Start with the gentlest method and move up only if it doesn't work. Most exposed brick walls need nothing more than water and a stiff brush. Acid and high pressure are the last resort, because both can damage brick and mortar for good.
Before you start
- Identify the stain. White powdery patches, green or black growth and grey cement smears each need a different method.
- Test a hidden patch and let it dry fully before judging.
- Pre-wet the wall before any cleaner, so the brick doesn't soak it up.
- Use a nylon or natural bristle brush. Wire brushes leave rust marks and scratch the face.
Cleaning method by stain
| Stain | What it looks like | What to use |
|---|---|---|
| Dust and dirt | Dull, grey surface | Water and a stiff brush. Mild detergent for greasy areas. |
| Efflorescence | White, powdery patches | Dry brush only. Do not wash. |
| Algae and moss | Green or black patches on shaded, damp walls | Biocide or diluted bleach, then rinse |
| Cement or mortar smears | Hard grey film or lumps | Scrape lumps, then dilute brick acid cleaner |
| Soot and smoke | Black, oily marks near kitchens or chimneys | Degreaser and scrubbing |
| Paint | Splashes or old coating | Paint stripper meant for masonry |
White salt patches (efflorescence)
Efflorescence is salt carried out of the brick or mortar by water. It is very common on new walls and after the monsoon. Brush it off dry and collect the powder. Washing dissolves the salt and brings more to the surface as the wall dries.
If it keeps coming back, water is getting into the wall. Look for a leaking pipe, a missing drip edge on the parapet, rising damp from a failed DPC, or garden soil heaped against the wall. Fix the source first.
Algae and black monsoon staining
- Scrape off thick moss with a plastic scraper.
- Apply a masonry biocide, or household bleach diluted with water as the label directs. Protect plants below.
- Leave it for the time on the label, usually 15–30 minutes. Do not let it dry in hot sun.
- Scrub and rinse with plenty of water.
Shaded, north-facing walls grow algae fastest. Trimming overhanging plants and sealing the brick after cleaning slows regrowth.
Cement and mortar smears
Cement smears are common on new exposed brickwork when masons don't wipe joints as they go.
- Let the mortar cure for at least two weeks.
- Scrape off lumps with a wooden or plastic scraper.
- Soak the wall with water.
- Apply a dilute brick acid cleaner as the label directs, on a small area at a time.
- Scrub with a nylon brush, then rinse thoroughly.
Acid can burn the colour out of some bricks and attack mortar joints. Always test first, wear goggles and gloves, and never mix acid with bleach.
Soot and paint
For soot, use a degreaser or strong detergent with a stiff brush, working from the bottom up so streaks don't run over dry brick. For paint, use a paint stripper made for masonry, follow the dwell time on the label, scrape with a plastic scraper and rinse. Old, thick coatings may need two or three applications.
Pressure washing
A pressure washer helps on large walls, but only at low pressure with a wide fan nozzle. See pressure washing brick walls for safe settings.
After cleaning
Let the wall dry for a few days. For exposed brick, a breathable silane-siloxane water repellent helps keep out rain and slows algae. Avoid glossy, film-forming sealers, which trap moisture and can cause the brick face to flake.
