Another subsidence question thread!

[Gary Prentice]

 

Foundations must suit the situation if through Council Building Regs the councila re liable if not suitable ,hence seemingly over engineered foundation depths requested. Don't know about NHBC suspect less robust.

 

Not knowing how all the computations are calculated I phoned an architect to discuss this. Architects get the depths from the engineers, who he thinks refers to a British Standard. So, presumably the NHBC follows the same guidelines.

 

It may be that the council engineer uses over engineered foundation depths to allow for a probability of new tree planting, whilst commercial builders choose to work to the cheaper alternative when no trees are present.

[daltontrees]

Not knowing how all the computations are calculated I phoned an architect to discuss this. Architects get the depths from the engineers, who he thinks refers to a British Standard. So, presumably the NHBC follows the same guidelines.

 

It may be that the council engineer uses over engineered foundation depths to allow for a probability of new tree planting, whilst commercial builders choose to work to the cheaper alternative when no trees are present.

 

Ah but unless the builders put a restriction in the title deeds or get the new owner to sign a disclaimer, with the effect that the builder is not responsible for subsidence if trees are planted, the builder could surely find himself exposed to claims from house owner. And if the builder has no control over adjacent land which is close enough for future tree planting to affect foundations, I think it is shortsighted to design and build foundations that are only suitable for a no-tree scenario.

 

I got the link about Kelly evidence, thanks, I will have a look. I recall the name Kelly form the case, he came across as a very credible witness.

[Gary Prentice]

Ah but unless the builders put a restriction in the title deeds or get the new owner to sign a disclaimer, with the effect that the builder is not responsible for subsidence if trees are planted, the builder could surely find himself exposed to claims from house owner. And if the builder has no control over adjacent land which is close enough for future tree planting to affect foundations, I think it is shortsighted to design and build foundations that are only suitable for a no-tree scenario.

 

I got the link about Kelly evidence, thanks, I will have a look. I recall the name Kelly form the case, he came across as a very credible witness.

 

I take it that you haven't had time to read that article:001_tt2:

 

"It is worth noting that a small number of buildings constructed in

line with NHBC guidance have failed during periods of drought.

A significant number of these failures can be attributed to new-build,

greenfield sites that were devoid of trees. The buildings were constructed on foundations suitable for a site without trees, but with no anticipation

that homeowners might plant trees in their gardens once they had

moved in."

 

Unfortunately it doesn't elaborate further, as to who got sued.

 

I would imagine that Oisin Kelly would be top notch, working under O'Callahans banner. I've met him once or twice and very much doubt that he'd suffer fools:lol:

 

Sloth, apologies for monopolizing your thread, although in the interests of furthering knowledge I'm feel you won't be too concerned

[sloth]

Sloth, apologies for monopolizing your thread, although in the interests of furthering knowledge I'm feel you won't be too concerned

 

No worries. I'm grateful for all the replies and prompts to get my grey matter earning its keep!

 

Made the visit today, it seems the bungalow was built 6 years ago, and the trees planted at that time. There is maybe 6 or 7m between the property and the boundary. On the boundary was planted a twisted willow, two ash and a red oak, with laurel hedging between the trees (I'm told the trees were 5-6ft tall when planted, now all are at least 30ft tall. That's pretty good going!).

There is no sign of subsidence. It seems the owners daughters house had a willow implicated in a case, which cost a lot of time and money, and now the elderly residents are worried about their home. They have spoken to the builders who told them not to worry (what else would they say!) as the foundations are designed according to the soil type, which sounds like nhbc guidelines.

I did not see the concerned neighbours, but told my friend to pass the message on that if they still have concerns they should get in touch with their buildings insurer. Also, as they are elderly, retired and less well off they will offer to prune back all over hang (up to 4 meters in places) as a good will gesture.

 

I wonder what you may have said in this situation?

 

Thanks again for all the replies, been a good thread

[daltontrees]

I take it that you haven't had time to read that article:001_tt2:

 

I have now! Reasonably good article, a bit pseudoscientific and partisan but makes a number of good points and refers usefully to other's documents elsewhere that have researched the question. I am not worthy to question the experienced-based judgement of the author but I am a few other people would contest the statement that it has been proven that pruning doesn't help and that there is no alternative to tree removal. It doesn't address the occasional situations where removal would cause heave but continued growth would cause subsidence and where regular pruning almost the only option. It doesn't deal with whether crown lifting would help, nor does it even mention root pruning. Root barriers are eventually dismissed on the basis that they might get breached. But I accept the generality that the tree will bounce back after pruning, with bigger leaves.

