House built on shrinkable subsoil

Chakhusi

Hi all,

We were planning to buy a house but the report says its built on shrinkable subsoil which can effect the foundation during long dry spells. Also it has a low risk of flooding in case of heavy rainfall. Though we liked the house very much but we are not desperate to buy it.
Please suggest if we should go ahead with this deal.

Thanks in advance

BitterAndTwisted

Half of the houses in the South-East of the country are built on clay which behaves in precisely the way: extreme wet conditions equals subsidence, extreme dry equals heave. But still people buy those properties quite happily.

lessonlearned

Same where we live - half the county is clay....

If there are no trees to make matters worse you should be ok.

We are currently buying a property and the survey came back with dark mutterings about shrinkable subsoil and flood risk.

I had already checked the flood risk websites so am not that concerned. 2% chance of a 50 year event.

Plus I know that the council is currently working on further flood defences and has a 20 year stragtegy in place.

The survey also flagged up an issue with a couple of trees but we had already spotted that and had decided they would be cut down.

The house we are buying is 250 years old. During WWII the Luftwaffe went off course and dropped bombs less than half a mile away.

The house is still standing and is obviously not going anywhere. If the might of the Luftwaffe couldn't destroy it then I'm not unduly concerned about the clay subsoil and a couple of trees.

Do your homework, check the Environment websites, check what provision your council has made for future flood events. You will then be better placed to make an informed decision.

My last house nearly flooded, nowhere near a flood plain, but Severn Trent had not maintained the drains properly.

Houses can flood even when they are nowhere a flood plain or a river.

Furts

With clay "sub soil" the odds are if it was built after the mid 1970's the foundations will be to a suitable depth. Before this and any house has a risk.

However, unless trees are nearby, or a drain or water pipe failure, the only real risk is if we have another summer like 1976. After waiting for 37 years this still has not happened!

Chakhusi

Thanks lessonlearned, infact the survey does talk about a couple of trees very close to the house. According to them those trees need regular pruning and at the same time we should not cut them down either as that may also shake the foundation. They are huge trees at just a few feet distance.
This house was built in 2007, so I guess the builder would have kept these things in mind.
Still would you recommend a proper structural survey as one of the engineers we spoke to mentioned that the type of tree also matters. Only some pose a risk because of their long and strong roots.
We don't want this house to become a dead investment for us because if we ever try to sell it, the buyers should not withdraw for these reasons.

lessonlearned

You would need to discuss pruning and or chopping down with a tree surgeon. Generally it is advised to cut down the trees in easy stages rather than one fell swoop to minimise the risk of soil heave.

The tree surgeon is the expert on this.

With regard to the house, given the trees were already in situ then the foundations should have been built with the trees in mind, however if you are in any doubt then get a structural engineer in.

If the trees were fully mature then you may find that they are subject to a Tree Preservation Order, in which case you may not cut them down completely buy you may be able to prune them back.

You must check to see whether a TPO is in place. It is an offence to disobey it.

Furts

Chakhusi wrote: »

Thanks lessonlearned, infact the survey does talk about a couple of trees very close to the house. According to them those trees need regular pruning and at the same time we should not cut them down either as that may also shake the foundation. They are huge trees at just a few feet distance.
This house was built in 2007, so I guess the builder would have kept these things in mind.
Still would you recommend a proper structural survey as one of the engineers we spoke to mentioned that the type of tree also matters. Only some pose a risk because of their long and strong roots.
We don't want this house to become a dead investment for us because if we ever try to sell it, the buyers should not withdraw for these reasons.

What is needed for full peace of mind is;

Identify each of the tree types
Measure the distance from each tree to the house
Correctly identify, or assume a worse case scenario, the type of clay
Calculate the required foundation depth
Apply a factor for the geographical location of the house
Trial dig to check the depth of the foundations

This procedure would not be the remit of a tree surgeon. A Chartered Building Surveyor, or Clerk of Works, or Structural Engineer would be fine.