Subsidence - is it always the fault of the tree?

[Allyzap]

Hi, Just looking for alternative view points with regards to minor (at the moment) ground movement of a single garage extension. Original house and extended garage are built of the common South East region high plasticity clay and we're at the top of a slight slope/small hill.

We had minor cracking appear in 2019, at the point of the extension. Repaired with Helical bars but cracks reoccured in different palces after the hot summer in 2022.

We have a 250 yrs old oak tree about 10 metres away, falling in the boundary of our neighbour. The tree has a protection order on it.

The engineering report and arborist report have both suggetsed the tree takes too much water from the ground during those drought times and should be felled. They don't seem to think heave will be an issue.

I guess I'm just looking for anyone who might have experience in doing something other than remove a beautiful old tree?

I do accept that we need to do something as repair works done so far are just to repair the minor cracks and not fix the route cause, and we ceratinly don't want to face a worsening situation as we get older. But is it always the case that the tree is the cause?

 

Thanks

A tree lover

 

Edited March 8 by Allyzap
typo

[Hughgo12]

In this case it sounds like the tree.

 

A 250 year old oak will have a very wide and significant route system which will draw alot of water. 

 

I do feel your pain as I'm sure its impressive and you'll be sorry to see it go. 

[Mr Punter]

If there is a TPO and it is not on your land I don't think that it can be easily removed.

 

If this is just for a garage, do a few cracks matter?

[Allyzap]

1 minute ago, Mr Punter said:

If there is a TPO and it is not on your land I don't think that it can be easily removed.

 

If this is just for a garage, do a few cracks matter?

The cracks are minor, less than 3mm. I guess it is more the fear of future ground movement and the best way to protect against that.

[Conor]

Ground movement will be seasonal, not progressive. No reason to expect the ground to subside significantly. It's a seasonal problem that won't go away, unless you want to start watering the tree with 1000s litres of water every day in the summer 🤣 

 

Joking aside, has the makeup of the ground (and therefore surface water movement) changed in the last few years? E.g. with lots of paving, and surface water directed to drains, will mean the tree will struggle to get water in the drier months, and instead take it from the deeper clays.

Edited March 8 by Conor

[Allyzap]

34 minutes ago, Conor said:

Ground movement will be seasonal, not progressive. No reason to expect the ground to subside significantly. It's a seasonal problem that won't go away, unless you want to start watering the tree with 1000s litres of water every day in the summer 🤣 

 

Joking aside, has the makeup of the ground (and therefore surface water movement) changed in the last few years? E.g. with lots of paving, and surface water directed to drains, will mean the tree will struggle to get water in the drier months, and instead take it from the deeper clays.

Not really much change although we did lose an area of lawn to parking immediately in front of our house...

The only other change is the physical growth of the tree itself.

I guess we maybe panicked when we first spotted cracking in 2019 and reported straight to our insurer at that point. They have been monitoring the ground level since early 2021 and movement is slight, hence they are not suggesting any preventative work, other than tree removal.

What do others do if they get regular cracking with seasonal movement? Self repair in some way?

[Temp]

My guess is the design of the foundations for the garage didn't take into account the soil type and tree. In some cases >3m deep foundations lined with compressible boards are needed. Sometimes piled foundations.

Do you know how deep they went?

 

 

 

[Allyzap]

17 minutes ago, Temp said:

My guess is the design of the foundations for the garage didn't take into account the soil type and tree. In some cases >3m deep foundations lined with compressible boards are needed. Sometimes piled foundations.

Do you know how deep they went?

 

 

 

Hmmm, will need to go back to documents to check but certainly what ever the building regs requirements were in 2005. 

[Russell griffiths]

35 years in the tree business 

single storey garage extension, probably built on inadequate foundation, cost value £12-15 grand. 
oak tree value £250,000??? There is actually a scale you can look up to value a tree. 
arborist report, blame the tree it’s an easy scapegoat. 
what was there first the tree or the garage 😂

garage should have been designed for its surroundings. 
conclusion, knock the garage down and build it better,

repair garage and move on. 

prune the tree lightly to try and lesson the water uptake, this is debatable if it actually has any bearing on water uptake, 

the tree is definitely the cause of the ground shrinkage, but is it the cause of the problem?
the actual problem is a building of inadequate design and construction has been built in an area of shrinkable soil leading to the building movement. 
 

if the garage hadn’t been built the problem wouldn’t exist. 
 

the amount of people I’ve seen who put a £5000 greenhouse under an apple tree, and then moan that the tree drops apples and breaks the glass, then wants the tree cut down. 
 

prune the tree and live with some minor cracks. 
when you can see the neighbours having dinner through the cracks in the house walll then start worrying. 
 

this must be bothering you a fair bit though to be at the stage of an engineer and arborist report. 