 

The article refers to NHBC guidance from 2003 and its original creation in the 1970s. As I thought the gudance has been around for so long that it is part of the whole industry.

[daltontrees]

Here's a hint to how NHBC deals with the problems that could arise from new planting. Basically, don't plant trees or shrubs without expert advice. So I guess the foundations have nothing to spare. Relevant section is p.19

 

http://www.nhbc.co.uk/NHBCpublications/LiteratureLibrary/HomeownerDocuments/filedownload,15900,en.pdf

 

It still doesn't deal with the issue of whether the foundations can cope with new trees on adjacent land that the owner or builder cannot control. Until I find out otherwise I will assume that no additional foundation is put in to cope and that thereafter the common law of nuisance would apply.

[daltontrees]

There is maybe 6 or 7m between the property and the boundary. On the boundary was planted a twisted willow, two ash and a red oak, with laurel hedging between the trees.

...

I wonder what you may have said in this situation?

 

Red Oak and Willow are on the NHBC list of high water demand, Ash is Medium water demand, as is Cherry Laurel. All in all, thirstier than a tree squad at the end of a summer's day shift. Zone of influence is 125% of mature tree height for High water demand. The Oak might already be influencing soil moisture levels at the new building. That is not to say it is a problem, but is good to be aware of it.

 

The main piece of info missing now is whether shrinkable clays are present. If not, end of problem in my opinion. If yes, it would be worth pencilling in an arb inspection in 3 years' time to assess. Hope this helps. I have enjoyed looking into it and learning a few things along the way.

Edited August 1, 2013 by daltontrees

[fagus]

(It may be that the council engineer uses over engineered foundation depths to allow for a probability of new tree planting, whilst commercial builders choose to work to the cheaper alternative when no trees are present.) I think that is a fair assumption.

Evidently it is legal requirement to build to the national building regulations so nhbc and local authority will both enforce these. I understand that local gvt officers system is more inspection based, at a cost, and NHBC rely more on warranty i,e Buildmark and builder registration.My experience is that certainly L Gvt Building Regs officers interpret the Regs and have been known to request a deeper foundation than was being prepared.

I question the reluctance against root barriers, yes they could be breached but takes years and good growth /soil conditions.Most of us know and see on a regular basis that root plates are not very deep.I think a root barrier is ok if the tree value and soil types are properly assessed.E.g not for a big willow on shrinkable clay.When the barrier trench is taken out it should be clear the no roots are anywhere near.

[Gary Prentice]

I have now! Reasonably good article, a bit pseudoscientific and partisan but makes a number of good points and refers usefully to other's documents elsewhere that have researched the question. I am not worthy to question the experienced-based judgement of the author but I am a few other people would contest the statement that it has been proven that pruning doesn't help and that there is no alternative to tree removal. It doesn't address the occasional situations where removal would cause heave but continued growth would cause subsidence and where regular pruning almost the only option. It doesn't deal with whether crown lifting would help, nor does it even mention root pruning. Root barriers are eventually dismissed on the basis that they might get breached. But I accept the generality that the tree will bounce back after pruning, with bigger leaves.

 

The article refers to NHBC guidance from 2003 and its original creation in the 1970s. As I thought the gudance has been around for so long that it is part of the whole industry.

 

 

I'm only aware of the testing carried out at West Malling, where if my memory is correct, there was no significant changes in water uptake until reduction

by 70%. I know that research is fairly ancient now and wonder if their is more recent research available.

 

That's a good point about retention at a given size, where removal would be likely to cause heave. Again more research is required, by myself. Every answer begs another question or more reading.

[Gary Prentice]

Red Oak and Willow are on the NHBC list of high water demand, Ash is Medium water demand, as is Cherry Laurel. All in all, thirstier than a tree squad at the end of a summer's day shift. Zone of influence is 125% of mature tree height for High water demand. The Oak might already be influencing soil moisture levels at the new building. That is not to say it is a problem, but is good to be aware of it.

 

The main piece of info missing now is whether shrinkable clays are present. If not, end of problem in my opinion. If yes, it would be worth pencilling in an arb inspection in 3 years' time to assess. Hope this helps. I have enjoyed looking into it and learning a few things along the way.

 

I thought there was still some debate on the water demand category? In that non-arbs require a pigeon hole for each genera whilst arbs recognise that there are simply far too many factors to take into account. Does a kilmarnock willow still fit into a high demand category the same as a white willow:lol:

 

Not to say that very broadly speaking, that they are not to be disregarded.