Cracking and building movement

Cracking is likely to occur if a structure is unable to accommodate this movement. For occupants, the appearance of distortions and cracks can be visually unattractive and disturbing, and they can affect the integrity, safety and stability of the structure if left untreated.

Effective treatment first requires an understanding of the causes of building crack. Only then can a building repair strategy be implemented.

Causes of cracks

The most common causes of cracking are:

  • Ground movement (beneath foundations) caused by clay shrinkage, landslip, vibration, subsidence, settlement, heave, sway, and so on.
  • Foundation failure due to the decay of soft clay brick, concrete erosion due to chemical contaminants, and so on.
  • The decay of the building fabric, due to woodworm, rust, and so on.
  • Moisture movement that causes materials to expand or contract, perhaps due to the presence of vegetation or faulty or damaged drains.
  • The thermal movement that causes materials to expand or contract as temperature increases or decreases.
  • Inherent defects, particularly in historic structures.
  • Faulty or damaged drains.
  • Suspended structures such as floors that deform under load.
  • Tree root growth.
  • Absence of foundations in older buildings.

Size of cracks

Experience engineering firm will assessed building properly and identified six categories of cracks together with the typical damage caused and the remedy required:

  • 0 – Hairline cracks: Less than 0.1 mm in width. No repair action required.
  • 1 – Fine cracks: Up to 1 mm in width. It is generally restricted to internal wall finishes and easily treated using simple decoration.
  • 2 – Cracks quickly filled: Up to 5 mm in width. Not necessarily visible externally, but doors and windows may require adjusting to prevent sticking. Can be masked by suitable linings.
  • 3 – Cracks that require opening up: Widths of 5-15 mm. Weather-tightness and service pipes may be affected. External brickwork may need repointing or, in some cases, to be replaced.
  • 4 – Extensive damage: Widths of 15-25 mm. Windows and door frames become distorted; walls lean or bulge noticeably. Requires breaking-out and replacement of wall sections.
  • 5 – Structural damage: Widths greater than 25 mm. Beams lose their bearing, walls require shoring, and the structure is generally unstable. May require major repair works.

Shapes of cracks

  • Hairline cracks are common in many buildings, particularly in plaster, prone to shrink and sensitive to movement.
  • Stepped cracks tend to follow the lines of horizontal and vertical joints in buildings, such as beds of mortar between bricks or blocks and may indicate structural movement.
  • Vertical cracks may indicate that structural components such as bricks or blocks have failed and can sign significant stresses within the building structure.
  • Cracks that are wider at the top or the bottom may indicate that there has been foundation movement, with the widening direction indicating the movement’s likely path.
  • Horizontal cracks may indicate that an element such as a wall is failing, presenting a safety concern.

Dealing with cracks

Designers of new buildings may try to ‘design out’ many of the causes that may lead to cracking by careful design of the foundations and superstructure, introducing movement joints, specifying materials that can tolerate movement, and so on.

The potential cause of movement and whether this is due to an external factor (such as subsidence), or is caused by the building’s fabric, should be investigated in existing buildings (such as a low grade of cement). It is necessary to assess if, and if so, to what extent and over what period, the cracking is likely to be progressive and deteriorate further.

Repair strategies can range from major interventions to limited cosmetic works. However, unless the underlying cause has been dealt with, further maintenance work will likely be required in the future. 

Movements caused by vibration or thermal expansion and contraction, for example, are likely to continue. In this situation, it is important to carefully choose repair materials; those with the potential to bend in a plastic way would be more effective than those that increase pressure around the crack and are therefore likely to cause the repair to fail.

For example, hydraulic or fat limes may provide enough plasticity to accommodate strain. Corrosion-resistant stainless steel reinforcements can be inserted into bed joints to reduce the risk of failure by redistributing strain and stress over a wider area.

Instruments such as boroscopes can be used to locate voids that may have opened up due to movement. Grouting voids to fill them with cementitious material can provide a solution if the underlying cause has been dealt with.

Where underpinning of foundations is required, several alternative techniques are available. 

Rather than simply covering up problems, it is important to understand why cracks have occurred, particularly if they are large, or if they are increasing in size, and it may be wise to seek expert advice.

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