Footings

[5125andrew]

Can anybody advise how deep the footings need to be on a single storey extension, which will be the size of a cloakroom. The footings will be over an area where my drains are already laid and I want to check how much soil is needed above to prevent them collapsing. Is there a building regs requirement on footings?

thanks

 

[Johndug]

Your local building inspector should have already seen and approved/rejected your drawings.

 

[dunc]

Agree with previous post, you should have building regs or you are buying your self a serious problem when you come to sell! Secondly they can take enforcement action against you and tell you to take it down.

Footings should be a minimum of 1000mm. ie from soil level to top of concrete. May need to be deeper depending on soil conditions. Concrete will need to be 250mm thick and 600 wide.

dunc

 

[Mr Winston]

the bottom of your foundations must be lower than the bottom of your drains. (if you put them above they would crush and block the drain!)

the actual depth of your foundations will depend on the ground conditions. a reasonable guide may be to the same depth of your existing foundations (providing they are on suitable ground)

as said previously the building inspector from the council must inspect the foundation trench - if its not ok he will tell you

 

[dunc]

Yep good point, assumed that 1000mm + 250mm would be beneath! But if not, you would not necessarily need to go deeper with the footings. The inspector might ask you to reinforce the footing and shutter the drain. Hope it doesn't come to that.
Every time I have looked at where I am going to put footings I always think where are the ***@@@!!! drains!!

Dunc

 

[suzzanne]

another option would be to use a raft foundation which can be kept at high level but needs good steel reinforcement in it. this is where the floor slab and footings are cast as one. it helps to spraead the load, but the sub grade would need to be adequete to do this.
you would have to concrete encase your drains and provide movement joints.
Binspector is the best person to speak to.

if you don't get bregs approval your house insurance may be invalid should anything happen and also if you sell your house who ever wants to buy it may not get a mortgage if the work does not have bregs approval!

 

[5125andrew]

Thanks. I do realise I need building regs approval, I'm trying to ascertain whether building the extension over an area where I know the drains run is going to lead to expensive complications.

I don't understand the comment about footings needing to be 1000mm and the concrete 250mm thick and 600mm wide. Does this mean you can use hardcore except for the top 250mm?

 

[suzzanne]

It doesn't have to be a problem but you will have to concrete encase the drains if they are beneath the building.
Are the drains private?
Can you move the drains?
How deep are the drains?
The 1000mm is a bit strange, you only actually need 600mm to the underside of the footing from the ground level but this will depend on the sub grade. if you are digging down 1m to the top of the concrete and then another 1m to good ground you need to ask yourself why?

 

[dunc]

Hi,
to clarify, typically the trench for footings will need to be 1250 deep. This can vary depending on the ground, [heavy clay being one of the worst] and the building inspector. The 600mm wide relates to the thickness of the intended wall typically 100/75cavity/100 ie total of 275. This wall needs to sit on a wider stable platform of concrete normally ie 600mm wide.

Ring your local authority and speak to your local BI, ring between 9-10am they are normally out on site after this! He /she will me a mine of useful knowledge and more importantly and my last piece of advice...... Get onside with your BI from day one and stay there!!!

Dunc

 

[chappers]

Its usually 1m including your concrete but as someone already said its dependant on soil conditions.
If you hit bedrock after a few inches then you wont need to go any further if however your still digging in sand afer a meter then you will need to go further, large trees in the proximity will also have an effect.
Only your BCO can tell you when you have dug sufficient out but my guess will be 1m min unless you hit rock.