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The Professor

Are oak tree roots likely to circumvent a 3.3m deep root barrier?

Question

I am in the final stages of purchasing a 1930's semi-detached house in south-east London. During the purchasing process, I have found out that the house had its rear wall underpinned in 2008 to stop subsidence which was identified as being caused by an circa 80-year old oak tree which is sucking all the moisture from the clay soil. The oak is located on an unadopted side road 16 metres from the house's back wall. The insurance company drilled 3 bore holes around the property and did DNA tests that revealed 80% of roots present were oak (it's the only oak in the area). An ornamental acer and cherry (the other 20% of roots found) were removed from the garden.

Unfortunately, in 2014, the property began to subside again, so the insurance company underpinned the house's rear wall once more with 3.3m of concrete, which they call a 'root barrier'. They tried to carry out works to the oak tree in 2014, but neighbours formed a campaign group which has resulted in the oak being given a TPO by the council.

The vendor, the the vendor's insurance company and the underpinning specialist that did the work all say that the oak roots can no longer get under the house to cause further subsidence as the oak's roots will not go that deep (3.3m), but my building surveyor says that its only a matter of a few years until the roots go under/through/around the barrier in their search for water. In your opinion, will the oak be able to circumvent the 3.3 metre deep root barrier? Any advice welcome! 

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It’s possible. If there is water and oxygen down there then yes. 

 

I’ve heard of instances in London where roots have been found down to around 5 metres. 

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I would also check that there are no leaking water or drain pipes in the area around yr house ( Thames Water are famous for that ) I wouldn't jump to the conclusion that Oak is the only culprit . Root barriers are not the silver bullet for soil dessication problems  involving trees and the rewetting of clays can take a long time before the layer  'stabilises' , heave can be an issue too . Pity yr not north of the river as I would like a look , K

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44 minutes ago, Khriss said:

I would also check that there are no leaking water or drain pipes in the area around yr house ( Thames Water are famous for that ) 

Unless there is a public sewer or Water Authority water main running within the boundary of the property it's got nothing to do with Thames Water. Unless the drains are section 24.

 

In Answer to the OP, I don't know. I have cleared/replaced drains that are deeper than 3.3m on many a occasion that where damaged by tree roots.

 

I would either spend a lot of money on specialist surveys, or walk away. 

 

Friends of ours in the village are having their house underpinned and the interior is being 'stitched' back together as I type. Costly, inconvenient job.

 

 

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Thanks for your advice so far. There is a sewer pipe running under the rear patio which had to be rebuilt while the 2008 underpinning was done.  The cast iron pipe was replaced with a plastic one and the access manhole brickwork re-pointed. I'm guessing that the original pipe may have been leaking and attracted the roots towards the house, but can't know for sure.

The extra depth added to the original underpinning was described to me as a 'root barrier' but I'll try to look up some extra information about it and post it here in a short while.

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Here's what seems to have been done during the second round of underpinning:

 

"Establish the edge of the existing underpinning projection and excavate down the side, a trench 1m wide to a depth of 3.3m. Install spreader boards and accro supports to support the side of the excavation and provide lateral support to the underpinning. In an agreed sequence, excavate under the existing underpinning bays to a depth of 3.3m and remove spoil from site. Carefully shutter and pour C30 grade concrete to form pins in sequence. Shuttering to be formed 200mm beyond existing face and the pins poured to 200mm higher than the existing pins to al low for any shrinkage."

 

Although the work has been described to me as a 'root barrier' the invoice from the company that did the underpinning simply refers to it as 'underpinning' - so does the building control sign-off certificate.

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When you say sewer pipe what exactly do you mean?

 

Is it a shared drain with neighbouring properties, or is it the drains from your house only.

 

When were the drains connected to the public sewer?

 

Dont assume that the drain is your reposibilty until you have the above facts.

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The sewage/waste-water/ drain pipe (I'm not sure of the exact terminology - the waste bath/toilet water feeds into it) starts at the rear of the neighbouring semi-detached and runs under the rear patio of the house that I'm potentially buying. The pipe then turns down the side alley of 'my' property down to the main sewer located under the main road at the front of the house. I believe that this waste-water pipe is actually the property of a Thames Water, as the next door neighbour's waste flows under the rear patio of my house.

The house was built in the 1930's, so I expect that these waste-water pipes have always been there since the house was built.

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1 hour ago, The Professor said:

Here's what seems to have been done during the second round of underpinning:

 

"Establish the edge of the existing underpinning projection and excavate down the side, a trench 1m wide to a depth of 3.3m. Install spreader boards and accro supports to support the side of the excavation and provide lateral support to the underpinning. In an agreed sequence, excavate under the existing underpinning bays to a depth of 3.3m and remove spoil from site. Carefully shutter and pour C30 grade concrete to form pins in sequence. Shuttering to be formed 200mm beyond existing face and the pins poured to 200mm higher than the existing pins to al low for any shrinkage."

 

Although the work has been described to me as a 'root barrier' the invoice from the company that did the underpinning simply refers to it as 'underpinning' - so does the building control sign-off certificate.

WHat you have described is not a barrier, it's underpinning.

3.3m is no an arbitrary number. I imagine someone specialising in clay soil subsidence has done something along the lines of a retrospective NHBC calculation and decided 3.3m is the appropriate foundation depth.

As purchaser I would be lookign for a guarantee that the work has been carried out in accordance with the specification, and an indemnity from whoever specified the works.

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From how you have described the drains it would be "public" from where the drain leaves the neighbouring property, and passes to your garden/property then it takes your wastewater etc, to the main sewer. To clarify this I'd suggest searching under "private sewer switchover" on anglianwaters web site where it is explained pretty plainly. All sewerage undertakers follow the same protocol, as its laid in law. 

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