How deep should the top of the footings be below ground level?

[brickierob1]

Hi everyone, 

I'm wondering the answer to the above question in regards to a rear extension, which is 1200mm deep by 800mm wide. I'm looking for a definitive answer, but I'm unsure there is one. Is their any factors that would contribute to the answer. 

 

Many thanks

[the_r_sole]

ground conditions determine the design of foundations so it varies from project to project

[A_L]

The bottom of the footing should be at least 450emm deep to prevent frost heave.

 

Bottom of Page 37 here https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/429060/BR_PDF_AD_A_2013.pdf

 

 

[nod]

1 hour ago, brickierob1 said:

Hi everyone, 

I'm wondering the answer to the above question in regards to a rear extension, which is 1200mm deep by 800mm wide. I'm looking for a definitive answer, but I'm unsure there is one. Is their any factors that would contribute to the answer. 

 

Many thanks

I filled our flush 

1800 deep It’s a toss up between 

the extra concrete used and the amount of brickwork and Labour you save 

 

Edited December 22, 2020 by nod

[markc]

Only minimum depth and thickness (design) as the rest would often be blocks or brick etc. then its a matter of aesthetics should the foundation be visible above ground level.

[Russell griffiths]

Work it out from damp course down, if you have blocks below dpc you don’t want to be cutting them. 

[Brickie]

Personally,minimum 450mm from dpc to top of concrete. 
That leaves enough room to get the pipework in with lintols over the voids before you’re coming out of the ground. 
You’d hope with those two bed joints on the blocks that you can get over any discrepancies in the concrete but you’re at the mercy of others commitment to accuracy there. 

[Oz07]

Is it the smallest extension on earth?

[Temp]

4 hours ago, Russell griffiths said:

Work it out from damp course down, if you have blocks below dpc you don’t want to be cutting them. 

 

+1

 

Remember when your school teacher taught you about "lowest common multiples" in maths ?

 

If building outer leaf in brick and inner leaf in block there is a height at which X number of brick courses is the same height as Y number of block courses. So if you fill the trench to that height below DPC you dont have to cut any bricks or blocks.