I’ve found a crack in my house, what should I do?
Cracks in houses can be caused by several factors. The following provides a guide of the types of cracking you would usually see with subsidence:
- Vertical and diagonal cracking concentrated in specific areas and tapering in width between the top and bottom of the property
- Cracks extending through the damp proof course, down into the foundations
- External cracking reflected internally in the same area of wall
- Rucking of wallpaper at corners between walls and ceilings
- Misaligned or distorted windows and door frames; causing doors and windows to stick
- Cracks appearing after a prolonged period of dry weather
- Seasonal opening and closing of cracks
If your property is exhibiting any of these traits, or you fear it is subject to subsidence, then the sooner it is diagnosed the better.
First, check that your buildings insurance covers subsidence. Most insurers will be helpful in dealing with a claim and they will recommend specialist advice. A Chartered Structural Engineer or Building Surveyor will be able to work out whether or not there is subsidence and what the likely cause is.
The insurance company will advise you on the next steps to be taken. Where appropriate, specialists will be appointed to investigate the cause of the damage and to arrange for repair work to be carried out. Note: even the effects of serious subsidence damage can often be rectified by means other than under-pinning.
In the most serious of cases, subsidence investigation (including monitoring of the movement causing the damage) can take a considerable period of time. Where monitoring and substantial repair work is required, the timeline may exceed 12 months.
Are all cracks in houses caused by subsidence?
Cracks in houses can be caused by a range of issues other than subsidence. Some house cracks are simply due to normal wear and tear, the age and construction of the property, or other non-foundation related structural issues. Common causes include:
Settlement after construction
When a property is built, its weight may cause the ground to consolidate and lose volume.
Thermal expansion and contraction of materials
Modern, rigid structures have less ability to expand and contract from changes in temperatures. Block work (which is generally used as the inner skin in most modern external cavity walls) is particularly prone to this effect, which results in vertical cracking.
Other materials prone to temperature shrinkage include modern mortars (cement based), re-constituted stonework, concrete lintels over windows and doors, metal and steel lintels and beams, timber and even plasterboard ceilings.
Lintels support the walls above windows and doors. They can deteriorate over time, resulting in a weakening of the lintel with resulting movement of the supported wall. Cracks usually appear above the window or door opening.
Degradation occurs in chimneys where the chemicals in the gases from the fire (sulphates) attack the brickwork. This leads to expansion and vertical cracking of the chimney stack.
If the roof is poorly constructed, if there has been structural deterioration, or if there has been an increase in weight on the roof (such as the installation of Solar Panels) then the roof may push the walls outwards, resulting in cracking.
Wall tie failure
Wall ties bind together cavity walls. They can corrode and expand; pushing out the external wall and resulting in horizontal cracking.
Why is clay so prone to shrink/swell?
Clay soils are highly susceptible to volumetric change, because of their molecular structure. Clays have a repeating arrangement of atoms (known as a crystalline structure) with weak bonds that hold the clay crystals together. These allow water into the spaces between the crystal and push them apart, causing swelling. Logically, the opposite is also true and results in shrinkage.
Clay is unique in containing water within its molecular structure. In all other soil types, water simply occupies the gaps between soil particles. To complicate matters further, there is more than one type of clay; each with different properties.
Montmorillonite is the most highly expansive clay, whilst kaolin is less expansive. The shrink/swell potential of clay is measured through the ‘Plasticity Index’ (the range of moisture content over which clay retains its plastic properties). An index of 20 denotes a soil susceptible to ‘medium’ shrink/swell.
What else can cause subsidence?
There are several man-made triggers that can result in subsidence, most of which are caused by vibration, or as a result of building on landfill.
In-filled sites are typically where pits or quarries have been filled in after excavation. They can be filled with a variety of materials, including refuse, and covered over. The fill can consolidate or degrade over time, causing a reduction in volume of the soil.
Excavations around the foundations (e.g. extensions, new drainage or work on adjacent land) of a property can cause a sideways collapse of the ground beneath the foundations.
Vibration can consolidate and settle soil, leading to downward movement of the foundations. This can be caused by heavy, constant traffic near properties; or pile driving on an adjacent site.