Cracks in walls: which are serious and how to repair them

Almost every house has cracks. Most are harmless and can be fixed with a good render repair; a few are the first sign of a structural problem. Knowing how to read a crack — its shape, direction, width and whether it is growing — is what separates a cosmetic repair from work that really has to be done.

Why walls crack

A crack appears when a material is forced to deform more than it can. Mortars and masonry are strong in compression but very weak in tension: small strains are enough to crack them. The most common causes are:

• Shrinkage of mortar and concrete as they dry.
• Temperature and moisture changes, which make materials expand and contract.
• Different behaviour of materials in contact (concrete, brick, plaster, timber).
• Deflection of slabs and beams over time (creep), which either load the walls below or stop supporting them from above.
• Differential settlement of foundations, when one part of the building sinks more than another.
• Rebar corrosion, which makes concrete burst from the inside.

Reading the crack: the most common patterns

Fine, shallow, map-like cracks

They look like “crocodile skin” or a map. They stay in the render or paint and result from mortar shrinkage (too much water or cement, fast drying, layers that are too thick). They have no structural significance.

Straight cracks where structure meets masonry

Horizontal under beams and slabs, or vertical along columns. Concrete and brick expand, contract and deform differently, and the joint between them opens. Very common in reinforced-concrete buildings with masonry infill walls.

Cracks on the top floor

The roof slab heats up and cools down much more than the rest of the building. As it expands it drags the top of the walls, creating horizontal cracks near the ceiling or diagonal cracks at the corners of the top floor. Improving roof insulation reduces the cause.

Diagonal cracks from the corners of doors and windows

The corners of openings concentrate stress. Fine, stable cracks may just be shrinkage; cracks that grow, especially when doors or windows no longer close properly, may indicate structural deformation or settlement.

Stepped cracks that are wider at one end

They follow the masonry joints in steps and open more on one side. This is the typical pattern of differential foundation settlement. They should always be assessed by an engineer.

Cracks along reinforcement, with rust stains

In beams, columns, balcony slabs or ceilings, cracks running parallel to the bars, sometimes with concrete “popping” and brown stains, indicate rebar corrosion. It is a durability problem — and, if advanced, a safety one.

How wide is the crack?

Width doesn’t tell the whole story, but it is a good starting point. A classification widely used internationally for damage to low-rise buildings (BRE Digest 251) relates crack width to damage severity:

Approximate width	Damage	What it usually means
Up to 0.1 mm	Negligible	Hairline cracks; dealt with when repainting
Up to 1 mm	Very slight	Fixed during normal redecoration
Up to 5 mm	Slight	Open up and fill; doors and windows may stick
5 to 15 mm	Moderate	Masonry repair, possibly work on window frames and services
15 to 25 mm	Severe	Rebuilding sections of wall; structural investigation
Over 25 mm	Very severe	Major structural repair; possible propping

More important than today’s width is knowing whether the crack is growing.

Monitor before you repair

• Mark and date the ends of the crack in pencil and photograph it with a ruler alongside.
• Use a crack-width gauge to measure the width, always at the same point.
• Fit tell-tales — small plaster seals or plastic crack monitors fixed across the crack. If they break or shift, the crack is active.
• Keep records for several weeks or months, ideally across different seasons: some cracks open and close with temperature without actually growing.

How they are repaired

Stable cracks in render

1. Rake the crack out into a “V” and remove loose material.
2. Clean and dampen (or prime, depending on the product).
3. Fill with a mortar compatible with the substrate, or a repair filler.
4. Where different materials meet, embed a fibreglass mesh in a new coat of render or filler, extending well beyond the crack on each side, to spread the movement.
5. Once dry, paint with a good-quality or elastomeric paint.

Cracks that open and close

When movement is unavoidable (for example due to temperature changes), filling with a rigid material is pointless: the crack reopens. The solution is to create a movement joint, sealed with a flexible sealant, in a controlled and visually acceptable position.

Structural cracks

First the cause is dealt with — underpinning foundations, strengthening beams or slabs, correcting loads — following a design by a structural engineer. Only then are the cracks repaired, for example by resin injection in concrete elements or stitching masonry.