Cracks in walls are one of the most common concerns for homeowners in London, and one of the most frequently misunderstood. The vast majority of cracks in residential properties are superficial and caused by normal movement, thermal cycling, or drying out of new plasterwork. However, some cracks are indicators of more serious structural movement that requires investigation and remediation. Knowing how to distinguish between the two can prevent unnecessary anxiety or, equally importantly, prevent a serious problem from being overlooked.
Hairline cracks less than one millimetre wide are almost universally superficial. They are caused by normal thermal movement of building materials, seasonal moisture changes, or the drying of plaster in relatively newly occupied or refurbished buildings. These cracks require no structural attention and can be filled and redecorated as a cosmetic matter.
Small cracks between one and five millimetres wide are still typically superficial but are worth monitoring. If they are stable and do not change over a period of months, they are unlikely to indicate active movement. If they appear to widen or extend over time, further investigation is warranted.
Cracks wider than five millimetres, cracks that step diagonally from the corner of windows or doors, cracks that pass through brickwork as well as mortar joints, or cracks that are accompanied by other signs of movement such as sticking doors or windows, sloping floors, or visible gaps at structural junctions are more significant. These types of crack pattern suggest that the structure is experiencing differential movement, meaning parts of the building are moving at different rates relative to each other.
The most common cause of significant cracking in London homes is subsidence, which is differential settlement of the foundations. London clay is particularly prone to shrinkage and heave with changes in moisture content, and the roots of mature trees near the house can remove moisture from the clay and cause localised settlement. Leaking drains can also cause erosion of the ground beneath foundations, leading to differential settlement.
If you see cracks that concern you, the appropriate response is to have the building inspected by a structural engineer or chartered building surveyor. They can assess the crack pattern, identify the likely cause, and advise on whether monitoring, investigation, or remediation is required. Acting early is always better than waiting until the problem worsens.