Classification of foundation and wall crack damage

Category of damage	Description of typical damage (Nature of repair in italic type)	Appropriate action (Relevance of underpinning in italics)
0	Hairline cracking which is normally indistinguishable from other causes such as shrinkage and thermal movement. No action required.	Remedial measures are generally unnecessary as cracks can be repaired as part of routine maintenance. Where cracks recur during periods of dry weather, consider pruning nearby trees and shrubs. Monitoring is needed to confirm that damage is caused by foundation movement. Underpinning unlikely to be cost-effective except in very rare circumstances, for example where there is recurrent damage to expensive wall finishes.
1	Fine cracks, which can easily be treated using normal decoration. Damage generally restricted to internal wall finishes; cracks rarely visible in external brickwork. Typical crack widths less than 1/16"
2	Cracks easily filled. Recurrent cracks can be masked be 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 width up to 3/16".	Cracks which appear at end of summer and close during subsequent winter can be repaired in spring and steps taken to reduce the risk of damage recurring, such as pruning nearby trees and shrubs. Where cracks are not seasonal, having taken steps to minimize the movement, monitoring should be used to establish extent, magnitude and rate of foundation movement. Underpinning is unlikely to be cost-effective, unless foundation movement is progressive or excessive and there is either a likelihood of recurrent damage which will be expensive to repair; or the potential for further movement (e.g. as a result of heave) will create excessive damage (say Category 4)*
3	Cracks which 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 up to 5/8".	Having taken steps to mitigate the cause of the movement, monitoring should be used to establish extent, magnitude and rate of movement; brick arches and other susceptible features may need propping to prevent deterioration. Underpinning is likely to be cost-effective, where movement is progressive or excessive and alternatives such as tree removal are impracticable.
4	Extensive damage which requires breaking-out and replacing sections of walls, especially over doors and windows. Windows and doorframes distorted, floor sloping noticeably. Walls leaning or disrupted. Typical crack widths are 5/8" to 1", but also depends on the number of cracks.	Unless there is risk of instability, monitoring should be used to establish extent, magnitude and rate of movement. Wherever practicable, steps to remove the cause of the movement should be taken prior to monitoring. Underpinning is needed to prevent instability where movement is progressive or excessive, unless the cause of the damage is obvious and can be easily removed; for example, if caused by a large tree and there are no impediments to its removal. This may be preferable to underpinning.
5	Structural damage, which requires a major, repair job, involving partial or complete rebuilding. Beams lose bearing, walls lean badly and require shoring. Windows broken with distortion. Damage of instability. Typical crack widths are greater than 1", but depends on number of cracks.	Temporary support (e.g. external shoring and/or internal propping) is probably needed to prevent collapse. Monitoring may be needed to give warning of instability, but is unlikely to aid selection of appropriate remedy. Underpinning or rebuilding on deeper foundations needed to reinstate affected areas †; work should be implemented rapidly to prevent unnecessary deterioration of the structure.