How do I clean brickwork without damaging it?
Start with the gentlest method that works, why the pressure washer is almost always the wrong tool, and how to shift mortar splashes, algae and dirt without wrecking the brick face.
- Difficulty
- intermediate
- Time
- 1 hr 30 min
- Read
- 4 min
- Safety
- warning
Short answer
Work from gentle to aggressive, and stop as soon as it is clean enough. Water and a stiff bristle brush first, then a biocidal masonry wash for organic growth, then a proprietary brick cleaner for mortar splashes and stains — always with a test patch. Pressure washing is a last resort at low pressure, and on soft old brick it should not be used at all.
A brick has a fired outer skin that is denser and much more durable than the material behind it. Once that skin is broken, the brick absorbs far more water, weathers faster and spalls. Almost every wrong decision about cleaning brickwork ends up breaking that skin, which is why the professional rule is to use the mildest method that achieves an acceptable result and stop there.
Safety
Pressure washing brickwork is the commonest way a householder permanently damages their own house — it strips the fired outer skin of the brick, drives water deep into the wall and blows out pointing, and none of that is reversible. Acid brick cleaners are corrosive, damage skin and eyes, destroy glass, aluminium and plants, and give off fumes. Never mix cleaning products. Working at height on a wall means the ladder and access rules apply.
Step by step
- Work out what the dirt actually is.General atmospheric grime, green algae, black spot lichen, mortar splashes, efflorescence, rust, paint, and soot are all different and want different treatments. Cleaning a wall with the wrong product is at best ineffective and at worst permanent.
- Find out how soft the brick is first.Scratch an inconspicuous face with a coin. Modern hard facing brick resists; soft handmade or Victorian brick powders. Everything about method follows from this — a soft brick will not tolerate pressure, abrasion or strong acid.
- Always do a test patch.Somewhere inconspicuous, at the size the product recommends, and left for a week to see how it looks dry and after rain. Damage from cleaning frequently does not show for days.
- Start with water and a brush.Warm water and a stiff bristle — not wire — brush, worked over the surface, shifts a surprising amount of general dirt. It is slow, and it does no harm at all.
- Use a biocidal masonry wash for algae, moss and lichen.Green film, black spotting and moss are living growth, and killing them is more effective and far gentler than scrubbing them off. Apply the biocide, leave it the stated time, then brush off the dead growth. It also delays regrowth for a year or two, which scrubbing does not.
- Fix why it is green before you clean it.Organic growth means a wall that stays wet — a leaking gutter, a blocked downpipe, overhanging planting, deep shade, splashback from a hard surface, or a saturating wall. Cleaning without changing that just books the job again next year.
- Use a proprietary brick cleaner for mortar splashes and stains.These are usually acid-based and formulated for masonry. Dampen the wall thoroughly first, because acid drawn into dry brick keeps working and burns the face. Work small areas, keep it off the pointing as far as possible, neutralise or rinse per the instructions, and never leave it to dry on the surface.
- Take mortar splashes off mechanically where you can.Fresh splashes brush off once dry. Set ones can often be flicked off with the edge of a trowel or a wooden wedge, working parallel to the face. That is far kinder than acid, and it is why bricklayers brush down as they go.
- Treat the pressure washer as a last resort.If it is used at all, it should be at the lowest effective pressure with a wide fan tip, held well back, kept moving, and never held on the pointing. On soft, old or already spalling brick, do not use one. The damage — a stripped face, blown joints and a soaked wall — is permanent.
- Consider a specialist for a whole building.DOFF steam cleaning, TORC and other low-pressure systems clean historic masonry effectively without stripping the face. For a whole elevation of a period house this is the right answer, and it is not something to attempt with hired equipment.
- Protect everything around the work.Mask windows, doors and any metalwork before using an acid cleaner, sheet plants and lawn, and keep run-off out of drains and ponds. Acid run-off kills planting and etches glass in seconds.
- Repoint after cleaning, not before.Cleaning will find every weak joint, and it is much easier to repoint a clean wall. Doing it the other way round means cleaning your new pointing.
Common mistakes
- Pressure washing a Victorian brick house — It strips the fired skin off soft brick, blows out lime pointing and saturates a solid wall. The wall then absorbs far more water, spalls faster and dries slower. It is the single most damaging thing regularly done to period brickwork.
- Using a wire brush — It scratches the face permanently and leaves fine steel particles in the surface that rust and stain within weeks. Bristle only.
- Applying acid cleaner to a dry wall — The acid is drawn straight into the brick and cannot be rinsed out. It keeps reacting, burns the face and causes staining and salt problems later. Soak the wall first.
- Cleaning without finding the cause — Algae, moss and staining are symptoms of a wall staying wet. Clean it and the growth returns within a season unless the water is dealt with.