Drains and Subsistence

In this post I discuss the issue of leaking drains and subsistence.

Subsistence is defined by the Institution of Structural Engineers as "the downward movement of a building’s foundation caused by the loss of support site beneath the foundation”. In this post I discuss common causes of subsistence, the signs of subsistence and action you may take to remedy this unfortunate problem, particularly with reference to your drains.

Common causes of subsistence

In the United Kingdom subsistence affects around 50,000 homes. The South East is the region most affected by subsistence. This is because many of the regions homes are built on ‘London clay’. This variety of clay is at a heightened risk of shifting, particularly following a dry and hot summer. Here a lack of moisture causes the cray to shrink leading to subsistence.

Another common cause of subsistence is over-grown trees. Trees located close to your property absorb water from surrounding soil by a process known as photosynthesis. This causes a moisture deficit in surrounding clay and soil. In turn soil and clay shrink. This shrinkage leads to subsistence. Around 65% of subsistence in the UK is due to tree photosynthesis. Oak and willow trees are the worst offenders given the sheer size of their roots.

The other major cause of subsistence in the United Kingdom is leaking drains in need of drain repair. Water omitted from damaged drains softens your home’s foundations. Over time the ground shifts and soil beneath your home’s foundation may totally wash away. Experts reckon around 30% of subsistence is due to damaged drains. A major cause of damaged drains is root intrusion.

Signs of subsistence

A classic tell-tale sign of subsistence is diagonal cracks appearing on inside plastered walls or bricked-up external walls. Cracks caused by subsistence tend to be at least 2mm thick, or around the same size as a ten pence piece. Cracks located around door and window-frames particularly suggest the occurrence of subsistence. Other tell-tale signs include sunk windows and doors.

Action to remedy the problem

The appropriate remedy for subsistence depends upon the cause of the problem. I therefore advise you pay for a complete structural survey to determine the cause and extend of the problem. You must then contact your home insurance company with this information. If you fail to contact your insurer you may invalidate your policy. This is because withholding this information is seen as a ‘failure to disclose material information.’ Insurers are renowned for turning down subsistence claims for this very reason.

If the problem is due to trees

If you believe surrounding trees are the cause, you must prune the trees’ foliage back. This action deceases the amount of water the tree will absorb from surrounding soil and therefore slow down or prevent further subsistence. Although this is not the most environmentally-friendly option, you may be advised to chop the offending tree down completely. Please determine the exact tree type of tree at issue becomes some types of trees are protected by law .

If the problem is due to drains

If you believe your drains may be the cause, we suggest you book a CCTV drain survey without delay. Here our engineer will determine the cause of the leak and repair drains accordingly. Once repairs are complete you are then advised to check your drains regularly.

Underpinning

As a measure of last resort your home may require ‘underpinning.’ This process strengthens your foundations. However underpinning results in substantial increases in the cost of insuring your home. Therefore underpinning should only be implemented after the above measures have failed to solve the problem of subsistence.

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