Assessing cracks in houses

Everyone is familiar with cracks in the brick or concrete block walls of houses. The presence of some narrow cracks is usually tolerated and even expected if the house is old, but it is not always obvious what level of cracking is acceptable and when building repairs are necessary. Rupert Pool explains how such cracks can be assessed.

Cracks in brick or concrete block walls can be caused by a number of factors. One of the most unnerving for the occupants is subsidence or foundation movement. Changes in the ground conditions under and around a house can cause it to move slightly, which can lead to cracks to developing in the walls.

Historically, claims on household insurance policies for repairs to buildings after subsidence have increased after periods of very dry weather. This prompted the industry to look in detail at how the cracks were assessed and what repair works were necessary.

In order to help building surveyors to identify when ground movement had occurred and when structural intervention was necessary, BRE carried out an assessment of 130 properties that had suffered from subsidence.

The results of the study enabled a way of categorising cracks to be developed, which would help building surveyors and insurance assessors to determine whether the cracks were likely to affect the property – and what repairs were appropriate. The results of the study and the conclusions were published in BRE Digest 251 Assessment of damage in low-rise buildings.

Six categories of crack were identified, which linked the width and number of cracks to the type of repair that was appropriate.

Damage categories with descriptions of typical damage. Ease of repair in italics.

0 - Hairline cracks of less than about 0.1 mm which are classed as negligible. No action required.

1 - Fine cracks that can be treated easily using normal decoration. Damage generally restricted to internal wall finishes; cracks rarely visible in external brickwork. Typical crack widths up to 1 mm.

2 - Cracks easily filled. Recurrent cracks can be masked by suitable linings. Cracks not necessarily visible externally; some external repointing may be required to ensure weather-tightness. Doors and windows may stick slightly and require easing and adjusting. Typical crack widths up to 5 mm.

3 - Cracks that require some opening up and can be patched by a mason. Repointing of external brickwork and possibly a small amount of brickwork to be replaced. Doors and windows sticking. Service pipes may fracture. Weather-tightness often impaired. Typical crack widths are 5 to 15 mm, or several of, say, 3 mm.

4 - Extensive damage which requires breaking-out and replacing sections of walls, especially over doors and windows. Windows and door frames distorted, floor sloping noticeably. Walls leaning or bulging noticeably, some loss of bearing in beams. Service pipes disrupted. Typical crack widths are 15 to 25 mm, but also depends on number of cracks.

5 - Structural damage that requires a major repair job, involving partial or complete rebuilding. Beams lose bearing, walls lean badly and require shoring. Windows broken with distortion. Danger of instability. Typical crack widths are greater than 25 mm, but depends on number of cracks.

In general, categories 0, 1 and 2 with crack widths up to 5 mm can be regarded as ‘aesthetic’ issues that require only redecoration. Categories 3 and 4 can generally be regarded as ‘serviceability’ issues, that is, they affect the weathertightness of the building and the operation of doors and windows. Category 5 presents ‘stability’ issues and is likely to require structural intervention.

BRE Digest 251, and in particular the table above, is now used widely in the industry as a way of categorising cracks and determining whether any intervention is necessary.

It should be stressed that these comments are a simplification of the assessment needed to properly classify damage to housing. Several factors, including whether the widths of the cracks are increasing with time, can affect the classification. BRE Digest 251 should be consulted when carrying out any assessment and a building professional should be consulted where damage is significant.

A copy or PDF of BRE Digest 251 Assessment of damage in low-rise buildings can be purchased at www.brebookshop.com