Footing Depths - Measured from where?

[Capable Noob]

Hi everyone, 

 

As per my username, I am capable of doing stuff but don't necessarily know all the regs etc so would welcome advice please.

 

Probably inviting ridicule for what may be seen as a stupid question but having searched all over I can't actually find an answer.

 

I'm doing a fairly large extension and my SE has advised footing depths of 1100mm.  My question is, where is that measurement taken from?

 

Are the stated foundation depths measured from:

 

a) the finished ground level (the patio will be built up by 450mm or so to meet the internal floor level)

b) the existing ground level

c) or from the top of the existing concrete foundations of the existing building 

 

Basically, are the depths stated by the SE the required thickness of concrete or just to the bottom of the trench (so where the existing footing of the building is 600mm below the surface, it would mean that the stated 1100mm depth footing would mean a 500mm depth of concrete to be poured)? 

 

Many thanks!

[joe90]

No such thing as a stupid question, stupid is not asking 👍despite what the SE says it’s the building inspector that makes the decision on ground conditions once the trenches are dug (and clean, scrapped flat and smooth). I would say dig 1100 from existing floor level, the thickness of the concrete poured should be specified by the SE but some do full fill pours (trench filled to top, saves laying blocks in a trench). 

[Capable Noob]

Thanks Joe.  My SE hasn't been specific with depth / thickness of concrete hence my question.   I'm hiring a digger so don't want to do it all and then have building inspector say it's not deep enough and have to then re-hire the digger (and conversely I'd like to avoid going way too deep and thus costing myself more unnecessarily in excess concrete). 

 

Regarding the laying of blocks in the trench, I thought you had to start below ground level due to frost etc? And doesn't the concrete have to be to the same level as the existing building? 

[George]

You can justify either a) or b), but I'd select whichever gives the deepest formation (underside of foundation) level.

 

 

 

 

[joe90]

16 minutes ago, Capable Noob said:

don't want to do it all and then have building inspector say it's not deep enough

You will have to get the trenches inspected by building control before you fill them!

[markc]

@Capable Noob nothing at all on the drawings to indicate a footing spec?

[Capable Noob]

30 minutes ago, joe90 said:

You will have to get the trenches inspected by building control before you fill them!

Sorry, I was meaning after I've dug the trenches and sent the digger back, I don't want to then hire it again to dig deeper if building control aren't happy.  I guess I can try to get them out to inspect while the digger's still on site but then they'll need to come again when all trenches are dug...

[Capable Noob]

54 minutes ago, markc said:

@Capable Noob nothing at all on the drawings to indicate a footing spec?

The spec is 1600mm depth and 600mm width.  Stepped up by 500mm when 1500mm further from tree location.  

 

What hasn't been clarified is where that depth is measured from (and, assuming the top level of the concrete matches existing foundation, would then tell me the thickness of concrete).

[Marvin]

25 minutes ago, Capable Noob said:

The spec is 1600mm depth and 600mm width.  Stepped up by 500mm when 1500mm further from tree location.  

 

What hasn't been clarified is where that depth is measured from (and, assuming the top level of the concrete matches existing foundation, would then tell me the thickness of concrete).

I thought it was from OGL - original ground level?

[Conor]

It would be local ground level as you have to account for any slopes. E.g. if you have a fall of 500mm over the length and used the existing floor level as the datum, then your trench would only be 600mm deep. So you use the local ground level and introduce steps so you maintain a consistent depth. Of you've a fall of 100mm, I wouldn't worry too much.

 

The whole point is to ensure you are down to firm, undisturbed ground and to allow enough depth for the concrete. It'll all change once you start digging!!!

[Temp]

Normally the DPC is roughly 150mm above ground level so I would dig  down 1600+150 = 1750 mm below the existing DPC.

 

Then after the BCO is happy fill them with concrete to a suitable multiple of brick or block courses below the DPC. For example 1 block course on the inner leaf and 3 brick courses on the outer leaf should be the same height (but check your bricks and blocks). Alternatively 2 blocks and 6 bricks etc.

 

Then when you lay those courses your DPC will end up at the same height as the house.

 

 

 

[Capable Noob]

Thanks everyone!  It's quite clay-y (is that a word?!?) so as you say Conor, it'll all change once I start digging.  The position of the new footings is all level ground so that's handy.  I've already dug a fair bit by hand - I've exposed all of the original garage footings, so they now need breaking up with the pecker to get them gone ready for the new footings.

 

Where I'm stepping up by 500mm as I get farther from the nearest tree, it would mean that the concrete would only be 200mm thick - having queried this with my SE this afternoon his reply was "Correct but you need minimum 450 concrete depth 1m below GL regardless".

 

I understand this to mean any concrete that extends beyond 1 metre depth must be a minimum of 450mm from the top of the concrete.... ORRRR does this mean that it must extend 450mm below the 1m point, ie the bottom of the concrete will be at 1450mm and the top will be matching existing footings level at 600mm below GL, therefore thickness of concrete is 850mm....??

 

Sorry to sound daft but obviously I just want to meet the requirements.

 

 

[Temp]

Clay soil can sometimes require special foundations. Depends if it expands and contracts with the seasons. Sometimes you need to line the trenches with compressible boards before filling with concrete. You BCO may have have a view on this. Sometimes they require a ground condition report/soil survey to support your proposed foundation design.

[Alan Ambrose]

Building control are usually quite happy to answer any questions in advance.

 

Alan

[spottycat]

Hi - In my experience it's kind of impossible to know in advance of a Building Control visit. I dug a trial pit and the inspector checked that and advised. My neighbour only a couple of hundreds yards from me had to go much deeper because the inspector saw clay. Until the BCO looks in the hole there is no way of knowing how deep you have to go. Here's my experience - https://diyhomeextension.co.uk/footings-and-foundations/

[Temp]

We were allowed 1.5m deep trenches in clay after an inspection pit and soil report. House a few doors down was told they needed piles and they ended up going very deep.