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Foundation depth - clay & trees

Discussion in 'Building Regulations and Planning Permission' started by Petra P, 7 Nov 2019.

  1. Petra P

    Petra P

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    Hi, we are building single storey extension of approx. 40m2. The soil is clay and there are two thuja trees (10m high) 6m from proposed foundations. We had a building officer visiting today and I missed him unfortunately, so he just spoke to our builder. He requires us to dig 2.5m deep foundations because of these 2 thuja trees. I already questioned that and waiting for his call back. Is this reasonable requirement? Does he have to show me some calculations he based his decision on? This requirement will cost us £4000 more :(
     
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  3. Notch7

    Notch7

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    Building inspectors work ftom tables for zone of influence in regards to trees and shrinkable soils.

    You can check yourself here:
    https://info.labcwarranty.co.uk/foundation-calculator

    Yes inspectors can dictate foundation depth. They dont have to prove with calcs.

    If the soil is not shrinkable and load bearing, ie say its mudstone not clay, then a soil test with a report will be accepted by building control.

    I had one with an oak tree -inspector want 2.5m footings, guy who did soil test said 800mm depth was fine.

    Im surprised the architect or builder didnt warn you: clay + trees is always going to add cost.

    £4k sounds a fair bit, but I guess there could be £1k in concrete, similar in skips, bigger machine hire, extra labour, maybe shuttering and or risk of trench collapse......the costs soon add up.
    Are the builders using grab lorry or skips?

    The building inspector might expect block abd beam suspended floor instead of slab - its best to find out as the beams take a while to get drawings done and theres a lead time.
     
  4. Mottie

    Mottie

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    I had a similar problem when I had my front extension built. The stupid architect didn’t show a tree in my neighbours garden and when the building inspector came round to check the footings he noticed it and gave me a chart which indicated that I needed foundations 2.5m deep. He did say that if I only went to 1m deep it would need to be removed before he could pass it off. My neighbour agreed that I could remove it, get my extension passed off and then replant a new tree which I did. That was 17 years ago and the tree I planted is now three times the height of the one it replaced! OP - any chance you could do the same?
     
  5. Petra P

    Petra P

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    Thank you for your reply. Well i went to LABC and did the calculations and it gave me 1m depth requirement. It’s a clay and both trees are going down today,which the officer was informed about. When I did the calculations for trees staying in place it gave me 2.5 metres. So I guess he didn’t consider we are going to cut them down even though we told him... I’m still trying to get somebody from the office to give me further info,because it really looks ridiculous to require 2.5metres...
     
  6. Notch7

    Notch7

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    Removing trees can cause heave due to reduced water take up, however that may be mitigated by using jabolite claymaster (-Im not a pro so may be wrong).
     
  7. Yes , NHBC still take trees into account in relation to foundations even when trees are removed . NHBC Standards, 4.2 I think it is, gives zones of influence/precautions relating to trees etc.
     
  8. DIYnot Local

    DIYnot Local

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