Extension foundations question

8 Mar 2012
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United Kingdom
We're planning a smallish two storey extension to a circa 1965 link detached house, going out 2.5m and about 4.5m wide across the back of the house. Its passed planning permission and is in for Building Regs approval. My architect has just informed that he has specified engineer designed mini-pile foundations instead of the strip foundations I and my builder were expecting. The house is on medium shrinkable clay (approx 23% if that means anything to anybody) and the worry seems to be a hawthorn hedge running alongside the boundary of the property (belonging to the local authority, not me!). The hedge is 2m high, 1m from the house and 1.5m from the nearest wall of the extension. I've spoken to the BCO who quotes NHBC regs and won't budge, though I've read elsewhere that hawthron isn't one of the thirstiest trees, and its a hedge not a tree.

My questions are:
1. Does this seem right. Is everyone being over-cautious? Don't want the extra cost if it can be avoided. (but also don't want a disaster!)
2. The house itself is on 1m foundations. Am I going to end up with a bomb-proof extension attached to a house that will move around. The architect has specified a 'movement joint' between the extension and the house but will I still see problems eg cracking between new and old walls and ceilings etc.

Any input gratefully received!
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A BCO can't normally argue with an engineers design, and an engineer is going to design with his PI insurance in mind

So this hedge, which may or may not grow bigger into trees or large shrubs at some future time, is going to influence the design however you look at it

An arboriculturist may be able to give some specialist comment, but the fact remains that the hawthorn is within influencing distance according to todays criteria

Tables seem to indicate a 1.6m foundation depth within 2m of a hawthorn tree. I don't think it would make any difference for an established hedge, but it could make things worse if anything
Common Hawthorn (Crataegus) is a high water demand broad leafed tree with a possible mature height of 10 metres.
23% volume change potential is classed as medium shrinkable soil.
Both your architect and local BC are correct, in that the tables for building within the zone of influence of trees, engineered foundations are required due to such close proximity of the hawthorn.
If the Hawthorn had been 3 metres away from the building, a 2.5 dig and Claysield would have been acceptable.
One word of advice, make sure that builder forms movement joint all as architects instructions, as if we have a dry summer there will be a lot of movement in the ground this year.
It is a pleasure to know that there are still a decent few architects about that know their job.
And if the Architect really knows his job, then the movement joint will be in the inside wall too in the plaster and not just the outside brickwork
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Thanks very much for your replies Woody and Oldun - I'm reassured that my architect knows what he is doing with the foundations. Waiting for quotes now from piling sub-contractors - eek.

Can I ask a question about the movement joints: the plans specify stainless steel wall ties and a mastic seal between old and new walls. Does that sound ok? Also, Woody mentions an internal movement joint: what would the construction of that be like? Thanks
Can I ask a question about the movement joints: the plans specify stainless steel wall ties and a mastic seal between old and new walls.

That sounds like a standard wall start tie arrangement

If your designer expects movement then a proper joint should be formed in the internal plasterwork too, or else the plaster will crack at the new to old junction. Or specify careful use of plasterboards to bridge any such joint - but still a risk of cracking for excessive movement

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