White powder on bricks: what efflorescence is telling you

The white powdery bloom on brickwork is efflorescence — salts that were already in the masonry, carried to the surface by water moving through the wall and left behind when that water evaporates. It is not mould, and the deposit itself does no harm. What it tells you is that water is getting into the wall and finding its way out through the face, and that is the part worth paying attention to.

Most of what ranks online for this is written for British and American housing, and a good deal of it attributes the damage to freeze-thaw cycles. That is a genuine mechanism in a cold climate and largely irrelevant across most of Australia. Here the escalation is salt attack, and the usual water source is the ground.

What is actually happening

Three things have to be present.

Soluble salts, which come from the bricks themselves, the mortar, the concrete, the soil, or in coastal areas from airborne salt.

Water, moving through the masonry.

An evaporation surface, which is the exposed face of the wall.

Water dissolves the salts, carries them through the pores of the brick and mortar, and reaches the surface. The water evaporates. The salts cannot, so they are left behind as a white crystalline deposit.

The deposit is a symptom. The water is the problem.

Efflorescence or mould?

These get confused constantly, and the responses are completely different.

Efflorescence is white to off-white, powdery or crystalline, and sits on the surface. It brushes off dry as a fine powder. It often appears or worsens after rain and can disappear on its own. It has no smell. It occurs on masonry, concrete and render — not on timber or plasterboard.

Mould is usually black, green or grey, though it can be white. It has a musty smell, feels slightly slimy rather than powdery when damp, and grows on organic surfaces — timber, paint, plasterboard, fabric, dust. It does not brush off cleanly.

The simplest field test: a small amount of water. Efflorescence dissolves and disappears momentarily as the salts go back into solution. Mould does not.

Where the water is coming from

This is the part that decides whether you have a passing nuisance or a problem.

Construction moisture in a new build. New brickwork contains a lot of water from mortar and from the bricks themselves, and it carries salts out as it dries. This is by far the most common cause on a house less than a couple of years old, and it usually resolves on its own — see how long a new house takes to dry out.

Ground levels above the damp-proof course. The most common cause on an established house. Soil, mulch, paving or a garden bed built up past the DPC lets ground moisture bypass the barrier and travel up into the wall above. Efflorescence appearing in a band low on a wall, particularly behind a garden bed, points here first.

Watering against the wall. Irrigation, a dripping tap, or a garden bed watered daily keeps one section of masonry permanently wet.

Poor site drainage. Ground falling toward the house, a downpipe discharging at the footing, or paving that ponds — all deliver water to the base of the wall.

Blocked weep holes. On brick veneer, weep holes drain the cavity. Covered by soil, render or mortar droppings, water accumulates inside the wall instead of draining out.

Failed or missing flashing. Above windows, at roof junctions, over openings. Water entering the cavity from above appears as efflorescence below.

A leak. A plumbing leak inside the wall, or a roof leak tracking down. Efflorescence in one localised patch, high on a wall, with no ground explanation, is worth chasing as a leak.

New build versus persistent

The distinction that matters most.

New build efflorescence appears within the first year or two, often across large areas fairly evenly, and diminishes each season. It is a nuisance rather than a fault, and the standard advice is to leave it alone and let the wall dry. Cleaning it early tends to bring more to the surface.

Persistent efflorescence keeps returning in the same place year after year, often in a band or patch rather than across the wall. That is an ongoing water source, and no amount of cleaning will change it until the water is dealt with.

If it is still appearing in the same spot in the third year, it is not construction moisture.

When it becomes a real problem

Efflorescence itself is cosmetic. What follows it may not be.

Salt attack — also called fretting or spalling — happens when salts crystallise within the brick rather than on its face. The crystals expand and break the surface apart from the inside. The brick face begins to flake, then crumble, and eventually erodes back. Once it starts it does not stop while the water keeps coming, and the damage is permanent.

Signs the situation has moved on: the brick face flaking or powdering rather than just a surface bloom, mortar joints eroding back, a deep white crust rather than a light dusting, and damp appearing on the inside face of the same wall.

At that point it stops being a cleaning question and becomes a damp investigation.

What not to do

Do not seal the brickwork. This is the most common and most damaging response. A sealer stops water leaving through the face but does nothing to stop it entering the wall. The moisture then evaporates deeper inside the masonry, and the salt crystallises behind the sealed surface — which is precisely the mechanism that destroys brick. Sealing efflorescent brickwork frequently converts a cosmetic problem into a structural one.

Do not paint over it. The paint will not adhere to a salt-laden surface and will blister and peel, and it creates the same trapping problem as a sealer.

Do not reach for acid without advice. Acid cleaners are used on masonry, but incorrectly applied they burn the brick face, damage mortar joints, and can mobilise more salt. If a wall needs acid cleaning, it is worth having someone do it who does it regularly.

Do not clean it repeatedly while it keeps returning. That is treating the symptom on a schedule.

What to do instead

1. Find the water first. Walk the wall. Check ground and mulch levels against the damp-proof course, look for a garden bed or paving built up against the brickwork, check where downpipes discharge, and look for weep holes that are covered or missing.
2. Lower the ground. If soil or mulch sits above the damp-proof course, cutting it back below is often the entire fix.
3. Clear the weep holes so the cavity can drain, and keep them above adjacent ground level.
4. Fix the drainage. Ground falling away from the building, downpipes discharging properly, no ponding against the wall.
5. Move the irrigation away from the base of the wall.
6. Then wait. Once the water is dealt with, the wall needs time to dry and to bring out the salts already dissolved in it. Cleaning before that is wasted effort.
7. Clean dry. A stiff bristle brush on a dry wall removes surface efflorescence without adding water. Wetting it dissolves the salt back into the brick, where it will reappear.
8. Get advice if the brick face is deteriorating. Flaking, fretting or crumbling brick is past the point where cleaning helps, and the underlying damp needs identifying properly.

Common questions

Is efflorescence dangerous?

The deposit itself is not. It is not mould, it is not a health hazard in the way mould can be, and it does no harm sitting on the surface. What it indicates — water in the wall — can become damaging over time.

My house is brand new. Should I complain?

Efflorescence on new brickwork is common and usually resolves within a year or two. Raise it with the builder so it is on record, but expect the answer to be that it is a normal part of drying out. If it is still there in the third year, that changes.

Will it come back after I clean it?

If the water source is still there, yes — and often more strongly, because cleaning with water redissolves salt and draws more to the surface. That is why finding the water comes first.

Is white on the inside of my wall the same thing?

It can be. Efflorescence on internal plaster or paint indicates moisture moving through the wall from outside or below, and is generally a stronger signal than the same deposit on the outside face. Worth investigating rather than repainting.

What about efflorescence on my concrete driveway or pavers?

Same mechanism, same causes — water moving through the concrete and evaporating at the surface. Usually a drainage question. It is cosmetic, and on new concrete it typically fades over the first year or so.

Does it mean I have rising damp?

Not necessarily. Rising damp is one possible source, but so are a leak, a blocked cavity, a failed flashing, or simply new construction. The position and pattern are the clue — a low band around the base of a wall suggests ground moisture; an isolated patch higher up suggests something else.

Where to go next

Efflorescence is one of the more useful things a house does, because it makes an invisible water problem visible on the outside of the wall.

Read next: how long a new house takes to dry out if the brickwork is recent, or the end-of-winter roof and gutter check for the drainage that usually causes it. Or start at Preventative Maintenance for the wider picture.

The white bloom is the wall showing its working. Read it, find the water, and the deposit takes care of itself.