Underpinning is a last resort. It's highly unlikely to be necessary. The insurance company will do all they can to avoid paying out to have it done, and it's not what you want either, even if they pay, due to the future issues with selling and remortgaging.
The crack at the moment is minor. No-one will be underpinning your house for such minor movement. As I said, most likely the insurance company will send out an assessor, who may also require further advice from a structural engineer.
I guarantee you, having worked on such issues on many occasions, the structural engineer will report on the crack, mention that it's wider at the top than the bottom, indicating potential foundation rotation caused by downwards movement at the far end of the extension. They will certainly mention the tree, and they will ask whether the extension predates the tree.
They will then ask for a trial pit to be dug to determine the depth of the existing foundation.
They will get soil samples taken from the underside of the foundation and have them sent away for analysis, to determine soil type (shrinkable or non-shrinkable), and if shrinkable, to determine volume change potential, modified plasticity index and desiccation. The first two will help to determine whether the foundations are deep enough and the last one will determine whether the soil is already so dry that it will have potentially shrunk.
They will get root samples analysed from the base of the foundation if there are any, to confirm what tree the roots belong to (probably obvious in this instance).
They will then make recommendations: If the extension predates the tree, they will recommend that the tree is removed. This will allow the soil to recover over an extended period of time, allowing the crack to close up.
If the tree predates the extension, particularly if it was mature or semi-mature, it's not quite so simple. If the tree was there before the foundation was built, your foundations should be deep enough to allow for a mature Ash 1m away. The engineer will be able to tell you if that's the case. They would need to be far in excess of the usual 1m though, and if high volume change soil then your extension should have been put on piles.
If the tree was already mature then a root protection zone should have been considered which would most likely have meant that the extension would have been put on piles.
For many reasons I suspect that the extension is probably older than the tree, but you need to confirm that??
The crack at the moment is minor. No-one will be underpinning your house for such minor movement. As I said, most likely the insurance company will send out an assessor, who may also require further advice from a structural engineer.
I guarantee you, having worked on such issues on many occasions, the structural engineer will report on the crack, mention that it's wider at the top than the bottom, indicating potential foundation rotation caused by downwards movement at the far end of the extension. They will certainly mention the tree, and they will ask whether the extension predates the tree.
They will then ask for a trial pit to be dug to determine the depth of the existing foundation.
They will get soil samples taken from the underside of the foundation and have them sent away for analysis, to determine soil type (shrinkable or non-shrinkable), and if shrinkable, to determine volume change potential, modified plasticity index and desiccation. The first two will help to determine whether the foundations are deep enough and the last one will determine whether the soil is already so dry that it will have potentially shrunk.
They will get root samples analysed from the base of the foundation if there are any, to confirm what tree the roots belong to (probably obvious in this instance).
They will then make recommendations: If the extension predates the tree, they will recommend that the tree is removed. This will allow the soil to recover over an extended period of time, allowing the crack to close up.
If the tree predates the extension, particularly if it was mature or semi-mature, it's not quite so simple. If the tree was there before the foundation was built, your foundations should be deep enough to allow for a mature Ash 1m away. The engineer will be able to tell you if that's the case. They would need to be far in excess of the usual 1m though, and if high volume change soil then your extension should have been put on piles.
If the tree was already mature then a root protection zone should have been considered which would most likely have meant that the extension would have been put on piles.
For many reasons I suspect that the extension is probably older than the tree, but you need to confirm that??
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