Cracks in Walls: When to Worry

Cracks in walls are one of the most common sources of anxiety for homeowners, yet the vast majority are entirely harmless. Understanding what you are looking at, crack width, direction, location and whether it is active or dormant, is the key to deciding whether to get the filler out yourself or reach for the phone to call a structural engineer.

The yardstick most UK surveyors and insurers use comes from BRE Digest 251, which groups cracks up to 5 mm wide as aesthetic damage that needs only redecoration.¹ Cracks wider than 5 mm, cracks that are still widening, or any crack where the two sides have moved out of line need professional assessment.¹

Types of Wall Crack and What They Mean

Not all cracks behave the same way, and their pattern tells you a great deal about the cause.

Hairline and shrinkage cracks are by far the most common. They appear in plaster, render, and drylining as the building dries out or as seasonal moisture changes cause slight thermal movement. They have no structural significance. In new homes, NHBC notes that shrinkage cracks appear as mortar, plaster and concrete dry out, and are usually a constant width of less than 2 mm.²

Diagonal cracks running at roughly 45° from the corners of windows and door frames are the next most frequent type. Openings concentrate stress, so a small diagonal crack here often comes from drying shrinkage, thermal movement or slight deflection of the lintel; NHBC notes that thermal movement cracks are commonly found around a concrete or steel lintel.² If the crack is less than 1 mm and has been stable for over a year, it is generally monitored rather than acted upon. A growing crack, or one with step displacement, warrants investigation.

Stair-step cracks following the mortar joints in brickwork are a classic sign of differential movement, one part of the foundation moving relative to another. Caused most often by subsidence, heave, or localised foundation failure, these should always be professionally assessed if wider than 2 mm.

Vertical cracks through brickwork or blockwork are often thermal or moisture movement, particularly in a long wall without movement joints, or, in older properties, the separation of a chimney breast, bay or other attached structure. NHBC notes that thermal movement cracks in walls are usually vertical and a constant width.²

Horizontal cracks in a wall need a closer look. In a cavity wall, horizontal cracks along the mortar joints are often the first sign of corroding wall ties.³ At or below damp-proof course level, they can also indicate lateral ground pressure or frost-damaged brickwork in older properties. Horizontal cracks in a basement or retaining wall may point to significant structural stress.

Crack Classification Table

The Building Research Establishment (BRE) Digest 251 provides a widely used classification system.⁴ The table below summarises its six categories:

In BRE’s own summary, categories 0 to 2 are aesthetic damage, categories 3 and 4 affect weather-tightness and the operation of doors and windows, and category 5 affects stability and is likely to need structural intervention.¹

Common Causes of Wall Cracks

Settlement is downward movement caused by the weight of the building compressing the soil beneath it, and it happens mainly in the first years after construction.⁵ It is usually slight and fairly uniform, producing mostly fine cracks at stress concentrations.

Subsidence is when the ground beneath a building sinks, pulling the foundations down with it, and it often affects one part of the building more than another.⁵ Clay shrinkage, often driven by tree roots drawing moisture out of the soil in dry spells, is one of the most common causes of subsidence.⁵ The British Geological Survey describes clay shrink-swell as the most damaging geohazard in Britain today.⁶ Other causes include leaking drains, mine workings, and inadequate foundations in sandy or made-up ground.

Thermal movement affects materials differently. Brick, concrete, and timber all expand and contract at different rates. Long, unbroken external walls without movement joints will crack at their weakest point, typically a narrow pier between windows.

Lintel failure occurs when the steel or concrete beam over a door or window corrodes, deflects, or was undersized from the outset. Corroding steel expands as it rusts and can force the surrounding concrete or masonry apart.

Damp and frost action cause localised spalling and cracking, particularly in older solid-wall properties where soft brick or lime mortar absorbs moisture and then expands on freezing.

How to Tell If a Crack Is Active

Monitoring is simple and cheap. Stick a small plaster or polyfilla tell-tale across the crack and mark the date. Check it monthly through summer and winter. If it breaks or the crack dimension changes measurably (more than 0.5 mm over 12 months), the crack is active and professional advice is warranted.

You can also photograph the crack next to a ruler at regular intervals. A crack that opens during dry summer months and closes again in wet weather is typical of shrinkable clay, which moves with the weather, especially near large trees.⁶

Costs and When to Call a Professional

Call a structural engineer, not a builder, if:

• Any crack is wider than 5 mm
• There is displacement (the two sides of the crack are at different levels or depths)
• Cracks are growing rapidly (more than 1 mm over a few weeks)
• You see stair-step cracks in brickwork and the ground nearby has trees or has recently had large trees removed
• Doors or windows have become difficult to open or close alongside new cracking
• You are buying or selling the property and want a professional opinion for the conveyancing process

Crack Repair Methods

For harmless cosmetic cracks in plaster, the standard approach is to open the crack slightly with a scraper or oscillating tool, brush out loose material, apply a flexible filler (an acrylic or fine surface filler rather than rigid Polyfilla for cracks that may move slightly), let it dry fully, and sand before redecorating. Rigid fillers applied to active cracks will simply crack again.

For hairline cracks in external render, a brush coat of exterior masonry paint often suffices, though flexible crack filler designed for render performs better.

For structural repairs, repointing, stitching with stainless steel helical bars, or lintel replacement, a competent builder or specialist masonry contractor is appropriate. Underpinning requires a structural engineer’s specification and an experienced groundworks contractor.

Homeowners should bear in mind that most home insurance policies cover subsidence, heave and landslip, but check your policy wording for exclusions such as settlement.⁵ If a claim is possible, document cracks thoroughly before any repair and notify your insurer before carrying out work.