How to Identify Subsidence Cracks in Internal Walls | 2025 Guide

Wall cracks are common in UK houses. In most cases, it’s a result of material shrinkage or house movement during temperature changes. However, some cracks can indicate more significant issues. In this case the type, size, and location are critical to helping to spot a subsidence crack over a cosmetic one.

A subsidence crack is a visible sign that the ground beneath your property is moving or sinking, often due to unstable soil, tree roots, broken drains, or long dry spells. When the soil under your foundations shifts, it can cause parts of your home to sink or drop unevenly. This movement puts stress on the structure and results in cracking in walls, ceilings, or uneven floors.

Not all wall cracks are caused by subsidence, which is why it is important to distinguish between a harmless cosmetic issue and a structural crack that could indicate a deeper problem. Cracks linked to subsidence tend to be wider, longer, and show up in key areas of weakness like corners, windows, or door frames. Spotting these early is crucial in preventing more serious (and costly) structural damage.

Subsidence cracks typically appear suddenly, in high-stress areas like the corners of doors or windows. Bay windows, porches, and small single-storey extensions are likely to show signs of subsidence cracks first. This is because these features are typically built on shallower foundations, making them more susceptible to movement.

These cracks may run diagonally or follow a stepped pattern in brickwork or plaster and can be more than 3–5mm wide.

Unlike minor cracks caused by shrinkage or seasonal movement, subsidence cracks in internal walls tend to look uneven or jagged and may be accompanied by signs like sticking doors, sloping floors, or external cracks.

Understanding the difference between a minor and major subsidence crack requires measuring and understanding the context of the problem:

• Minor subsidence cracks: Likely hairline cracks less than 2mm wide. Not likely to be structural and could be cracked plaster because of settlement or seasonal changes
• Moderate subsidence cracks: Between 2-5mm should be monitored to see if they grow over time; especially if you notice sticking doors or windows
• Major subsidence crack: Over 5mm wide, jagged, or associated with visible shifting in the building. These are more likely to be subsidence cracks and need immediate attention.

Even small cracks can become worse over time, so monitoring change is key. If you’re not sure whether a minor subsidence crack needs attention, don’t hesitate to get in touch.

You need to assess the full extent of subsidence cracks in internal walls to identify whether it’s the result of thermal movement or material shrinkage, or if it’s something deeper. Smaller cracks on the walls plaster are straightforward and typically only needing a cosmetic repair.

However, larger cracks in walls that display the below characteristics may indicate weaknesses or movement within the soil layers below your property’s foundations. According to RIAS, subsidence cracks tend to show these signs:

• Cracks are wider than about 5mm (or half a centimetre)
• Large cracks that start at windows, doorways or corners of buildings
• Cracks that are jagged, horizontal or vertical, zig-zagged, stepped in brickwork, or follow mortar lines
• Visible movement in bay windows, extensions or porches pulling away from the main structure
• Sinking or sloping floors, jammed or misaligned doors and windows, or skirting boards pulling away from walls or floors
• Changing widths along the length of wall crack can indicate if your home is sinking (wider at the top) or heaving from below (wider at the bottom)

The Building Research Establishment (BRE) provides more detailed guidance on categorising the severity of wall cracks in their Cracking in Buildings Guide, which is commonly used by surveyors and structural engineers across the UK.

For subsidence cracks in internal walls, DIY is not the right solution. While you can fill and repaint minor cosmetic cracks, structural issues require a professional approach. Patching over a structural crack without fixing the root cause will allow the problem to worsen over time, risking further damage and increased repair costs.

If a crack is showing signs of being a major subsidence crack then it’s time to call in an expert. A professional inspection can determine whether your foundations are affected and what steps to take next.

Don’t wait until minor issues become major structural faults.