Foundation depth

N

nm7642

New Member
Evening all,

We're planning on a single storey extension under permissable development. The issue is our neighbours trees will be circa 2m from the edge of our foundation and so building control said for that as is we'd be looking at a raft foundation. I'm not sure how this was overlooked by the architect by we move on.

The neighbours aren't opposed to removing some trees however they want an exact answer on how far as building control have said there can be a 1.9m trench if trees are removed.

My question is, how far away to the trees need to be removed from the foundations under if it's

Clay soil
Lawson Cypress circa 7m tall

Many thanks!!
 
Nice one thanks, so if I got them cut back to 5m from edge of foundation would I be correct in saying that that would make it a 1m dig?

I found that earlier but what has be puzzled is when I spoke to building control he mentioned about the roots being still left in there and heave so would it still require a deeper foundation as they have been there recently and haven't always been 5m away if that makes sense?
 
There's a couple of misconceptions here.

Nowt to do with PD/planning this is pure building Regs. A raft is one solution where strips would be too deep, but another is piles

The goodish news is that the Lawsons Cypress is actually the least bad conifer to have nearby (but not by much)

The reality of ground movement under foundations is that it doesn't matter if one bit is going up or down relative to the other, the net result is cracking above. The usual situation is tree sucks water out of ground, ground shrinks.. However if a tree is removed ground water will migrate back into the dryer ground to reach equilibrium with the surroundings causing expansion and again cracking.

So if you remove the trees then BC will expect the founds to match the current 7m high/2m distant situation which is about 2.2m. Go back 5m and leave the trees and the criterion will be max chart height (18m) at 5m which will be about 2m. The good thing is that you can step the foundations as the distance increase will need a suspended floor as you're over 1.5m deep. The effect of the removed tree will reduce with time, but in theory this could be years (ground water movement has caused collapse in many rail cuttings for instance which have stood for 100 years). In this case you wouldn't need to worry as the remaining trees would be critical.

You need an SE either way: a slab will typically be 225/250 thick with 2 layers of heavy mesh over 250/300 subbase. There will be a thickening round the edge and typically a step detail to form the bottom of the cavity. Dependant on the size you'd probably need 6 piles plus ground beams then a beam and block floor. The piled approach will probably involve less excavation/disposal.
 
nm7642 said:
Nice one thanks, so if I got them cut back to 5m from edge of foundation would I be correct in saying that that would make it a 1m dig?

I found that earlier but what has be puzzled is when I spoke to building control he mentioned about the roots being still left in there and heave so would it still require a deeper foundation as they have been there recently and haven't always been 5m away if that makes sense?
Click to expand...
I am not an expert on these things but BC are correct. The roots will remain at its existing size and a fast re growth may occur. Non coniferous trees can be pollarded but not sure if this applies to conifers.
 
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