The direction a crack runs is one of the best clues to what caused it. Diagonal and stepped cracks usually mean one part of a wall has moved relative to another, often around doors and windows. Horizontal cracks in brickwork point towards bowing, roof thrust or corroding wall ties. Vertical cracks are most often found at junctions, where two parts of a building move differently.
Direction is only one clue, though. Width, taper, whether it’s growing, and the building’s age and setting all matter just as much. Here’s how surveyors read them together.
Why the direction of a crack matters
Masonry is strong when squashed but weak when pulled. A wall cracks along the line where it is being stretched, so the crack forms at right angles to that pull. That means its direction tells you which way the stress was acting:
- one end of a wall dropping or lifting produces diagonal cracks
- a wall being pushed outwards at the top, or the outer skin being forced up, produces horizontal cracks
- two sections pulling apart side to side produces vertical cracks
In brickwork the crack usually follows the weakest route — the mortar joints — so a diagonal crack becomes a stepped crack, zig-zagging through the bed and perpend joints like a staircase.
Crack direction at a glance
| Crack pattern | Common causes | Usually | Look for |
|---|---|---|---|
| Diagonal / stepped, from corners of doors and windows | Foundation movement (subsidence, heave, settlement); weak or missing lintels; shrinkage in new work | Worth investigating if tapered, wide or growing | Taper, both faces of the wall, sticking doors, trees, drains |
| Horizontal, repeated every few courses | Corroding cavity wall ties | Needs investigation | Cavity wall built before the 1980s, bulging, cracks in several bed joints |
| Horizontal, single line near the top of a wall | Roof spread pushing the wall out; bowing; lack of restraint at floor or roof level | Needs investigation | Wall leaning out, sagging ridge, gaps at ceiling level inside |
| Vertical, at a junction (extension, bay, party wall) | Differential movement or thermal movement; bay foundations shallower than the main house | Often stable, but check | Width the same top to bottom, or tapered? Is it growing? |
| Vertical, in the middle of a long wall | Thermal or moisture movement with no movement joint | Usually not structural | Uniform width, seasonal opening and closing |
| Fine random or “map” cracking | Shrinkage of render or plaster | Cosmetic | Confined to the finish, not the masonry |
What do diagonal and stepped cracks in walls mean?
Diagonal cracks in walls are the pattern most associated with subsidence, because they’re what you get when one part of a building loses support and drops, or rises, relative to the rest. They typically start at the corners of door and window openings, which are the weak points in a wall, and run up or down at roughly 45 degrees.
Stepped cracks are diagonal cracks in brick or blockwork that follow the mortar joints. They’re very common and have many causes:
- Ground movement — clay shrinkage near trees, leaking drains washing out the ground, or mining legacy, which is a real consideration across much of West and South Yorkshire
- A failed or missing lintel over an opening, letting the masonry above sag — usually with cracks spreading up from both top corners
- Shrinkage in newer blockwork or extensions as materials dry out
- Old, stable settlement in a house that moved decades ago and stopped
The detail that distinguishes them is taper. A crack that is wider at one end than the other shows that part of the wall has rotated — and the wide end points towards where the movement is greatest. A crack wider at the top often suggests the ground under that end has dropped; wider at the bottom can suggest upward movement (heave) or the corner of the building moving outwards. A crack the same width along its length is more consistent with shrinkage or thermal movement.
What do horizontal cracks in walls mean?
Horizontal cracks in walls are less common and usually worth taking seriously, because they often reflect something pushing on the wall rather than the ground moving beneath it.
The main causes:
- Wall tie failure. In cavity walls built before the 1980s, the steel ties linking the inner and outer skins can rust. Rust takes up more room than steel, so the ties expand and lift the brickwork, producing horizontal cracks along the bed joints where the ties sit — typically repeating every few courses up the wall. We cover this in detail in our guide to wall tie failure.
