Cracks in house walls: is this one serious?

What does a crack tell you?

Width is what people ask about first. It is one of six clues.

• Width. A hairline is rarely structural; the wider it is, the more it needs explaining.
• Taper. Even width suggests drying or temperature. Wider at one end means something moved.
• Route. Stepped through the mortar, straight through bricks, diagonal from a corner, or level along a joint.
• Where. Over an opening, where newer work joins, under the eaves, below the damp-proof course.
• When. After a dry spell, a felled tree or building work, or in a new extension’s first years.
• Movement. Opening in late summer and closing in winter points to clay and trees.

What usually causes cracks in walls?

A handful of causes account for most of them, and each leaves a recognisable pattern.

Diagram drawn for this site. Not to scale, and not a diagnosis.

1. Subsidence. Diagonal, wider at the top, rising towards a corner and down through the damp-proof course: that corner dropped. Often clay drying under trees, or a leaking drain.
2. A sagging lintel. Rising from both ends of the lintel to meet above it, with nothing below the opening.
3. Rusting wall ties. Level cracks in the mortar every few courses, as steel ties in the cavity rust and swell. Thin wire ties can fail with no sign at all.
4. Roof spread. A level crack under the eaves: the roof is pushing the top of the wall outwards.
5. Settlement. Vertical, where newer work joins, as two foundations settle differently. Common in an extension’s first years; it usually stops.
6. Shrinkage and temperature. Fine and even, often under a window.

Sources: Association of British Insurers, “Subsidence”; Property Care Association, “Cavity wall ties”.

Key to the drawings

• Existing masonry A flat tone: everything already there, cut or seen.
• Concrete Aggregate triangles and fine dots, scattered at random: foundations, slabs, padstones, lean-mix cavity fill, encasement.
• Undisturbed ground Basket weave, as a band that follows the ground line and fades out.
• DPC Heavy dashed line, as Approved Document C draws it.
• Hidden, beyond or removed Out of the cut, beyond it, or taken out.
• By others, beyond Faint: frames, coverings. Not our structure.
• Cloud Confirm on site. Never a revision mark.
• Break line The drawing stops; the building does not.

Clay, trees and a dry summer

On clay the ground itself moves: it shrinks as it dries and swells as it wets. Tree roots dry it deepest near the tree, and often reach further than the tree is tall. A shallow footing in that dried clay drops in a dry summer and recovers in winter, and the walls above crack.

Source: NHBC Standards 2024, chapter 4.2. Diagram drawn for this site. Not to scale, and not a design.

Heave is the reverse. Fell a large tree, cut its roots or let a drain leak into dried clay, and the clay swells and can lift the house; its cracks often taper the other way. So ask before a tree near a cracked house comes down. On sand and gravel, a leaking drain can wash the ground away instead. The same clay sets how deep a new foundation goes.

How serious is it?

Engineers grade visible damage from 0 to 5. BRE, whose Digest 251 sets out the scale, groups them: 0 to 2, up to about 5 mm, are aesthetic, generally a matter of redecoration; 3 and 4 affect weather-tightness and doors and windows; 5 is about stability. How many cracks there are, and whether they are growing, count as well as width.

Sources: names and widths from HS2 Phase 2a Information Paper C14, Table 1.1 (after Burland and others, 1977), which warns that width “should not be used on its own”. Groups: BRE, “Assessing cracks in houses” (2014).

When should you call an engineer?

• A crack wider than about 5 mm, or several smaller ones together.
• It is wider at one end, or runs diagonally from a door or window corner.
• It shows on both sides of the wall, or runs below the damp-proof course.
• Doors or windows have started to stick.
• It is new, still opening, or came back after it was filled.
• There is a large tree, a leaking drain or recent digging nearby.

Not sure? Send photographs first; they often show whether it needs a visit.

How do you tell if a crack is getting worse?

Watch it before anyone fills it. Filling destroys the evidence: its width, its taper and whether it still moves.

• Photograph it square on with a ruler across it, and note the date.
• Pencil a dated line across each end, to see if it runs on.
• For small changes, a tell-tale across it, or fixed points either side, measured each time.
• Keep going for a full year: one reading cannot show the seasons.

Illustrative shapes, not measurements. Drawn for this site.

What does our visit and report cover?

We visit homes across London, Surrey and Sussex.

• We look at every crack inside and out, and measure and record it.
• We read the house around it: age, construction, doors, windows, floors, trees and drains.
• If the answer lies below ground, we say what would settle it: a trial hole, a drain survey, a tree survey, or monitoring through the seasons.
• Then a written opinion in plain English: what it is, the cause, how serious, and what to do. If the honest answer is to watch it, we say so.

What we need from you

• Photographs of each crack, close up with a ruler and from further back.
• When you first noticed it, and whether it has changed.
• What changed around then: a dry summer, a tree felled or cut back, building work, a leak.
• Where the nearest large trees are, yours and next door’s.
• Any earlier reports, and whether you have told your insurer.

Source: Association of British Insurers, “What to do if subsidence happens”.