Requirements for foundations for an extension

[squarenut9]

I am starting a new extension in a few weeks. The drawings were done by an architect and the drawing shows the footings cast in a 900mm deep trench 600mm wide. But the concrete is only shown 250mm deep. I always thought when pouring footings you filled the trench allowing about 5 courses of bricks up to the damp course.

Any ideas anybody? Somebody said it was a cheaper option as ready mixed is quite expensive, however there is a fair amount of labour involved trying to build a cavity wall in a trench. Plus it can't be too easy working in a confined space.

Or is it up to the local building control?

 

[noseall]

The bare minimum amount of foundation concrete thickness, for many domestic extensions is 225mm.

However mass filling, i.e. filling the trench so that the least amount of brickwork is laid, is by far the cheapest option.

We would normally set the concrete levelling pegs at 525mm below dpc.
This equates to about 600mm thickness of concrete.

There is no comparison between adding more concrete as opposed to laying more bricks/blocks.

Mass fill every time.

 

[freddiemercurystwin]

Nout wrong with what's been drawn, you can do deep mass filled footings or thinner strip founds as have been drawn. There is no technical reason not to do one or the other assuming you're ground is not unusual. Some will say its cheaper to do one or the other depending on who's doing the bricklaying or who's supplying the concrete. Ultimately the Inspector will stick his thumb in the ground to satisfy himself the grounds firm enough, he won't care either way.

 

[squarenut9]

Nout wrong with what's been drawn, you can do deep mass filled footings or thinner strip founds as have been drawn. There is no technical reason not to do one or the other assuming you're ground is not unusual. Some will say its cheaper to do one or the other depending on who's doing the bricklaying or who's supplying the concrete. Ultimately the Inspector will stick his thumb in the ground to satisfy himself the grounds firm enough, he won't care either way.

I guessed bricking in the trench must be more labour intensive and cost more.
I have a 5.5mt deep well about 8mts from the proposed extension. I looked down it and the sides seem to be solid rock. The guys that dug it in the late 40's must have been built like Charles Atlas. There is a hell of a lot of rock around our place. Its not far from the moors (Dartmoor) so maybe I'll be lucky and only have to go down a meter and no more. The other problem is the weather, I have the windiest, rainiest place on earth and I'll have to fill as soon as the trench has been inspected. How long do the BCO's take to come out. The worst thing I can imagine is having a trench full of water.

 

[freddiemercurystwin]

Usually BCO will come out next day.

 

[jeds]

As noseall says the quoted thickness of concrete is just a minimum and is a bit misleading really. The trench will never have an exactly level bottom so the actual thicknes of concrete in any one area is going to vary. It's far better to peg out the dpc level and work downwards and let the actual thickness take care of itself - obviously making sure it is at least 225.

 

[garyo]

My BCO will visit same day if I call it in before 10am.

Gary

 

[squarenut9]

As noseall says the quoted thickness of concrete is just a minimum and is a bit misleading really. The trench will never have an exactly level bottom so the actual thicknes of concrete in any one area is going to vary. It's far better to peg out the dpc level and work downwards and let the actual thickness take care of itself - obviously making sure it is at least 225.

So when I phone round the ready mix companies and state what the mix is for will they automatically give me a cost for a 'correct foundation mix' or do I have to tell them what mix I require? I need approx 5.5 m3. At a guess I would say it will cost about £600.

I have been on sites in the past and I have seen site agents take samples of a mix while it is being poured and putting it a little cast iron mould to be sent away. Is this a commercial requirement or have I got to do this to prove to BCO that the concrete is the correct strength? I have been out of the trade for 10 years now and am well behind with regs.

 

[jeds]

It should be a Gen 3 mix but just tell them what it's for and they'll give you the right stuff. If there is any possibility of contaminants in the soil you should consider a mix suitable for sulfate conditions. No need to sample the mix. Water content is important - the mix should be just wet enough to be workable. You don't want it like runny pea soup. Cost at the mo is about £90 per cube. But obviously that varies depending on quantity and region. Most wagons carry 6 cubes so you are leaving them with a small amount that they probably won't be able to get rid of on the same trip.

 

[squarenut9]

It should be a Gen 3 mix but just tell them what it's for and they'll give you the right stuff. If there is any possibility of contaminants in the soil you should consider a mix suitable for sulfate conditions. No need to sample the mix. Water content is important - the mix should be just wet enough to be workable. You don't want it like runny pea soup. Cost at the mo is about £90 per cube. But obviously that varies depending on quantity and region. Most wagons carry 6 cubes so you are leaving them with a small amount that they probably won't be able to get rid of on the same trip.

Thanks for the info. I did check a local company. Something about people in Devon, seem very cagy when it comes to giving a precise cost. But the figure was about right. I was pleasantly surprised.

 

[masona]

Just finish pouring my footing with http://www.pro-mix.co.uk/index.htm don't know if they are available in your area, only pay for what you need, no left over or mess approx £90 cubic metre