Edited March 8 by Russell griffiths

[joe90]

17 minutes ago, Russell griffiths said:

built on inadequate foundation,

Is my guess, can’t blame a 250 year old tree. Should have been piles or raft.

[Allyzap]

14 minutes ago, Russell griffiths said:

35 years in the tree business 

single storey garage extension, probably built on inadequate foundation, cost value £12-15 grand. 
oak tree value £250,000??? There is actually a scale you can look up to value a tree. 
arborist report, blame the tree it’s an easy scapegoat. 
what was there first the tree or the garage 😂

garage should have been designed for its surroundings. 
conclusion, knock the garage down and build it better,

repair garage and move on. 

prune the tree lightly to try and lesson the water uptake, this is debatable if it actually has any bearing on water uptake, 

the tree is definitely the cause of the ground shrinkage, but is it the cause of the problem?
the actual problem is a building of inadequate design and construction has been built in an area of shrinkable soil leading to the building movement. 
 

if the garage hadn’t been built the problem wouldn’t exist. 
 

the amount of people I’ve seen who put a £5000 greenhouse under an apple tree, and then moan that the tree drops apples and breaks the glass, then wants the tree cut down. 
 

prune the tree and live with some minor cracks. 
when you can see the neighbours having dinner through the cracks in the house walll then start worrying. 
 

this must be bothering you a fair bit though to be at the stage of an engineer and arborist report. 

Had to laugh at the bit about seeing the neighbours eating their dinner through the cracks 🙂

 

What you say is true of course; the tree was there long before any of the housing went up, let alone our garage extension 12 years later. The reason for the engineer and arborist report was all driven by our insurer to be fair. They have been very thorough, but their conclusion still comes down to the tree, which I am loath to see come down. Our fear comes from lack of experience of building construction and subsidence issues  and a very real worry that this will become a bigger problem just as we are getting too old to be able to deal with it.

[saveasteading]

I agree completely.

The rule of thumb I was taught was if a pound coin does not fit in the crack then don't worry.

It won't fall down suddenly.

Helical bars are overused IMHO. They move the stresses to a solid bit of wall.

 

I'd want to see the situation but:

It is on a hill so rain runs away. Can you put in a trench or swale or french drain to hold the water?

And divert roof rain towards the tree, using a land drain or soakaway detail to encourage all the water into the ground.

 

Or a pond.

 

It's great that you don't blame the tree.

 

[Allyzap]

2 minutes ago, saveasteading said:

I agree completely.

The rule of thumb I was taught was if a pound coin does not fit in the crack then don't worry.

It won't fall down suddenly.

Helical bars are overused IMHO. They move the stresses to a solid bit of wall.

 

I'd want to see the situation but:

It is on a hill so rain runs away. Can you put in a trench or swale or french drain to hold the water?

And divert roof rain towards the tree, using a land drain or soakaway detail to encourage all the water into the ground.

 

Or a pond.

 

It's great that you don't blame the tree.

 

Thanks for these tips; I like the idea of redirecting rain water etc - will look into this! 

Yes, I really want to keep the tree if possible (in fact it was me that instigated the tree preservation order when the first set of cracks appeared in 2019 - my worry even then was that the quickest/cheapest option suggetsed by the insurers would be to fell the tree.)

The TPO might of course now be coming back to bite me....

[saveasteading]

The tree needs water but won't mind where it comes from. Clay takes a long time to recover from summer drying, and water has to very gradually migrate downwards. So if you can catch it and give it every opportunity to go down over a large area then this could work.

A trench along the contour with a perforated land drain pipe and gravel or rubble surround is probably easiest, cheapest and best.

[Allyzap]

Just now, saveasteading said:

The tree needs water but won't mind where it comes from. Clay takes a long time to recover from summer drying, and water has to very gradually migrate downwards. So if you can catch it and give it every opportunity to go down over a large area then this could work.

A trench along the contour with a perforated land drain pipe and gravel or rubble surround is probably easiest, cheapest and best.

Thank you so much 🙂

[Russell griffiths]

Don’t listen to me, but. 
I built a large two storey extension to a house in London, solid clay, foundation down to 2.3m because of a row of conifers in my neighbours garden. 
in the summer when you could see the grass developing cracks I used to go to bed and leave the outside tap running 😉I’m sure the few thousand litres of Thames waters finest kept that clay nice and moist. 
 

im probably going to hell anyway 😂😂

[saveasteading]

25 minutes ago, Russell griffiths said:

Don’t listen to me, but. 

But I do.   

Conifers are low water demand.   I usually avoid saying anything that suggests an overdesign  could have been avoided.  but I assume these trees were very close.

[George]

I would approach with these options as required.

 

First get the drains checked. They are a likely culprit and a repair/replacement should reduce effects on the soil. The issues are often linked because tree roots can ruin drains, but I've seen many times a tree isn't causing problems for a long period of time then suddenly it does - this to me indicates the tree has damaged drainage rather than the tree suddenly causing dramatic volume change, even if the weather is unusually dry/wet (this opinion was actually proposed by a correspondent in a recent issue of he IStructE magazine...).