- Roof spread. If a roof isn’t properly tied, its rafters push the tops of the walls outwards, often causing a horizontal crack just below the eaves and a gap where ceiling meets wall inside. See roof spread and sagging roofs.
- Bowing walls. Solid walls with poor connections to floors and roof can bulge outwards over time. A horizontal crack may form at the point of maximum bulge.
- Retaining walls and basements. Ground pressing against a wall from one side — common in hillside Pennine terraces with cellars or split levels — can cause horizontal cracking.
- Expansion of materials. Less commonly, sulfate attack in mortar or concrete expands and can lift brickwork.
What do vertical cracks in walls mean?
Vertical cracks are often the least worrying, but not always.
- At junctions — where an extension, porch or bay meets the main house — they’re usually differential movement: a newer structure on shallower foundations settling or moving with the seasons. They matter more if they’re tapered (wider at the top suggests the extension is tipping away) or growing.
- At bay windows, a crack where the bay meets the main wall is very common in Victorian and Edwardian houses. Bays often have shallower foundations than the main walls and can move independently. Many have been stable for a century; some are still moving.
- In long walls without movement joints, a vertical crack can simply be thermal or moisture expansion and contraction. These often open and close slightly with the seasons.
- Through a chimney breast or at a party wall, a vertical crack may reflect alterations, such as chimney breast removal, so it’s worth checking the history.
How wide is too wide? The BRE damage categories
UK surveyors, engineers and insurers grade cracking using BRE Digest 251, which places damage on a scale from 0 to 5 based mainly on crack width and how easily it can be repaired.
| Category | Description | Approx. crack width |
|---|---|---|
| 0 | Negligible | Hairline, less than 0.1mm |
| 1 | Very slight | Up to 1mm |
| 2 | Slight | Up to 5mm |
| 3 | Moderate | 5–15mm (or several cracks up to 3mm) |
| 4 | Severe | 15–25mm |
| 5 | Very severe | Over 25mm |
Categories 0–2 are generally regarded as aesthetic damage, category 3–4 as affecting serviceability (doors sticking, weathertightness), and category 5 as potentially affecting stability. Width isn’t everything — the digest considers the overall damage and ease of repair, not a single measurement — but it gives everyone a common language. Our cracks in walls checker walks you through where a particular crack sits.
How to read a crack yourself
- Note the direction — diagonal, stepped, horizontal or vertical.
- Check for taper. Measure the width at both ends with a ruler or a crack-width gauge.
- Look at both faces. A crack visible inside and out at the same point goes through the wall.
- Look around it. Trees, drains, gullies, recent building work, a sagging roofline, sticking doors.
- Monitor it. Photograph it with a ruler, date it and repeat monthly. Progression matters more than anything else.
Our guide to what causes cracks in walls goes deeper into separating shrinkage, thermal movement and historic settlement from active ground movement, and what subsidence actually is covers the signs of ground-related movement.
What if you’re buying a house with cracks?
Don’t let a single crack frighten you off, and don’t accept “it’s just settlement” without evidence. A survey will describe the cracking and give a view on whether it looks historic and stable or needs further investigation. If there’s a specific concern, ask the seller about any previous insurance claims, monitoring, or repairs, and get the cracking assessed before you exchange.
When to get an assessment
Get professional advice if a crack is tapered, wider than about 3mm, stepping through brickwork near an opening, visible on both faces of a wall, horizontal and repeating up a cavity wall, or demonstrably growing.
Our subsidence assessment looks at the pattern, the likely cause and whether movement is ongoing, and gives you an independent view — we don’t sell underpinning or repairs. The fee is fixed and quoted for the property. Contact us with the address and a photo of the crack with a ruler alongside.
Sources and further reading
- Assessing cracks in houses — BRE
- BRE Digest 251: Assessment of damage in low-rise buildings — BRE
- Cavity wall tie repairs — Property Care Association
- Mining reports — Mining Remediation Authority