What a diagonal crack in brickwork usually looks like
In brick walls, diagonal cracks commonly follow the mortar joints in a stepped pattern, which is why they are called stair-step cracks. Sometimes the crack runs through the bricks themselves rather than around them. They often start near the corner of a window or door, at a corner of the building, or near the base of the wall, and may be wider at one end than the other.
Diagonal cracking is one of the patterns most associated with uneven movement in the structure, so it is sensible to take it seriously. That does not mean every diagonal crack is a serious problem.
Are diagonal cracks always subsidence?
No. Settlement or subsidence (the ground under part of the building moving) is one possible cause, but others exist. A crack is more likely to be related to ground movement if it:
- is wider than about 1/4 inch, or is clearly widening over time,
- is tapered, wider at the top than the bottom, or the reverse,
- runs through several courses of brick, or continues into the foundation,
- appears with matching cracks inside, or with sticking doors and windows,
- is accompanied by displaced bricks or a wall that appears to tilt.
Thin, stable cracks, particularly in a house less than a few years old, can be due to normal settling or to brick veneer movement, and are often judged minor by inspectors. A discussion among home inspectors on a single crack in a young townhome, for example, treated it as minor in the absence of other signs inside. That is not a rule, only an illustration that context matters.
Other causes
- Thermal movement. Brick and mortar expand and contract with temperature, and long walls without movement joints can crack.
- Moisture movement. Bricks can expand slightly as they absorb moisture.
- Lintel or opening problems. A missing, undersized or corroded lintel over a window or door can lead to cracking at the corners.
- Tree roots and drainage. Moisture changes in clay soils, leaking drains, or large trees close to the building are commonly cited as contributors to ground movement. A leaking supply pipe can soften soil in the same way; see the signs of a leaking pipe.
- Foundation issues. Uneven bearing, inadequate footings or soil pressure.
- Impact or vibration. Vehicle strikes or construction work.
- Mortar or wall tie failure. Deteriorated mortar or corroded ties in cavity walls.
How to assess a crack
- Photograph it with a ruler or coin beside it for scale, and note the date.
- Measure the width at several points. Fine cracks under a few millimetres may be minor; those over about 1/4 inch (roughly 6 mm) deserve professional attention.
- Map the pattern. Does it follow the mortar? Is it wider at one end? Does it continue to the ground?
- Check inside the same area for matching cracks, sticking doors and sloping floors.
- Monitor it for several months through at least one seasonal change. Crack gauges or tell-tales are inexpensive; a professional can install them too.
- Look at the surroundings. Check for gutters or downspouts discharging near the wall, leaking pipes or drains, and large trees nearby.
Warning signs that need an engineer
Contact a structural engineer or other suitably qualified professional if you see a wide or growing crack, a bulging or leaning wall, displaced bricks, cracks that reappear after repair, or any crack accompanied by matching internal damage. If bricks look loose enough to fall, keep people away from the area below. Bulging, leaning or bowing masonry should not be left unattended, since it may be unstable.
Bear in mind that foundation repair companies also sell repairs. An independent engineer’s report can be a useful second opinion.
Repair options
What is appropriate depends entirely on the cause, so identify and address the cause first.
- Cosmetic repair of stable cracks. Raking out and repointing cracked mortar with mortar compatible with the existing brick and mortar is common practice. Using mortar that is much harder than the original can cause new damage in older soft brick, so seek advice on the right mix for older buildings.
- Replacing cracked bricks. Individual damaged bricks can be cut out and replaced by a mason.
- Crack stitching. Stainless steel bars set into mortar joints are used by specialists to tie masonry across a crack.
- Underpinning or piers. Where foundation movement is confirmed, specialists may install push piers, helical piers or underpinning. This is major work that needs an engineer’s specification. See issues to expect after foundation repair for what follows.
- Drainage and ground fixes. Repairing leaking drains, redirecting water and managing nearby trees are often part of the solution.
For a related discussion of how masonry and concrete cracks are patched, see repairing a crack in a concrete wall. For cracks inside, see interior wall cracks: when to worry.
Hairline cracks, leaks and wavy walls
Hairline cracks in brickwork. Fine cracks in mortar are common as buildings age. Monitor them, and repoint if mortar is crumbling or water gets in. Fine cracks that stay unchanged are generally less of a worry than growing ones.
A leaking brick wall. Water usually gets in through failed mortar, cracks, damaged flashing or blocked weep holes, or from gutters spilling onto the wall. Fixing the entry point comes first. Applying a sealant over a wall without finding the cause can trap moisture.
Wavy brick walls. A wall that looks rippled or bowed may be due to uneven bricks and mortar from construction, or to movement. If a wall has recently started to bulge or lean, have it assessed.
Insurance
Policies differ. Some include cover for subsidence or ground movement, others exclude or limit it, and cover often depends on the cause. Read your policy and speak to your insurer early, before any repair work starts. Keep dated photographs and measurements.
FAQ
How do I fix a leaking brick wall?
Find where the water enters, such as failed mortar, cracks, blocked weep holes or overflowing gutters, and repair that. Repointing is common for mortar loss.
What causes wavy brick walls?
Uneven construction, differences in brick and mortar thickness, or movement. New bulging is a reason for an assessment.
Are diagonal cracks always subsidence?
No, but they are one of the typical patterns of movement. Width, taper, growth and companion signs help distinguish them.
What should I do if I have hairline cracks in my brickwork?
Photograph and monitor them. If they stay stable, they are usually minor. Repoint if the mortar is deteriorating or water is entering.