Summary

Cracked mortar is one of the most common brickwork callbacks a builder gets, and it's also one of the most frequently misdiagnosed — because "the mortar is cracking" can describe anything from a cosmetic hairline crack that's been stable for a decade to the first visible sign of a failing foundation. The distinction matters enormously: repointing a cosmetic crack is an hour's work; repointing over an active structural crack just delays the real problem and wastes the customer's money.

The starting principle for any mortar diagnosis is the same one that governs mortar specification in the first place: mortar is designed to be the weakest and most sacrificial part of a masonry wall. It's meant to crack, flex, and be repointed over the building's life rather than force the bricks themselves to crack or spall. So a certain amount of fine cracking in old mortar isn't a defect — it's the mortar doing its job of absorbing movement that would otherwise damage the brick. The problem cases are where cracking is excessive, active (still widening), patterned in a way that indicates a structural cause, or accompanied by other symptoms like brick spalling, damp, or door/window distortion.

This article covers the specific mechanisms that cause mortar to crack and how to tell them apart on site, without repeating full mix-ratio guidance — see mortar mixes for designation and mix selection, and cracked walls if the cracking pattern suggests the brick or the wall itself, not just the mortar, is affected.

Key Facts

Quick Reference Table

Spending too long on quotes? squote turns a 2-minute voice recording into a professional quote.

Try squote free →
Crack Pattern Location Likely Cause Structural? Typical Fix
Fine, random hairline Joint surface only Rapid drying, over-strong mix, poor curing No Monitor; repoint if widening
Regular vertical, full-height Long unbroken wall runs Missing/inadequate movement joint No (but recurring) Cut and install movement joint
Horizontal, mid-joint External wall, any level Sulphate attack Not immediately, but progressive Rake out, confirm sulphate source, repoint with SRPC
Horizontal, regular intervals (~300mm) External wall, both leaves Cavity wall tie corrosion Yes Wall tie replacement, then repoint
Diagonal, staircase along joints Corners, extension junctions Differential settlement Yes Monitor/structural engineer before repointing
Diagonal from window/door corner Around openings Lintel deflection or local settlement Usually yes Investigate lintel/opening before repointing
Fine crazing, entire face New pointing, first weeks Curing too fast in heat/wind No Cosmetic; rarely needs remedial work
Cracking with brick face spalling Individual bricks Mortar too strong for the brick No (mortar/spec issue) Rake out hard mortar, repoint with matched-strength mix

Detailed Guidance

Diagnostic decision tree

MORTAR CRACK FOUND — START HERE
              |
              v
IS THE CRACK IN THE MORTAR JOINT ONLY, OR DOES IT
CONTINUE INTO THE BRICK/BLOCK?
              |
     ---------+----------
     |                    |
     v                    v
JOINT ONLY            INTO THE BRICK
     |                    |
     v                    v
WHAT PATTERN?        Almost certainly structural —
     |                see fault-finder/structural/
     |                cracked-walls; STOP mortar-only
     |                diagnosis and get engineer input
     v
  -----------+-------------+--------------+---------------
  |          |             |              |
  v          v             v              v
RANDOM    REGULAR       HORIZONTAL     DIAGONAL
hairline  VERTICAL      pattern        from opening
  |       (full height)    |              |
  v          |             v              v
Curing/   No movement   Sulphate OR   Check lintel/
mix       joint present  wall tie     settlement —
issue —      |           corrosion —  NOT a mortar
monitor      v             |          defect
          Install         v
          movement    Confirm cause:
          joint       mid-joint =
          (see        sulphate;
          expansion-  ~300mm bands
          joints)     = wall ties

When it's genuinely just the mortar mix

A mortar that's too strong for its context is a common, entirely avoidable cause of cracking and spalling. The classic mistake is using a hard 1:3 OPC mortar (M12 designation) on soft handmade or reclaimed facing bricks, or on a pre-1919 solid wall that needs a lime-based system. Hard mortar doesn't accommodate the brick's own thermal and moisture movement — the stress goes somewhere, and because the brick is softer than the joint, it's usually the brick face that cracks or spalls rather than the joint itself. If you see cracking accompanied by spalled brick faces (flaking, crumbling brick surface next to intact-looking mortar), suspect an over-strong mix before anything else. The fix is raking out the offending mortar and repointing with a designation matched to the brick — see mortar mixes for the matching table.

The opposite problem — mortar that's too weak — shows up differently: soft, friable, easily-picked-out joints, often with surface erosion rather than defined cracks. This is a durability problem more than a cracking problem, but weak mortar can also crack under normal loading it should be able to withstand.

When it's a missing movement joint

Long, unbroken runs of clay brickwork need vertical movement joints because clay brick undergoes irreversible long-term expansion after firing, plus ongoing thermal and moisture-driven movement. Without a joint to absorb this, the wall cracks at a weak point instead — often at a corner, a change in wall thickness, or simply wherever the accumulated stress finds the path of least resistance. This shows up as a full-height (or near full-height) vertical crack, sometimes at surprisingly regular spacing along a long elevation if there are multiple weak points.

NHBC Chapter 6.1 sets typical spacing of 4.5–9m centres for movement joints in clay brickwork, narrower for more exposed or darker-coloured brick (which experiences greater thermal movement) and wider for more sheltered, lighter brick. If a wall longer than roughly 6m has no visible movement joints at all, that's the first thing to check when cracking appears on an otherwise sound wall. See expansion joints for joint spacing, width, and correct detailing — critically, a movement joint must never be filled solid with mortar; it needs a compressible backer rod and flexible sealant, or it stops functioning as a movement joint entirely.

