extension foundations

[jeff186]

hi guys I hope you can help

I am rebuilding my extension which was build over 19 years ago very poorly I might add. planning permission was granted 19years ago

I would like to know what size for the foundations the building is ( H 2.6m x W 3.8m L 6.1m)

it will have a 75mm cavity double skin brick/breeze block. I think the currant foundations are 750mm deep not sure how wide they are.

I was thinking 900mm x 900mm or would that be too big

any help would be great

thanks

jeff

 

[r896neo]

To be honest you need to use concrete for foundations. Currants are definately not a suitable load bearing medium. Concrete is better for many raisins.

The foundation should be at least twice the width of the wall, so in your case 600mm would be adequate. 900 is overkill. In reality when dug with a 2 foot bucket they will come out nearer 700mm

You can use full filled trenches which will cost a lot more but save on labour or pour strip footings and bring the footings up in block.

As for depth that will depend on ground conditions. You should assume your trench will need to be minimum of 1m deep to satisfy building control who will want to inspect it before the pour.

How are you planning to insulate to regs? 75mm cavity is too small for standard partial fill celotex etc.

 

[noseall]

A single storey extension will sit happily on a foundation 220mm thick.

The width of the trench (and subsequent concrete) will need to be 150mm wider than the proposed wall. This equates to a 150mm 'toe' or projection of concrete beyond the wall faces.

The foundation depth will depend upon local soil conditions. 1m minimum depth is a typical dig, though nearby drains, trees and shrubs, made up ground etc, can often mean the trench needs to go much deeper than a metre.

We mass fill out trenches with concrete, reducing the amount of masonry and labour needed to get the walls up to DPC.