Raft foundation?

[HMC2000]

We're planning an extension this year and had a much recommended builder round to have a look on Friday. It's a straightforward single storey extension, and our soil is clay. We have two trees in the garden which have recently been much reduced and pruned - a beech tree and a yew tree - and they're marked on the architect's plan. Building regs have approved the plan subject to the technical drawings for beams/steels; those were sent off to them yesterday.

But the builder is concerned both by the trees, and a shrub that is right up next to the planned building. We were thinking of getting rid of this, and he advised us to do so, as he thinks the building inspector is likely to ask for really deep foundations to avoid issues with the roots. He mentioned that they might do that anyway, because of the yew tree.

After he'd left, he emailed to say that it had occurred to him that a raft foundation might be the best way forward, even if we get the shrub dug out. He suggested we ask our architect and tech guy about it. I'm not keen on asking architect, as we got them for drawings only, and they haven't been exactly brilliant, but I have emailed the guy who did the calculations.

But... I've had the best advice from this nest of vipers on pretty much everything! So, people - is a raft foundation going to be an expensive option? More time consuming? Pros and cons?

If your architect does it the way that most do, the drawings will have a note on the foundations to say, basically, that their design is indicative, and you need to do whatever the Building Inspector requires on site. Building Control will happily sign off the drawings on this basis, so don't assume that because the trees were shown on the plans, and the plans have been approved, that all is OK.

When the foundations are dug, the Building Inspector will be looking for evidence of root penetration into the trench, and will require that the foundations are taken below the lowest sign of roots. If you're on shrinkable clay, he or she will also doubtless insist upon clay heave protection.

Raft foundations (or piles) are an alternative to deep strip + clay heave, and which is most cost effective will depend on the depth of the roots - something no-one can tell you until a trench has been excavated.

You could go to the cost of having a raft designed by a Structural Engineer, then find that the roots of the trees run in a different direction. Conversely, you could stick with the assumption of strip footings, but then find that you need to dig ridiculously deep when it comes to forming the trenches. I'm afraid that no-one on here will be able to second-guess that for you.

The LABC provides a FOUNDATION DEPTH CALCULATOR that might help you make an informed decision (but bear in mind that trees often haven't read BS5837:2012, and have an inconvenient habit of not following the rules, on occasion).

The obvious way to be absolutely certain is to dig an investigative trench, then agree with Building Control and/or a Structural Engineer what the best/most cost-effective solution is.

Finally; bear in mind that removing trees or shrubs does not immediately cure the problem. On shrinkable clay, the ground will go through several seasonal cycles of swelling ('heaving') and contracting, over a number of years, before finally settling down. For this reason, do NOT remove any trees until the foundation design has been settled, as Building Control will wish to see with their own eyes what size and species the trees are, to assess the shrinkage potential.

Thanks Sensus. Yup I suppose what I just don't have a clue about is cost of raft vs cost of standard strip vs cost of deep strip. (Don't worry I'm not planning to make decisions on the basis of mumsnet 😊). I think I was hoping that the differential would be minor enough that committing to a raft foundation without checking via trench wouldn't be too big a risk. But I know it's the way forward really - there have been tall conifers nearby previously, and the soil is completely clay.

Thank you also for reminding me of the words "structural engineer" : I am full of cocodamol today, and somewhat off with the fairies. As may be obvious from my first post!

I'm confused as to what a raft foundation will achieve, unless they mean a raft on piles.

An assessment of the trees/bushes should be part of the calculations, and it should take account of what is taken out as well. The trees you've had pruned should be assessed as if they were at their full mature height.

Did the structural engineer visit site at all? Or were they only employed to design a few beams (my company refuses to do work without seeing the site for this sort of reason).

We are very close to a large mature oak tree. It is ours and it’s about 12 ft from the house. Our extension (oak frame) has foundations 6ft deep. We are on shrinkable clay. DH is a Structural Engineer.

I would think shrubs won’t affect the depth of the foundations a great deal and if you don’t cut down the other trees, a Structural Engineer can take account of them and the soil. Neighbours trees can make a difference too.

I would suggest piles would be far more disruptive and more expensive than a traditionally dug trench foundation. However, if that’s the best solution, then so be it. Some trees are more disruptive to foundations than others. Yews are slow growing, so you know what you’ve got. It won’t get as big as a sycamore or a London plane. Beech have shallow roots. They are not particularly long lived, unlike the yew.

Do get a Structural Engineer to advise because you will also need to consider how the new extension joins to the existing building to eliminate cracking if there are differing ground conditions, construction methods and foundations.