The wall must be thoroughly pre-wetted, the product applied for a short dwell period, scrubbed with a stiff brush and thoroughly rinsed. Do not allow the acid product to dry on the surface as this causes white scum deposits that are harder to remove than the original mortar staining.

Iron and manganese staining

Appearance: Orange-brown streaks (iron oxidation, similar in appearance to rust) or dark brown to near-black staining (manganese). More common on certain brick types, particularly buff and pale bricks.

Cause: Iron compounds are drawn to the brick surface by moisture movement and oxidise when exposed to air. Manganese dioxide is added as a colouring agent during brick manufacturing and can be released at the surface in wet conditions. Rust from nearby metal fixings, railings or guttering can also cause brown-orange streaking on brickwork below the metal element.

How it is removed: In many cases, light iron and manganese staining will weather away naturally over time and a wait-and-see approach is recommended before any treatment. Persistent staining can be addressed with specialist masonry stain removers formulated for iron and manganese deposits. Treatment of rust from metal fixings requires addressing the source of the rust (repainting or replacing the metal element) before treating the staining on the brickwork.

Masonry paint and coatings

Appearance: Uniform or partially covering paint layer over the brickwork, often applied in the mid-20th century to modernise the appearance or conceal damaged brick.

Cause: Previous decoration using masonry paint. Non-breathable paints and renders trap moisture in the wall and cause significant long-term damage to the underlying brick and mortar. Removing these coatings to expose the original brickwork is a restoration objective for many London period properties.

How it is removed: Paint removal from brickwork requires specialist treatment and should not be attempted with standard pressure washing, which will damage the underlying brick. The correct approach depends on the paint type. Plastic-based paints respond to DOFF steam cleaning combined with specialist paint softeners. Lime-based paints can sometimes be removed with chemical treatment. The condition of the underlying brick must be assessed before removing paint, as the brick may have deteriorated beneath the coating and not be suitable for full exposure.