When it's sulphate attack

Sulphate attack is a slower-developing, chemically driven cause of cracking that's often missed because it doesn't look like conventional shrinkage or movement cracking. Soluble sulphates present in some clay bricks (or in groundwater/contaminated ground) react with the tricalcium aluminate in ordinary Portland cement over months to years, forming expansive compounds (notably ettringite) that grow within the mortar and crack it from the inside — typically producing a horizontal crack that appears mid-way through the joint depth, sometimes with a slightly powdery or crumbly texture and white crystalline deposits.

This is most common in chimneys (repeated wetting/drying cycles plus flue condensate), below-DPC brickwork, retaining walls, and any masonry in prolonged contact with damp ground. Confirming sulphate attack usually requires a lab test of a mortar sample, though the horizontal, mid-joint crack pattern combined with a damp or below-ground location is a strong site indicator. The fix is raking out the affected mortar fully and repointing with sulphate-resisting Portland cement (SRPC to BS EN 197-1) — using ordinary OPC again will simply repeat the failure over the following years. See mortar mixes for SRPC mix guidance.

When it's wall tie failure — not really a mortar problem at all

Regular horizontal cracks appearing at intervals of roughly 4–5 brick courses (approximately 300mm) across a cavity wall are the signature of corroding steel wall ties, common in properties built between the 1920s and 1980s with galvanised steel ties that have since rusted through their protective coating. As the steel corrodes, the resulting rust occupies significantly more volume than the original metal, and that expansion physically splits the mortar bed at tie level in both the inner and outer leaves.

This is critical to recognise because repointing this crack pattern without addressing the ties is pointless — the mortar will crack again at the same level as the remaining ties continue corroding, and in the meantime the wall's structural tie between leaves is progressively weakening. Confirm with a borescope inspection or an electromagnetic wall tie survey, then arrange stainless steel remedial tie replacement before any repointing. See cavity wall ties for the full diagnosis and replacement process.

When it's curing or weather damage during construction

Mortar that dries too quickly — from direct sun, wind, or an overly absorbent (unwetted) brick pulling water out of the mix before it can hydrate properly — doesn't achieve its designed strength and develops a network of fine shrinkage cracks, sometimes visible within days of laying. This is largely a workmanship issue: bricks with high suction should be dampened before laying in hot weather, and freshly laid brickwork should be protected from direct sun and wind for the first few days.

Frost damage is the cold-weather equivalent: mortar that freezes before it has gained sufficient strength (typically within the first 48 hours, at temperatures below approximately 2–5°C) suffers irreversible loss of bond and strength as ice crystals disrupt the curing cement matrix. Frost-damaged mortar often looks fine initially but crumbles, cracks, or fails to bond properly once thawed. Winter brickwork needs either an accelerating admixture appropriate to the mix, protective covering/heating, or a decision to postpone laying until temperatures are reliably above freezing overnight.

When the mortar crack is telling you about something worse

The most important diagnostic skill in this whole topic is recognising when "cracking mortar" is not actually a mortar problem at all — it's the visible symptom of movement in the structure underneath. Stepped diagonal cracks following the joint pattern, cracks that widen at one end (tapering), cracks associated with sticking doors or windows, or cracks that continue from the mortar joint straight through the brick itself are all signs to stop treating this as a repointing job and start treating it as a structural investigation. See cracked walls for the full crack-pattern decision tree and RICS crack category thresholds — any crack in this category above roughly 5mm, or one that's actively widening, needs a structural engineer before any repointing work is specified.

Frequently Asked Questions

Is a small amount of hairline cracking in old mortar something I should always fix?

Not necessarily. Mortar is designed to be the sacrificial, flexible element of a masonry wall, and some fine, stable hairline cracking in old pointing is normal wear rather than a defect. The distinction is stability: mark and monitor the crack for a few months (a simple pencil tell-tale works) — if it isn't widening and there's no associated damp, spalling, or opening distortion, routine repointing as part of general maintenance is sufficient rather than urgent remedial work.

Can I just repoint over a cracked joint without raking it out first?

No — repointing directly over cracked or failing mortar without raking out to a proper depth (typically 15–20mm minimum, more for full-depth failures) doesn't create a proper bond, and the crack will reflect straight back through the new pointing within one or two seasons. Rake out fully to sound mortar, brush out dust and debris, dampen the joint, and repoint with a mix matched to the surrounding brick strength and location.

My new brickwork's mortar has cracked within weeks of laying — is this a workmanship fault?

Often yes, though not always. Cracking this soon after laying most commonly points to rapid drying (unprotected work in hot sun or wind, or unwetted high-suction bricks) or an over-strong mix that hasn't been given time to cure properly before load or movement was applied. It can also indicate the wrong mortar designation was used for the brick type. Ask what mix was used and whether the brickwork was protected during curing — both are reasonable, answerable questions for a recently completed job.

Does painting or sealing cracked mortar joints fix the problem?

No, and it can make diagnosis harder later by hiding an active crack under a coating. Surface sealants don't address the underlying cause (movement, sulphate attack, or a weak mix) and can trap moisture behind them in some cases, worsening frost or sulphate-related deterioration. Diagnose and repoint properly rather than masking the symptom.

How urgently do I need to act on cracking mortar in a chimney?

Chimney mortar cracking should be treated with more urgency than cracking on a garden wall, because chimneys combine severe weather exposure, thermal cycling, and — critically — potential sulphate attack from flue condensate, and because chimney failure carries a fall-of-masonry risk at height. Inspect chimney mortar cracking promptly, use sulphate-resisting mortar (SRPC) for any repointing, and if cracking is extensive or the chimney shows any lean, get a structural assessment before working at height on it.

Regulations & Standards