DIY Doctor

Foundation depth rejected...

Postby jasel » Thu Aug 16, 2007 2:02 pm

I am part way through my first project part of which involves a small 2mx3m side extension with architects plans that say the depth of the foundations required are 1.1m. I have been away and returned to find that the foundations were dug and the builders had drilled a hole ready for the inspector to come and look at... The inspector has subsequently come round and stated that a depth of 2m is required as there is a Willow tree 20m away that may be an issue in the future.

I would be grateful for advice on:
1) Is there any options to overcome having to dig 2m?
2) If the builders have to dig 2m vs. 1.1 what would the cost differential typically be. i.e. Had a builder been quoting for 1.1m and then had to quote for 2m for a 2m x 3m space, what would the difference for the digging and concreting only be...

Total newbie would be grateful for any advice you can offer...

Many thanks,
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Simply Build It

Postby mikehutch59 » Tue Sep 04, 2007 4:11 pm


I know how you feel!,building inspector came round and saw that we had large leylandi trees 6 m from the trench,he got out his book and said 2.8m! :evil:
for a single story office,how ever i found 6" of greensand 300mm under bottom of trench and he has since been back and passed them,they are 1.4 m deep,so alot of extra cost has been avoided, :D.

Good luck

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Postby thedoctor » Fri Sep 07, 2007 6:38 pm

Ask the builder Mike, anyone giving you an answer to a question like that without knowing any of the conditions is guessing and could be thousands of pounds out. If you don't like the price the builder gives you, get another builder or three to also quote. Its your house, its your extension, make sure you know what is going on and how much it is going to cost.

Building Inspectors work from a table which tells them how much damage any tree can do at any distance with roots etc. See our project on foundations. They dont do things like this for a laugh and always go down the safest route for you. Your house would be worth nothing if in 5 years time it started to break up because of a root problem.
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Postby andysarah » Sat Oct 13, 2007 10:04 pm

what the problem is is the clay soil, the only way of getting around the deep footings are to;
1 demonstrate that the natural water table is high and the soil will not suffer from drying out, i.e. if you are on a marsh etc.
2. you can collect soil samples and have them tested at a soil lab for plasticity and moisture content, compair this with the NHBC guide section 4 and you may be able to get the clay classified as low shrinkable as opposed to moderate or high this could say 500mm to a 1m?

typically this is the kind of approach a structural engineer will take, if you have a chat with a local indipendent Engineer they will normally give a reasonable fee for the work and i normally recon i can save the fee or more in economic design.
Andy MIStructE
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Postby Buzzfan » Tue Feb 12, 2008 4:55 pm

can anyone advise how much engineers typicall charge, or they paid, for foundation calculations? assuming a simple structure, on poor clay.
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Postby Fishie42 » Fri Feb 15, 2008 9:50 am

Assuming a simple structure you are looking at around £500ish for a structural engineer to produce design for foundations. This should include a site visit and you should also recieve a set of the final calcs produced. They should/may even submit them for building regs themselves.

Hope that helps
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