Where this starts
A brick wall is not a monolith. It moves with temperature and moisture, it settles, and it gets wet and dries out repeatedly. The mortar is the part designed to absorb all of that: softer than the brick, able to accommodate small movements without cracking the units, and permeable enough that the wall dries through the JOINTS rather than through the brick faces.
THAT IS WHY SOFT IS CORRECT. A traditional lime mortar is far weaker than the brick beside it, and it is meant to be — it erodes slowly, is renewed by repointing every few decades, and in the meantime it protects the bricks. Replacing it with a strong cement mortar reverses the relationship: the joint becomes harder than the brick, so the movement and the salt and the freezing go into the brick instead, and the brick faces start to blow. A wall that was losing a little mortar is then losing its bricks, which cannot be repointed.
SO THE FIRST QUESTION IS WHAT IS FAILING. Mortar receding evenly across an elevation over a long period is normal weathering and is a repointing job when it gets deep enough — commonly when the loss approaches the depth of the joint's thickness. Brick faces spalling, flaking or sounding hollow is a different and more serious finding.
THE SECOND QUESTION IS WHERE. Failure concentrated below a defective sill, under a leaking gutter, at the base of a wall, or in one patch is being fed by water arriving, and repointing it without fixing the feed simply restarts the clock. Failure on one elevation only usually means the weather side.
THE THIRD IS HOW FAST. A wall pointed five years ago that is already failing is a materials or workmanship question — the wrong mix, pointing done in frost, or joints that were not raked out deep enough to hold.
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.
Ordinary weathering over a long period
Most commonEven, shallow recession across a whole elevation, worst on the exposed face, on a wall that has not been pointed in decades and with the bricks themselves sound.
How to check: Whether the bricks are intact. Mortar going and brick staying is the wall working as designed.
A previous repair in too strong a mix
Most commonHard grey pointing standing proud of eroded brick faces, brick edges spalled away beside intact joints, and often a visible patch where the repair was done.
How to check: Which is harder, the joint or the brick. Pointing that outlives the brick beside it is the cause of the damage, not a repair of it.
Water being fed to one area
CommonFailure concentrated under a sill, a coping, a leaking gutter or a downpipe, or in a band at the base of the wall, with staining or growth nearby.
How to check: Look directly above the worst area for a water path. Damage under a defect is being fed rather than weathering.
Freeze-thaw on a saturated wall
CommonDamage that worsens markedly after cold winters, on a wall that stays wet — north-facing, shaded, or behind heavy planting.
How to check: Whether the wall dries between spells of rain. A wall that never dries is being broken up from inside each frost.
Salts crystallising in the joint
Less commonWhite deposit with powdery, friable mortar behind it, often near ground level or where a chimney has been used, and recurring after cleaning.
How to check: Whether a white bloom keeps returning. Salt moving through and crystallising in the joint breaks it up mechanically.
Pointing done in the wrong conditions
Less commonRapid failure of recent work, sometimes across a whole repointed area, often traceable to work done in frost, in strong sun, or into joints not raked out deeply enough.
How to check: How old the pointing is. New work failing quickly is a workmanship question rather than a weathering one.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
Mortar receding with sound bricks is the wall behaving correctly. Spalled or hollow-sounding brick faces are damage to the units, and usually mean a hard repair is driving it.
- 2
It settles the hard-mix question without tools. A joint you cannot mark beside a brick you can is the wrong way round.
- 3
Defective sills, copings, gutters and downpipes feed local failure, and repointing beneath one of them buys a few years at most.
- 4
Repointing is usually warranted when recession approaches the joint's own thickness; shallower than that is cosmetic, and raking out a sound joint does more harm than leaving it.
- 5
Recent work failing is a mix or a workmanship question; nothing for fifty years is simply due, and the two lead to completely different conversations.
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.
When to stop and get somebody in
- Where brick FACES are spalling, flaking or sounding hollow, since that is damage to the units and repointing over it traps the problem.
- Where the building is old, solid-walled or listed, because the mortar specification is a conservation decision and a cement repair on a lime wall causes the damage it was meant to prevent.
- Where failure is concentrated at a structural point — under a beam bearing, at a lintel end, or in a line rather than an area — which is a movement question rather than a weathering one.
- Where the wall is a chimney, a parapet or a retaining wall, all of which are wet on more than one face and fail differently from an ordinary elevation.
- Where the affected area is at height, because repointing is slow work at arm's length and the access usually costs more than the pointing.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.