Subsidence in a building

Subsidence usually shows up first as a crack that gradually becomes longer and bigger. Some new build extensions will have bedding in cracks that after six months to a year should stabilise.
When stripping old wallpaper and a large crack is unearthed most of the time there is no cause for concern, it may be something that happened a long time ago such as bomb damage from the war.

Some properties are built on clay and in severely wet years the clay can expand, or shrink in dry years causing movement in the walls or outbuildings of a house.
An extension built with a poor foundation or even no foundation at all and built over soft ground will eventually sink down causing cracks and often severe damage with the only cost effective way to remedy this would be to knock down and rebuild with a proper foundation.
If a stream diverts and begins to run near to or under a house then erosion will occur and subsidence can set in.
Tree roots from large trees too close to the house can over time cause damage too.

Over time the mortar in older houses, often in stone, and usually near to a busy road will crumble and sink and bow out, this is not always a sign of subsidence, especially if it was built upon rock as some were, but the only remedy for this kind of damage, once it has gone too far is to support the roof and interior walls and floors then re-build, tying in to the existing building as it goes up. This can be costly.
A crack that is suspect can have a thin strip of glass bonded to either side of it and if over a period of months the glass cracks then you can be sure there is movement, often knocking out and re-bedding every other course of brick or stone will solve this, and also using ties binding the wall inside and out can be used as well.

A surveyor when examining a property looks for a number of obvious and not so obvious symptoms such as floors that have dropped where the beams underneath are supported by a load bearing wall; windows an doorways that are not square and have sticking doors. Cracks in the wallpaper, sagging roofs, and standing back to view walls from a distance to see if there are any tall tale cracks running any which way and sometimes corbelled that are not obvious from directly below.
Really bad subsidence can leave walls hanging dangerously over nothing. Subsidence is not something that can be patched over and hope for the best because if it’s happening and on the move then expert and professional help will have to be called in immediately.

It’s an extremely expensive business to right a house with subsidence and usually involves clearing the ground floor of everything including floors to excavate and bridge the walls by underpinning with whatever the subsidence engineer has specified.
Doing it yourself is really not a good idea, and most especially without knowing the cause.
There are many causes of subsidence and your house buyer should bring to your notice the most obvious ones such as mineshafts, areas of clay, old stone houses near to busy roads or works near to the house that could affect it.
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