Subsidence occurs when the ground sinks unexpectedly. This can be due to:

• Changes in the soil moisture levels because of the climate, which can cause the ground to shrink or swell. This can be a particular problem in the clay-rich soils found in areas like Dublin, Meath and Cork

• The roots of nearby mature trees, which can have a similar impact because these suck moisture from the ground, again causing it to shrink

• Heavy traffic local to a property can cause sub-surface vibrations which can result in subsidence

• Leaking drains which wash away soil particles, particularly in sandy soils, can destabilise foundations

• Historical land use such as mining or peatland drainage in rural areas can lead to ground settlement

When the soil under a home's foundation shifts, it can cause parts of your home to sink or drop. This movement puts stress on the structure which appears as cracks in walls and ceilings, or it can cause a concrete floor slab to start to slope.

The type, size, and location of a crack are critical in helping establish whether a crack is a sign of subsidence or is simply cosmetic.

Subsidence cracks typically appear suddenly, often in areas like the corners of doors or windows. They also occur around bay windows, porches, and small single-storey extensions because these features are typically built on shallower foundations, making them more susceptible to movement. Unlike minor cracks caused by shrinkage or seasonal movement, subsidence cracks in internal walls tend to look uneven or jagged; they may run diagonally or follow a stepped pattern in brickwork or plaster and can be 3–5mm wide. Cracks may also be accompanied by of signs of subsidence, like doors or windows sticking or floors starting to slope. Sometimes cracks can also appear externally.

Spotting subsidence cracking early is crucial in preventing more serious (and costly) structural damage.

Documents like BRE Digest 251: Assessment of damage in low-rise buildings are often referenced by Irish engineers and building surveyors.

Why crack size is important

The size of a crack is an indication of the seriousness of the problem. Smaller cracks may only need cosmetic repair. Larger cracks, however, may indicate weaknesses or movement within the soil layers below your property’s foundations.

• Minor subsidence cracks: These are hairline cracks less than 2mm wide and not likely to be structural. They could be a crack in the plaster finish, for example, from minor settlement or seasonal changes in ground conditions.

• Moderate subsidence cracks: Between 2-5mm should be monitored to see if they grow over time; especially if you notice sticking doors or windows or skirting boards pulling away from walls or floors.

• Major subsidence crack: Over 5mm wide, jagged, or associated with visible shifting in the building. These can appear at junctions as window bays, extensions and porches start to pull away from the main structure. 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). These are more likely to be subsidence cracks and need immediate attention.

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

Can you fix internal wall cracks yourself?

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 risks further damage and increased repair costs.

If a crack is indicating subsidence, then it’s time to call in an expert. A professional inspection can determine the cause, whether the foundations are affected and the next steps.