What is it?

Subsidence and heave are caused when the ground beneath a building moves significantly.

• When the ground moves downwards, it is called subsidence
• When the ground moves upwards, it is called heave.

Causes of subsidence

Potential causes of subsidence are:

• Clay soils normally hold a significant proportion of water and maintain this moisture content over many decades. If these soils lose much of this water, either during a long spell of warm, dry weather, or due to a tree drawing the water out, the clay can collapse under the weight of the foundations.
• Soils that contain a high proportion of sand or gravel can be washed away if a damaged drain allows leaking water to flood through. This reduces the volume of the subsoil, allowing the weight of the property to distort the supporting ground.
• Soil on sloping ground is not as stable as that sitting on horizontal ground and can sometimes become unstable and start to slip downhill.
• Old mining shafts cut deep into the ground can start to shift and settle, which causes the ground on top of them, often quite some distance away, to sink.
• Soils incorporating a large quantity of organic matter, such as peat, remain very stable when they sit beneath the water table, but if the water table should fall, this matter dries out and begins to decompose. This causes the volume of soil to reduce so that foundations can crush the subsoil.

Heave is most often caused when a mature tree dies or is removed and its root network no longer draws water from the subsoil. The ground therefore accumulates water in the absence of this dehydrating effect and swells, forcing the foundations on top of it upwards.

How to spot it

It is not uncommon for most homes to develop superficial cracks in the walls, due to changes in temperature and humidity. These can be repaired with filler and repainted.

You should also consider the risk of subsidence if you notice wallpaper rippling or doors and windows sticking in their frames, neither of which are caused by damp.

How to fix it

Firstly check whether your buildings insurance policy covers subsidence; if it does, notify your insurance company as soon as possible. Fixing subsidence can take up to two years, as cracks may need to be monitored, and the cause of the movement established. Shifting soil can also cause secondary problems such as breaking drains and piping.

Firstly, the cause of movement must be addressed:

• Fix any broken pipes, drains or water mains.
• Trees should not necessarily be removed as this may make the problem worse. Seek professional advice from a chartered surveyor as to whether the tree(s) should be removed or simply cut back to a size that will reduce its water consumption.

Secondly, if the foundations have shifted significantly or are at further risk, any further movement must be minimised:

• Underpinning utilises large concrete supports, or pins, that are placed beneath the existing foundations and extend deep into the ground beneath to reach more stable soils. It is a disruptive and expensive process (£5,000 up to £50,000), but highly effective.
• Piling, or mini-piling, uses long concrete or steel poles that extend down in to the stable soil, but are placed around the properties existing foundations to prevent them from moving and take the full weight of the building.

Finally, any damage to the fabric of the building should be repaired:

• If the structural, load-bearing parts of the property have been damaged, these are should be repaired with the inclusion of strengthening steel rods that are fixed into place with cement to reinstate the structural integrity of the building.
• Structural brickwork may need to be dismantled and rebuilt, or more cosmetic areas can effectively be glued back together with resin in the cracks.
• Superficial elements should be filled and re-weatherproofed where necessary.

Managing new and existing trees

Ensure that existing trees are well managed and new trees are planted at a safe distance from the exterior walls of the property.

• Place pine, birch and cherry trees at least 10m away.
• Place beech and sycamore trees at least 15m away.
• Place maple, lime and horse chestnut trees at least 20m away.
• Place oak and elm trees at least 30m away.
• Place willow trees at least 40m away.

Further information and useful links

• Find a Chartered Surveyor near you
• Find a Tree Surgeon near you
• Find a Masonry Repairs Service near you