When seeking advice with respect to the impact of cracking to walls in buildings it is important that the advice is based upon sound engineering principles and recognised methods of classifying the damage. It is essential to carry out a thorough survey from which records of damage and distortion should be clearly presented to the building owner. Assessment and categorising the damage should be carried out by an experienced Structural Engineer or Building Surveyor, with BRE Digest 251 in mind, taking into account the nature, location and type of damage.
Possible Causes of Cracking in Buildings
Associated with the External Environment:
- Trees surrounding the building may influence the existing foundation causing subsidence and loss of bearing pressure underneath the existing foundation.
- The removal of existing trees may cause progressive movement of the surrounding soil as the clay soil re-moisturised.
- Settlement or heave of floor slabs on unsuitable or poorly compacted in-fill beneath the slab.
- Instability of sloping ground.
- Movement due to consolidation of poor ground or made ground.
- Movement caused by nearby excavations or undermining.
- Chemical attack on foundation concrete
- Erosion of fine soil particles due to the passage of water (for example from a leaking pipe).
Associated with the Structure:
- Material shrinkage and creep.
- Corrosion or decay.
- Differential thermal movements in dissimilar materials.
- Poor detailing, design or workmanship.
Three broad categories of damage should initially be considered: ‘aesthetic’, ‘serviceability’ and ‘stability’.
‘Aesthetic’ comprises damage which affects only the appearance of the property and includes Categories 0, 1 and 2.
‘Serviceability’ includes damage that impacts the weathertightness or other function of the wall (e.g. sound insulation of a party wall being impaired), fracturing of service pipes and jamming of doors and windows when opening or closing. This level of damage is covered by Categories 3 and 4.
‘Stability’ describes a scenario where there is an unacceptable risk to the robustness of the building where some part of the structure will collapse unless preventative action is taken. This is covered by Category 5.
Extract from BRE Digest 251 “Assessment of Damage in Low-Rise Buildings”
|Category of damage||Approximate crack width (mm)||Classification of visible damage to walls, with reference to the potential serviceability/seriousness||Classification of visible damage to walls with reference to type of repair and rectification|
|Definition of cracks and description of damage||Definition of cracks and repair types/considerations|
|0||Up to 0.1||Cracks defined as HAIRLINE; generally considered to have negligible structural implications and can be expected to occur in almost all buildings at any location. They are not generally related to subsidence / foundation movement.||HAIRLINE – Internally cracks can be filled or covered by wall covering and redecorated. Externally, cracks rarely visible and remedial works are rarely justified.|
|1||0.2 to 2||Cracks defined as FINE. These cracks may occasionally have some structural significance but are not usually deemed serious. Often these cracks are more visible inside buildings than in external brickwork. Would generally be located at points of structural weakness in a building e.g. window/door openings. Indicates slight foundation movement, particularly if isolated. An array/series or large number of closely located fine cracks is unusual but could signify more substantial foundation movement.||FINE – Internally cracks can be filled or covered by wall covering and redecorated. Externally, cracks may be visible, sometimes repairs required for weather tightness or aesthetics. Note: Plaster cracks may, in time, become visible again if not covered by a wall covering.|
|2||2 to 5||Cracks defined as MODERATE. These cracks are likely to have some structural significance and will almost always occur at points of weakness or hinge points. Generally, cracks will be visible internally and externally and will indicate foundation or other structural movement enough to distort door and window frames and make doors and windows stick. Weather tightness may be an issue that needs to be investigated as may the structural integrity of the building.||MODERATE – Internal cracks are likely to need raking out and repairing to a recognised specification. May need to be chopped back, and repaired with expanded metal/plaster, then redecorated. The crack will inevitably become visible again in time if these measures are not carried out. External cracks will require raking out and repointing, cracked bricks may require replacement.|
|3||5 to 15||Cracks defined as SERIOUS. There will almost certainly be some compromise of the integrity of the structure and weather tightness may be impaired. Serious distortion may be occasioned to door and window frames, and glass fracturing is possible, as could be service fractures and strains.||SERIOUS – Internal cracks repaired as for MODERATE, plus perhaps reconstruction if seriously cracked. Rebonding will be required. External cracks may require reconstruction perhaps of panels of brickwork. Alternatively, specialist resin bonding techniques may need to be employed and/or joint reinforcement.|
|4||15 to 25||Cracks defined as SEVERE. Structural integrity severely compromised – floors, sloping walls leaning or bulging, bearings of beams, lintels suspect. Pipe fractures and straining likely – windows broken.||SEVERE – major reconstruction works to both internal and external wall skins are likely to be required. Realignment of windows and doors may be necessary.|
|5||Greater than 25||Cracks defined as VERY SEVERE. Potential danger from failed or fractured structural elements and for instability. Safety issues must be considered.||VERY SEVERE – Major reconstruction works, plus possibly structural lifting or sectional demolition and rebuild may need to be considered. Replacement of windows and doors, plus other structural elements, possibly necessary. Safety issues must be considered.|
For domestic dwellings, which constitute the majority of cases, damage at or below Category 2 is not of structural concern and does not usually justify remedial work other than the restoration of the appearance of the building. Unless there are clear indications that damage is progressing to a higher level it may be expensive and inappropriate to carry out extensive work for what amounts to aesthetic damage.
Category 2 defines the stage above which repair work requires the services of a builder. This level of damage also merits further investigation to determine the make-up of the supporting sub-soils and to conduct detailed examinations of the structure. This investigation work allows for an informed decision to be made with respect to the extent of any required remedial works.
In many cases, early intervention can curtail the damage from escalating above Category 2 and can lead to substantial cost savings against future remedial works.
Early intervention can include:
- All gutters, flashing and service pipes should be maintained in good condition, free of leaks and repaired when damaged.
- All surface water drainage should be collected in effective gutters, downpipes and gulley’s and discharged to surface water drains.
- In clay soils, no large trees should be allowed to exist closer than their height to the building. This distance increases to approximately 1.5 times the height of the tree for group or line planting.
- Any ivy in close proximity to the foundations should be removed immediately as its presence has a severe effect on the integrity of masonry joints.
- Air to Water units should be collected and discharge to surface water drains well clear of the building foundations.
- The ground around the building should be graded to slope away to prevent ponding adjacent to the base of the walls.
- Usually a concrete footpath is constructed to the perimeter of a dwelling to collect additional rainwater run-off and direct it away from the building. Not having this in place to may have allowed additional moisture into the founding subsoils.
An assessment of a damaged building should be carried out by an experienced Structural Engineer to ensure that the building owner is in receipt of appropriate advice for the level of damage observed.
Chancery Group Engineering Team is available to field any questions that you may have with respect to damaged buildings in your ownership. To find out more about our services, contact us via email firstname.lastname@example.org or call us on 057 9352340.