Introduction
It is never nice to see cracking in your property however a crack need not always be concerning. In this blog we will look at some of the types of cracks that can occur, what they mean and what type of repair is necessary.
The BRE 251 Digest; Assessment of Damage in Low-Rise Buildings
A good starting point for any review has been prepared by the BRE, formerly known as the Building Research Establishment and now known simply as BRE who undertook a survey of 130 houses that had suffered from subsidence. From this they were able to prepare a damage category list which is outlined below (ease of repair is noted 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.
Cracks below 5 mm in width can be considered aesthetic only and therefore can be repaired by a painter/decorator. Cracks over 5 mm will generally require a structural repair which in many cases will need to be specified by an engineer. As cracks become very significant, it will be necessary to break out sections of brickwork and possibly rebuild whole portions.
So what causes cracking?
We have seen that cracking from 0.1 to 5 mm will only require a decorative repair and we will now look at some of the main causes of cracking in houses.
Subsidence:
This can be defined as the downward movement of the soil beneath a house. By far the most common cause of subsidence is the action of tree roots on shrinkable soils such as clay. If a tree is allowed to mature in relatively close proximity to a house which sits on clay, the tree will draw up large amounts of water which will cause the clay to shrink and the house to subside.
Another common cause of subsidence are broken drains which cause water to leak out into the clay which over time weakens it significantly. There are more rare instances of subsidence due to coal mining or other mining activity beneath a property however this is less common and usually highlighted as part of a legal review prior to your purchase.
Subsidence cracks are most commonly wider at the top of a property, and the cracks will generally travel diagonally in opposition to the downward movement, this can be seen in the image below.
A common repair for subsidence is to remove the offending tree in stages so that heave does not occur (explained below) and underpin the property, repairing significant cracks and repointing.
Settlement:
This is defined as the downward movement of a property which has been newly formed. This can also occur if you extend your property and cause additional load to be applied to the soils. This is a process of soil consolidation and tends to occur once over a period of time and then cease. This must be carefully controlled in the design of a new property, but generally should not be considered worrying unless there has been an error in construction or design.
If cracking has been accommodated then decorative repairs only will normally be necessary. In some cases it may be necessary to stitch brickwork back together.
Heave:
This is the upward movement of soil and can broadly be considered the opposite of subsidence. This is caused as a result of dry soils expanding due to water being reintroduced. The most common example of heave his where a large tree is removed and the water which it would have drawn up is now introduced back into the soil. This is why it is best to remove a large tree over time so that there is no sudden heave.
Where heave occurs cracking is normally wider at the bottom of the property and tapers as it rises and cracks generally occur at right angles to the diagonal and upward movement. This can be seen in the diagram below.
Thermal movement:
All materials to some extent expand and contract as they warm up and cool down. In modern construction this is dealt with by movement joints so that cracking occurs exactly where you want it to rather than anywhere where it would cause a problem however with older buildings thermal movement can cause problems where large expanses of brickwork or flat roofs are subject to direct sunlight.
Thermal movement may be difficult to analyse due to the materials, angle of the material relative to the sun and its age and there are various forms of repair or mitigation which can be undertaken to minimise the impact of thermal changes or reduce temperature change altogether.
Seasonal movement:
In the United Kingdom we are clearly subject to the four seasons each of which has different characteristics which will affect shrinkable soils (clay and silts) in different ways. As in the case of subsidence, in the summer clay will shrink as it dries out, particularly in times of drought. Conversely some soils will expand during the winter months with heavy rainfall. Older properties such as those built in the Victorian period will have relatively shallow foundations and therefore will be more subject to seasonal movement and you may find that these properties have cracks which open and close across the course of one year. These cracks are rarely worrying and generally can be dealt with through normal decoration.
Synchronous or Diachronous inspection?
These terms are sometimes used by surveyors or engineers however put simply synchronous inspection is a snapshot of your property taken there and then. This is a great way of assessing what kind of movement has taken place but not whether continued movement is occurring. For this diachronous inspections will be required and again put simply this means that more than one inspection is required to assess whether movement is continuing. In the case of subsidence it is common to employ a form of monitoring where repeated visits by a special technician show whether movement is continuing or if it has ceased and therefore repair works can be undertaken.
If you are in any doubt feel free to give The Hopps Partnership a call and one of our expert surveyors will be able to attend your property in the first instance to assess the type of cracking that has occurred. The list above is not exhaustive and it may be that neighbouring construction work is to blame however this will become clear on closer inspection.
