Horizontal cracking that follows mortar joints can be associated with corroding wall ties, but other causes include thermal movement, lintel action, restraint and local masonry defects. Bulging, loose masonry or a widening crack warrants prompt professional assessment.
When this guide fits
Clues worth recording
- A stepped or horizontal line through mortar joints
- The wall face is no longer flat
- Cracks repeat at regular vertical intervals
- Doors, windows or roof edges show related movement
Start with the pattern
Record where the crack begins and ends, whether it passes through bricks or mortar, its approximate width and whether it appears on other elevations. A dated photograph with a ruler is more useful than a close-up with no context.
Possible wall tie corrosion
Where an older mild-steel tie corrodes, expansion in the mortar bed can produce horizontal cracking. The PCA identifies this as a common external indicator, while noting that some modern tie failures can have few visible symptoms.
Other explanations
Cracks around openings can involve lintels; long elevations may be affected by thermal movement; changes in materials or historic alterations can concentrate stress. Ground movement more often creates different patterns, but only an inspection can distinguish the causes.
Avoid filling or covering a crack before it has been recorded. The pattern may be important evidence.
When to act quickly
Keep people away from visibly loose or bulging masonry and seek urgent professional advice. For stable fine cracks, gather photographs and previous surveys and arrange proportionate assessment rather than assuming the worst.
Questions homeowners ask
What people usually want to know
Is a horizontal crack more serious than a vertical crack?
Direction alone does not determine seriousness. Width, change over time, wall shape, location and cause are all relevant.
Can I monitor the crack myself?
Dated photographs and simple width records are useful, but monitoring is not a substitute for assessment where masonry is bulging, loose or changing quickly.
Should I repoint the crack?
Not until the cause is understood. Cosmetic repair can hide useful evidence and may fail again if movement continues.
Sources & review
Evidence behind this guide
Reviewed 1 September 2026. We use primary guidance where available and label trade-body advice clearly.