Foundations

[roythegrass]

I have obtained five quotes for a conservatory at the rear of the house.
There is a mature oak tree (with a TPO) in the garden approximately 8 m from the proposed rear wall/foundations of the conservatory.
Three companies have allowed for 1m foundations saying this will be adequate and two have quoted for piled foundations insisting this will be necessary given the possible spread of the tree roots.
To add to the confusion my preferred builder –who allowed for 1m foundations- has included in his fixed price quote a proviso that further costs may occur depending on ground conditions when they start digging. It is my view that any roots 8m away from the tree will be either too deep or sufficiently slender to be easily severed whilst excavating without harming the tree or the foundations – but I have no previous experience of this situation.
Any thoughts would be appreciated.

 

[Beefpotnoodle]

Personally I wouldn't touch any part of the tree unless it need urgent pruning, that's what a tpo is about! And a builder should know better as I they are cutting the root they'll be the ones in trouble/fined.
The trees roots could still be big enough to interfere with the footings, but you need to have a dig and have a look. Your preferred builder sounds a good bet to me

 

[jeds]

The foundation needs to be properly designed. With the greatest respect to them - the average builder is probably not equipped to do that. I recently dealt with an Oak 12 metres away which resulted in a 1.7m deep foundation. But your soil may be different. The maximum you can reasonably go is 2.5m deep. After that you need special foundations. Your builder that says it will be ok at 1m is an idiot. Strike him off the list. The one that says 1m deep but reserves the right to charge for more, KNOWS it will be more. 100% for sure! So he is taking a flyer that it will be more than 1m deep but less than 2.5m deep. But he has absolutely no way of knowing for sure.

So where do you go from here. First question is; does it need building regs? If so you will have to do whatever you need to do to comply. That will be a minimum of proper assessment and design so you might as well crack on with that. My guess is you will be piling. If it doesn't need building regs you could take a view on it, but if you do that and get it wrong you run the risk of damage later on. And you can't just go on what it looks like. On my example above there was no sign whatsoever of roots in the trench.

 

[roythegrass]

Thanks for the input folks.
The subsoil is clay. The conservatory is 4.8 x 4.0m and within permitted development and the builder has undertaken to comply with LA building control requirements. (They are well established with a good feedback and history)
There is an existing 450mm dwarf retaining wall between the patio and the higher level garden which coincidentally is located the same distance from the house as the rear wall of the proposed conservatory. This wall is as old as the house -27 years- and shows no signs of movement or cracks although I would guess this was built on shallow footings.
Having researched this matter extensively and with the advice here I will be instructing a recognised firm of consulting civil and structural engineers to advise on suitable foundations. Fortunately the foundations being 8m from the tree are just outside the root protection area.
Roy

 

[roythegrass]

Update
Consulted a structural engineer and given the location of the tree, it's anticipated maximum height and the clay soil his calculations indicate 2.7m deep x 600mm wide mass fill trench foundations.
That seems to me to be excessively deep but I'm not qualified to question his advise. More importantly I would be reluctant to have foundations that may be deeper than those of the house. Perhaps a second opinion ?

 

[tony1851]

his calculations indicate 2.7m deep x 600mm wide mass fill trench foundations.
That seems to me to be excessively deep

That seems to me to be excessively silly. Calculations?? probably off the top of his head.

 

[jeds]

No, that's not correct. Oak is a high water demand tree with a mature height of 20m. If the soil is high change potential the depth would only be 2.3m and you can reduce that a further 50mm for geographical location. It would be worthwhile having the soil analysed. If your soil is medium change potential your foundation would reduce to something like 1.95m and if it is low it will be more like 1.6m. In your location there's a pretty good chance it will be low.

 

[roythegrass]

No, that's not correct. Oak is a high water demand tree with a mature height of 20m. If the soil is high change potential the depth would only be 2.3m and you can reduce that a further 50mm for geographical location. It would be worthwhile having the soil analysed. If your soil is medium change potential your foundation would reduce to something like 1.95m and if it is low it will be more like 1.6m. In your location there's a pretty good chance it will be low.

Thanks for that jeds - that's pretty much what the engineer said about change potential so he was looking at worst case scenario. I'm going to consult the LA building surveyor and also to see if the council still have the drawings showing the foundation depth when the house was built in 1987.
I will take some convincing that excavating below the existing foundations is both necessary and safe.
PS We,re now in Bexhill-on-Sea, East Sussex not Surrey.