Settlement and subsidence: what you need to know

Settlement and subsidence are terms that are often misunderstood. Both terms describe the downward movement of a building, resulting in cracks, but there are important differences, with varying outcomes.

structural-engineers-reportsIt’s important to be able to distinguish between settlement and subsidence cracks as settlement is normally historic, not normally a problem but not insurable whilst subsidence, might be an ongoing problem and can derail the buying and selling process if not reported properly.

What is settlement?

Settlement is downward movement caused by the weight of a new building squashing the soil beneath it. Settlement normally occurs in the first few years following construction and tended to occur in period properties many years ago.

Once a property has been standing for about five years, settlement should be complete. This means that any further signs of movement will probably be due to subsidence.

Modern properties and extensions, which have been built correctly, should not experience much settlement, provided the foundations have been designed properly by a qualified structural engineer.

Current building regulations require minimum foundation depths and soil bearing pressures to limit settlement; however local authorities do not always insist on future proofing foundations against foreseeable tree root growth on clay soils. So, although the foundations comply with regulations at the time of construction, in time they might experience subsidence if they have not been designed properly by a qualified chartered structural engineer.

Some conservatories and other minor structures, which don’t need to comply with building regulations, can experience settlement if their foundations are not deep enough.

Settlement is not normally an “Insured Peril” under household buildings Insurance whereas subsidence normally is, but you should check for exclusions.

What is subsidence?

Subsidence is the downward movement of the soil beneath a property’s foundation and is independent of any weight loading. Most subsidence is minor and can be easily repaired but on occasions it can lead to more significant problems. Our job is to assess the extent of the problem and the future risks involved.

Which buildings are most affected by subsidence?

All types of buildings can be affected by subsidence including old and new properties.

Older properties built on clay soil are particularly susceptible. This is because clay soils tend to shrink in dry weather causing the building to subside. Properties built on sand or gravel can also be liable to erosion and subsidence but this is far less common than when clay soils are involved.

What causes subsidence?

The main cause of subsidence is the drying shrinkage of clay soil in periods of draught, normally made worse by tree roots sucking moisture from the soil.

Less common is the washing away of fine soils like sand and silt when drains are defective.

Clay soil shrinkage

Clay soil has a high-water content, typically 30%. Warm weather and drought can cause clay soil to shrink and move downwards as the water within in is removed and its volume reduces. This, in turn, leads to distortions of the building, normally tilting and rotation with consequent cracking to brickwork and finishes.

Poor foundations

Building regulations have changed in recent decades to ensure modern houses have greater resilience from subsidence. Period properties may be at greater risk because of shallower foundations and poor ground conditions.

settlement or subsidenceTrees

Trees growing near and beneath a property’s foundation can cause the ground to shrink and subside. Tree roots can extend as far as the height of the tree. Ie the taller it gets the further the roots spread.

Many street trees are located within three to five metres of the frontage of many houses. They are often pollarded by local authorities to reduce the risk of subsidence, while also maintaining the amenity benefits of trees.

Defective drainage

Another common cause of subsidence is a defective drainage system, typically when sited in sandy soil or other loose material such as gravel. If the drainpipes are displaced, with open joints, loose soil enters the drains and is washed away, leading to erosion and voids beneath foundations with consequent loss of support causing subsidence.

How can you tell if a house has subsidence?

Some of the tell-tale signs of subsidence include corresponding interior and exterior cracks, often diagonal and normally emanating from the TOP (not bottom) of a wall. Cracks might start small and can easily be repaired IF it is foreseeable that they will not continuous reoccur and become much worse over time. It is the role of the Chartered Structural Engineer to assess these risks and advise accordingly.

Structural Engineer’s General Movement report will determine the nature and extent of the subsidence, advise on future risks of a recurrence and make recommendations for repairing any damage that has been caused, and give an indication of the likely costs.

How is subsidence fixed?

The scope of repairs is very dependent upon the severity of the damage, the cause of the damage and the risks of it reoccurring if remedial actions are not taken.

Most period property experiences minor damage from time to time and simple cosmetic repairs might be all that is needed. If large trees are involved (on clay soils) or if the drains have failed (on sandy lose soils), works to said trees and drains may be necessary. Only in the most extreme cases where trees cannot be controlled (Eg Due to not owning the tree or preservation order etc) and where damage is severe, underpinning may be necessary. This involves the “ deepening” of the foundations with concrete.

Pole Structural Engineers can assess and report, helping you decide how best to deal with the matter; indicating the nature of the damage, the likely cause and what actions are necessary to resolve.

If you suspect subsidence, please contact us so we can instigate a thorough inspection.