Types of Cracks in Walls and What They Mean

The four common crack patterns

Cracks are a message from the structure about how it is moving. The direction of a crack is the single most useful clue, because masonry cracks at right angles to the force pulling it apart. The diagram below shows the four patterns that cover most cracks in London homes.

Vertical cracks in walls

Vertical cracks are the most common and usually the least concerning. Plaster, mortar and masonry all shrink as they dry, and buildings expand and contract with temperature and moisture through the year. That movement often shows up as a narrow vertical crack, typically uniform in width from top to bottom, and frequently at a junction between different materials or at a weak point such as a chased-in cable run.

A vertical crack earns more attention when it is wider than about 3 mm, when it is clearly wider at one end than the other (tapering suggests rotation or settlement rather than shrinkage), or when it keeps reopening after being filled. Those features move it out of the cosmetic category and into the investigate category.

Diagonal and stepped cracks

Diagonal cracks, and particularly stepped cracks that follow the mortar joints through brickwork like a staircase, are the classic sign of differential movement: one part of the building moving relative to another. They often radiate from the corners of windows and doors, because openings are the weakest points in a wall, and they are the pattern most associated with subsidence, heave and foundation movement.

Not every diagonal crack means subsidence. Lintel failure over an opening, thermal movement in long walls and settlement of a new extension against an older building all produce diagonal cracking too. The distinguishing questions are whether the crack is growing, whether it tapers, whether doors and windows nearby have started sticking, and whether there is a plausible cause such as a large tree or a leaking drain close by. Our guide to signs of subsidence in London homes covers those warning signs in detail.

Horizontal cracks in walls

Horizontal cracks are the pattern a structural engineer takes most seriously, because they can indicate a wall being pushed or pulled sideways rather than simply shrinking. The common structural causes are:

• Failed cavity wall ties. When the metal ties connecting the two leaves of a cavity wall corrode, they expand and crack the mortar beds in long horizontal lines, and the outer leaf can bow.
• Roof spread. A roof without adequate restraint pushes the tops of the supporting walls outwards, cracking the wall high up and sometimes lifting the ceiling line.
• Ground or water pressure. Basement and retaining walls under earth pressure can crack horizontally as they bend.
• Floor or wall plate movement at the level where a floor bears onto the wall.

Because each of these mechanisms involves the stability of the wall rather than its finish, a long horizontal crack, particularly with any bowing or bulging, should be assessed by an engineer rather than filled and watched.

Hairline and cosmetic cracks

Hairline cracks, under about 1 mm wide, are almost always cosmetic: plaster shrinkage, minor thermal movement, settlement of a newly decorated or newly built room, or vibration. Fine cracking around ceiling edges, above doors and along plasterboard joints falls in the same category. These can be filled and decorated over, and if they stay closed, that is the end of the story. The exception is a hairline crack that keeps returning in the same place and growing, which is behaving like the start of a pattern rather than a blemish.

How wide is too wide?

Width bands are a general guide only, based on established industry crack classification. Pattern, location and change over time matter as much as width; a growing 2 mm stepped crack can be more significant than a static 6 mm one.

Structural cracks: pulling the clues together

A crack is more likely to be structural when several clues line up: it is wider than about 5 mm or growing, it tapers or steps through brickwork, it appears on both sides of the wall, doors and windows nearby stick or no longer square, floors slope, or the crack sits in a pattern that matches a mechanism, diagonal from an opening (movement), horizontal along a bed joint (lateral force), or vertical at a junction (separation). Our companion guide, when are cracks structural, walks through how an engineer reads these clues on site.

How structural cracks in walls are fixed

The repair always starts with the cause, not the crack. Filling a crack while the movement continues just resets the clock. Once the cause is understood and dealt with, the common repairs are:

• Crack stitching. Helical stainless steel bars bonded into slots cut along the mortar joints, restoring the wall's continuity across the crack.
• Repointing and localised rebuilding of cracked or displaced masonry.
• Lintel replacement where a failed or corroded lintel over an opening has caused the cracking.
• Wall tie replacement where corroded cavity ties have cracked the bed joints.
• Restraint, such as tying walls to floors and roofs, where walls have been pushed out of line.
• Foundation works, such as underpinning, only where foundation movement is confirmed and continuing, which is the minority of cases. Our guide to when underpinning is needed explains how that decision is made.

These repairs are specified through an engineering-led repair process: diagnosis, a repair specification, and inspection of the completed work. At EMA Structures we carry out crack repairs across London as well as diagnosing them, so the same team takes the wall from assessment to completed repair.

Worried about a specific crack?

A photo is often enough for us to give an initial view. Send us a photo of the crack with a note of where it is and how long it has been there, and we will tell you whether it looks like one to monitor or one to inspect. A structural inspection with a full written report costs £495 + VAT, see our fees guide, correct as of July 2026.