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What to check, and in what order

There is a white powdery deposit on my brickwork

The deposit itself is rarely the problem — what matters is whether it keeps coming back. Salts are carried to the surface by water moving through the masonry and left behind when it evaporates, so a deposit that returns after cleaning is telling you water is still moving. Removing it treats the symptom.
  • 6Candidate causes
  • Most commonMost likely
  • No — candidates onlyA diagnosis

Where this starts

A white bloom on masonry is salt, and salt on the surface means water has travelled through the wall and evaporated there, leaving behind what it was carrying. That is the mechanism for every cause below, and it explains why the useful question is not what the deposit is but whether it is still being produced.

NEW BUILD deposits are normal and self-limiting. Fresh masonry contains soluble salts from the bricks, the mortar and the water used, and the first months of drying bring them to the surface. It looks alarming, it is cosmetic, and it usually weathers away over a season or two without intervention.

RECURRING deposits are the ones that matter. A wall that is cleaned and blooms again is still passing water, and the salts are the visible end of a process that is also wetting the masonry, potentially freezing in it, and possibly reaching the inside. Here the deposit is a diagnostic, not a defect.

Two things are commonly confused with it. LIME BLOOM — sometimes called lime run — is calcium carbonate from mortar, harder, often in runs rather than a general bloom, and it does not simply brush off. And the crystalline salt band low on an internal wall is a different phenomenon associated with rising damp, which is diagnosed rather than assumed.

What it could be, and what tells them apart

These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.

  1. Normal drying out of new masonry

    Most common

    On work less than a year or two old, general across the elevation rather than local, brushing off dry, and reducing with each season.

    How to check: How old the work is, and whether it is getting better or worse. New masonry blooming and fading is doing what it does.

  2. Water getting in from above

    Most common

    Concentrated below a defect — a failed coping, a blocked gutter, a cracked sill, a missing flashing — rather than spread across the wall, and worse after rain.

    How to check: Look directly above the affected area. A deposit in a vertical band usually has something above it.

  3. Saturation from persistently wet ground

    Common

    Low on the wall, sometimes with the ground level raised above the damp-proof course, and present regardless of recent rain.

    How to check: Compare the external ground, paving or render level against the damp-proof course. Ground above it bridges the barrier.

  4. Water in the cavity or a bridged cavity

    Common

    A patch at a consistent height rather than at the base, often near a wall tie line, an opening, or where a cavity has been filled.

    How to check: Whether the band is at a consistent level across a run. A horizontal line of deposit often traces a course of ties or an obstruction.

  5. Lime bloom rather than efflorescence

    Common

    Harder, often in runs or streaks below joints rather than a general bloom, and it does not brush off dry.

    How to check: Try a dry brush. A deposit that resists it is carbonated lime and needs a different treatment from soluble salts.

  6. Salt contamination in the materials or the ground

    Less common

    Persistent, heavy, often crystalline rather than powdery, and sometimes with the brick face spalling behind it.

    How to check: Whether the masonry surface itself is breaking down. Crystallisation within the brick rather than on it is a different severity.

What to check, in order

The order matters — each step is here because of what it rules out for the ones after it.

0 of 5 checked
  1. 1

    It is the only observation that separates a cosmetic drying-out deposit from an active water path, and everything else follows from the answer.

  2. 2

    Low and level points at ground or damp-proof course; a vertical band points at something failing above it; a horizontal band mid-wall points at the cavity.

  3. 3

    Most local deposits have a source uphill — a coping, a sill, a gutter, a flashing — and it is usually visible from the ground.

  4. 4

    It separates soluble efflorescence from lime bloom, which look similar and are removed by completely different methods. Reaching for acid first can damage the masonry.

  5. 5

    Salts crystallising within the brick rather than on it cause spalling, and a face that is breaking down changes this from appearance to fabric.

The calculations behind this

Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.

Brick Calculator →Cavity Wall Weep Hole Spacing Calculator →Gutter Calculator →

When to stop and get somebody in

  • Where the deposit returns repeatedly after cleaning, which means water is still moving through the wall and the source has not been found.
  • Where brick faces are spalling, flaking or breaking down behind the deposit — that is damage to the fabric rather than a stain on it.
  • Before using any acid-based cleaner on masonry, which can damage bricks and mortar and can make salt problems worse by introducing more of them.
  • Where the deposit appears on the INSIDE of an external wall, which means moisture has crossed the wall rather than evaporated from its face.

This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.