Foundation Depths

[PGM]

I'm in the middle of a refurb of our cottage and want a small corner infill extension on the back for a utility room and en-suite above. It measures 3m by 2.5m and is two storey.

The Building Control officer has told me I need foundations of 2.5m depth due to being in clay and there being an Oak 10m away.

This seems very deep to me. Does anyone have access to the NHBC calculator/tables for foundations adjacent trees so I can check this. What would the corresponding width of the foundation be required?

Also what would the depth be for a single storey extension or conservatory I'm thinking about at the front of the property?

Thanks in advance
Paul

[camp freddie]

An oak is a high water demand tree and in the absence of a site investigation will assume the clay is of high volume change ie lots of potential to shrink and heave. Also if the oak tree is to remain the bco will take the full mature height in his calculation.

It does seem about right based on the above assumptions

[PGM]

Thanks for that, I pretty much have resigned myself to the inevitable.

Obviously I should say that I do want it to be correct and not have to fix it at a later date, the BCO does know the local area well.

Just confirming really to avoid any wasted work and money!

[camp freddie]

That's alright. The front foundation may not need to be so deep if it's further away from the tree, so you maybe able to save a little there

[TheEnd]

Dig a big hole now, and fill it with sand, and backfill it.
Next time he comes out for the front, dig a little hole, "oooh, look, sand!" and the job's a good 'un.

[PGM]

[mk1fan]

What does your Structural Engineer say?

[Morgoth]

that has got to be the most idiotic reply I have ever seen, the BCO will have to see the complete excavation before concrete , so a pointless excersize. As a Building Contol Inspector I would need to see the entire excavation, if you disagree get in toyuch with an engineer, but he will still use the same calculator for the foundation depth, if you do dig a hole and fill with sand, you would only be harming yourself

[Busamav]

Morgoth said:

that has got to be the most idiotic reply I have ever seen,

building inspector late to the job

The daywork sheets are on you

[markjo]

Dont skimp, that sounds about right, our current house was underpinned to 8m about 15 years ago due to a oak tree that was 10m away, the original foundations of pre 1920's houses are only on average 1 - 1.5m deep
Do it right or you will get problems in the future..

[Digga]

In clay areas we've also seen a lot of footings being increased in width from traditional 2' (600mm) to 2'6" (760mm).

[Spudler]

Morgoth said:

that has got to be the most idiotic reply I have ever seen, the BCO will have to see the complete excavation before concrete , so a pointless excersize. As a Building Contol Inspector I would need to see the entire excavation, if you disagree get in toyuch with an engineer, but he will still use the same calculator for the foundation depth, if you do dig a hole and fill with sand, you would only be harming yourself

Typical BCO...no sense of humor.

I should hope the OP has poured his foundations by now.

[Morgoth]

Busamav said:

building inspector late to the job

The daywork sheets are on you

24 hours notice required if we are not there by then carry on but if you carry on before that then the day work sheets will be on you

[MOTORVATOR]

And welcome to PH.

We usually like to have a whip round to get a welcoming present for the newbies, how do you feel about parrots?

[Busamav]

Morgoth said:

24 hours notice required if we are not there by then carry on but if you carry on before that then the day work sheets will be on you

I know the score very well Morgoth , I was just noting you responded to a question , 9 months late !

[MOTORVATOR]

Give him the parrot you meanie.

[andy43]

BCO is usually late, but 9 months...?

[PGM]

Even doing it all DIY at weekends and not having done anything since November due to weather the roof is going on this weekend!

Progress at Christmas time:


Foundations went down to 1.2m in the end by 600 wide. The trees in the vacinity were removed in the end. The local BCO has been extremely helpful during the whole build.