Trench fill foundation level below FFL?

21 Nov 2005
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United Kingdom
I have dug (1m deep) the foundations for our new two storey rear extension and they have passed their inspection by the local building officer. I have the concrete coming this weekend and would like someone to confirm the level to fill the trenches to.

It is an old house built into a slight slope. So the existing floor level of the kitchen is 150mm below the ground level where the foundations are. I need to tie the new extension to the floor level of the kitchen. So what level do I fill the trenches in with concrete. I think the building inspector said 225mm below the kitchen floor. Is this correct and fairly standard? I have read elsewhere that it should be minimum 150mm below the ground level. The finish of the building along one wall is block rendered, the other wall is stone with a bi-fold door?

Any advice will be greatfully recieved. Thanks
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So long as you observe the absolute minimum thickness of concrete required, you can work up from there.

It is normal to establish the finished floor level, which is also the dpc level and gauge the foundation concrete down from this datum working in courses of whatever masonry units you intend using.

You also need to bare in mind any fluctuating ground levels and the need for drainage gullies and their respective pipework. The last thing you want is the foundation concrete interfering with any drainage products.

So, let's say the ground around the building is fairly level. It would be fairly typical to set the finished concrete foundation level at two courses of block or six courses of brick below dpc.

We set ours at seven courses of brick but that's another story...:cool:

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