Repairing Cracks in London Homes: A Homeowner's Guide

The Building Research Establishment (BRE) provides a widely used classification system for cracks in domestic properties. Category 0 cracks are hairline — under 0.1mm wide, essentially invisible without close inspection. Category 1 (up to 1mm) and Category 2 (1mm to 5mm) are generally considered cosmetic and can be repaired without structural investigation. Category 3 (5mm to 15mm) may need investigation depending on location and pattern. Category 4 (15mm to 25mm) and Category 5 (over 25mm) are serious and require structural assessment before repair. However, width is not the only indicator of seriousness — a narrow but rapidly growing crack may be more concerning than a wide but stable one.

Vertical cracks in external brickwork often indicate thermal movement or minor settlement — they are typically stable and cosmetic. Horizontal cracks in brickwork at mortar joint level can indicate failure of a lintel, differential settlement or lateral pressure from the ground. Diagonal cracks at 45 degrees from the corners of windows and doors are the classic indicator of differential settlement or subsidence. Stair-step cracks following the mortar joints are common in older brickwork and usually represent minor movement that has stabilised over time. Internal vertical cracks in plasterwork often follow plasterboard joints where the boarding has moved or dried out. Ceiling cracks radiating from a corner or following a straight line may indicate floor movement or joist deflection above.

Seek professional structural assessment if: cracks are wider than 5mm at any point; cracks are diagonal and running from the corners of openings; you notice distortion of door or window frames (sticking or no longer squaring up); cracks are progressing over time (photograph them monthly and measure width with a ruler to track); cracks appear in both internal and external faces of the same wall; or the property has known proximity to tree roots or drainage issues. A structural engineer's initial assessment typically costs £300 to £700 and will confirm whether the cracks are cosmetic or indicative of a structural problem requiring further investigation.

Before repairing any crack that might be structural, monitor it for at least three to six months. The simplest method is to fix a tell-tale across the crack — a small plastic device calibrated in millimetres that shows whether the crack is opening, closing or stable. Alternatively, mark the ends of the crack with pencil and date them, then photograph monthly. A crack that is genuinely stable can be repaired with confidence. A crack that continues to grow requires investigation before repair — filling a growing crack repeatedly is expensive and pointless until the underlying movement is addressed.

Hairline cracks in plasterwork can be filled directly with a proprietary fine surface filler, feathered out with a damp sponge, sanded and decorated. Wider cracks in plasterwork need to be raked out to create a V-shaped channel for the filler to key into — if you try to fill a narrow crack it will fall out quickly. For cracks at plasterboard joints, the correct fix is to apply joint tape embedded in joint compound, feather out and sand. Where plaster has detached from the substrate (sounds hollow when tapped), it needs to be hacked off and the area replastered rather than simply filled. Cracks in corners are often best addressed with flexible acrylic filler rather than rigid gypsum-based products, as corners are subject to minor ongoing movement.

Stable cosmetic cracks in brickwork mortar joints can be repaired by raking out the old mortar to a depth of 15mm to 20mm and repointing with a mortar that matches the original in strength and composition. On Victorian and Edwardian brickwork this means a lime-based mortar — using a hard cement mortar on soft Victorian brick causes the brick faces to spall as moisture is forced through the harder mortar joint into the softer brick. A crack running through the brick itself (as opposed to along the mortar joint) requires either brick replacement or, for structural cracks, may need helical tie reinforcement designed by a structural engineer.

Cracks in external render may indicate movement in the underlying masonry or may simply be the result of thermal movement in the render itself, insufficient movement joints, or render applied in cold weather. Hairline cracks in silicone or flexible render can often be left — the render's own flexibility accommodates minor movement without water penetrating. Cracks in hard sand-and-cement render wider than 1mm should be filled — rake out, prime and fill with a flexible exterior filler or re-render the affected area. Avoid filling render cracks without investigating whether there is hollow render nearby, as water may be tracking behind the render and the hollow area needs to be re-bonded or replaced.

Where crack repairs are widespread or the plasterwork is generally poor condition, a full replastering of the affected area gives a better long-term result than multiple patch repairs. A skilled plasterer can cut out a section of damaged plaster, apply bonding coat to the substrate and finish with a single or two-coat plaster system that blends well with the surrounding finish. Older properties may have lime plaster — if this is in good condition it is worth repairing like-for-like with lime rather than switching to modern gypsum, as the different movement characteristics of the two materials can cause cracking at the interface.