Large Crack in Wall of House

Discovering a large crack running through your wall can trigger immediate panic. You stare at it, wondering if your home is about to collapse or if you’re looking at a simple cosmetic issue that needs nothing more than filler and paint.

The truth? Size alone doesn’t tell the whole story.

Cracks wider than 3mm need professional assessment. The Building Research Establishment classifies damage from 0-5, with Category 3 starting at cracks between 5-15mm wide. At this point, you’re dealing with potential structural concerns that go beyond surface-level repairs.

What the Width Tells You

Hairline cracks under 1mm wide rarely signal danger. They appear in most homes as materials expand and contract with temperature changes. You’ll spot these thin lines around door frames, along plaster joints, and near windows.

Cracks between 1-3mm deserve monitoring. Take photos with a coin for scale reference. Check them monthly to see if they’re growing. Static cracks that stay the same size for six months probably won’t cause problems.

Anything above 5mm requires immediate professional inspection. These cracks often run diagonally across walls, suggesting movement in your home’s foundations. The wider the crack, the more urgent the assessment becomes.

Direction Matters More Than Width

Vertical cracks along mortar lines usually stem from minor settling. Horizontal cracks across brick faces indicate serious stress, particularly if they appear in load-bearing walls. Diagonal cracks forming a stepped pattern through brickwork signal foundation movement.

Stepped cracks follow the mortar between bricks like a staircase. They typically start narrow at one end and widen as they progress. This pattern points to differential settlement, where one part of your foundation drops whilst another stays put.

Internal vs External Cracks

External cracks expose your home to water penetration. Rain seeps through, saturating the wall cavity and potentially reaching internal walls. This moisture can weaken structural elements and create damp problems that cost thousands to remedy.

Internal cracks might only affect plaster, or they could mirror external structural damage. Check both sides of the wall. If you find cracks in matching positions inside and out, you’re looking at a through-wall issue that affects the masonry itself.

Where the Crack Appears

Cracks near corners often result from thermal movement. Buildings expand in summer heat and contract in winter cold. Corner walls, being less restrained than mid-sections, move more freely and develop cracks at stress points.

Cracks around openings concentrate stress. Windows and doors create weak points in walls. Weight from above pushes down, and these openings can’t support the load as effectively as solid wall sections. Cracks radiate from corners of openings when lintels fail or settlement occurs.

Ground floor cracks close to soil level suggest foundation problems. Upper floor cracks typically relate to roof spread, lintel failure, or wall tie corrosion. The height matters when diagnosing the cause.

Testing if Cracks are Active

Buy tell-tales from builder’s merchants for £10-15. These simple glass or plastic strips bond across cracks. If movement continues, the tell-tale cracks, giving you clear evidence of active problems. Leave them in place for three months minimum, covering both wet and dry seasons.

Photograph cracks next to a ruler every month. Date each image. This visual record proves invaluable when discussing the issue with surveyors or insurers. Static cracks over a full year rarely need expensive intervention.

When Age Provides Context

New builds settle for the first few years. Minor cracking during this period is expected as the structure adjusts. Developers typically offer two-year warranties covering defects that appear during settling.

Older properties have usually completed their settlement. Large cracks appearing in homes over 50 years old suggest a new problem has developed. Tree roots, leaking drains, or soil changes often trigger movement in previously stable buildings.

Properties built between 1920-1950 might have shallow foundations. These homes prove more vulnerable to subsidence because their foundations sit only 600-900mm deep, compared to modern standards requiring 1000mm minimum.

DIY Diagnosis Limits

You can monitor and photograph cracks yourself. You can check if they’re getting bigger using tell-tales. You can even research common causes online.

But you can’t see underground. You can’t measure foundation depth from inside your home. You can’t determine if soil has washed away beneath your footings or if tree roots have desiccated the clay.

That’s when professionals bring specialised equipment. Levels detect which areas have dropped. Soil analysis reveals moisture content and bearing capacity. Structural calculations determine if walls remain safe or need underpinning.