 

Secondly, if the drains are fine and the cracks aren't causing critical damage you could consider installing a root barrier. This will lessen the effect the roots can have on the subsoil supporting the structure. They generally aren't used because they're not infallible, especially for the design life of a habitable structure, but for a garage and to protect a valuable tree it is a good option.

 

Lastly, or in combination, crown reduction can reduce the water demand. This creates ongoing maintenance but done right can keep a tree healthy for longer. Indeed - if I think a tree does need removing I would generally recommend it ideally is done over 2 or 3 years to allow the soils to gradually adjust (although the weather is a bigger factor).

Edited March 8 by George

[Gus Potter]

10 hours ago, Allyzap said:

I guess I'm just looking for anyone who might have experience in doing something other than remove a beautiful old tree?

I do accept that we need to do something as repair works done so far are just to repair the minor cracks and not fix the route cause, and we ceratinly don't want to face a worsening situation as we get older. But is it always the case that the tree is the cause?

Hope this helps a bit. Have attached an old document that explains a bit about structural cracks. Have a read.. it will be the best thing you can do.

 

Before you attribute all the blame to the tree you need to look at other things that could be exacerbating the problem.. it may be that you can address these and leave the old tree to do it's thing.

10 hours ago, Allyzap said:

Repaired with Helical bars

Often this type of solution just transfers the problem to another area as you create a stiff point that doesn't move so the movement happens at the end of the stiff point and often makes things worse.

 

With this sort of thing I think.. ok things are moving about.. many historic buildings behave this way but is the building a safety risk.. going to fall down?  If not then let's see how we live with it moving about and do something that reflects that. Internally we can force the movement (once understood to say a shaddow gap in the interior decor, externally concealing by some clever movement joints or say a planting trellise.. with maybe some careful cutting of stress relief joints.. first let's to see if the whacky and sympathetic ideas work first before going the full Bhunna / sledgehammer to crack a nut.

 

Structurally we maybe want to do some flexible ties that lets the thing move but not fall down in the wind.... here we are saying.. yes it needs some bracing to deal with the movement. This could even be some tensions wires like you see on sail boats.. that can work well if you want to give a room a Nautical feel or something more contemporary. As a rough guide a 6.0mm stainless steel tension wire with some connections to the wall will set you back say £500.00 quid, wire and turnbuckle / fork end  is about £150 then the builder and resin anchor fixings takes you up to the £500.00 quid.

 

I would dig well into my box of tricks before considering cutting down a tree like that. Once you cut the tree down there is no going back.

 

I suppose it boils down to this.. are you happy enough to live with the tree and enjoy what that brings.. shade ect, a bit of life that attacts nature and in doing so accept that you have a bit of semi concealed movement inside and out or do you want to have a house that does not move and sacrafice the tree? If you go abroad to say France or Spain or other parts of Europe.. their houses move all the time.. in the UK we tend to worry more!

 

You mention you concern that things may get worse. But if the tree is that old is it going to grow much more and the root ball expand? .. why not just say we are all of a certain age and lets live with each other.. as we get older and wiser?

 

Be careful though as if you cut off one source of nutriants that old tree could get a dose of Viagra and send out other roots that may cause you another problem! Did your experts tell you that?

 

 

 

 

 

BRE Digest 251 Assessment of damage in low-rise buildings.pdf

[Gus Potter]

2 hours ago, Gus Potter said:

Hope this helps a bit. Have attached an old document that explains a bit about structural cracks. Have a read.. it will be the best thing you can do.

Help ma boab.. looks like I have had an IT breakdown and posted an unedited version and a woke one for the faint hearted. Never mind.. if you dont like one read the other. Any complaints PM me and I'll sort it out.

BRE Digest 251 Assessment of damage in low-rise buildings.pdf

[Allyzap]

7 hours ago, Gus Potter said:

Help ma boab.. looks like I have had an IT breakdown and posted an unedited version and a woke one for the faint hearted. Never mind.. if you dont like one read the other. Any complaints PM me and I'll sort it out.

BRE Digest 251 Assessment of damage in low-rise buildings.pdf 263.17 kB · 3 downloads

Wow, thanks for all of those things to consider and I'll take a read through the attachment 🙂 I'm feeling much more confident now, from everyones responses, that just living with the movement of the extension over the seasonal variations and keeping the tree (which really is beautiful) is viable, as it does not necessarily follow that the building is at risk of falling down!! 

[joe90]

I remember a TV programme years ago about a house that had massive cracks everywhere due to trees, drains were blocked and the ground sunk. They removed a few tress but not all and replaced some with Himalayan birch , repaired the drains and the ground recovered, and the cracks in the house shrank back. I was amazed that could happen.