Foundations for a window coming forward ?

[Jake2021]

Merry xmas all,

So in jan I’ll be starting the foundations for a front dwarf wall which my windo will sit on and a 80 cm x 2m h block wall … it’s going to be connecting to my side wall of my front porch and supporting pillar… it’s not actually taking any weight as the insulated canopy is already there but will tie it to it just for strength

I just wondered if anyone can advise of foundation depth

Will be using 7.3 dense blocks outside , king span in cavity and thermal inner blocks

Thanks
Jake

 

[George_engineer]

Best bet is to match the existing foundation depth.

 

[Jake2021]

normally would but my issue is the porch I’ve just built I prob went overkill on 1m x 60 cm however the house foundation is on a 6 inch slab which you hit around 50 cm down
Best wishes
Jake

 

[noseall]

normally would but my issue is the porch I’ve just built I prob went overkill on 1m x 60 cm however the house foundation is on a 6 inch slab which you hit around 50 cm down
Best wishes
Jake

What deemed it overkill?

One would dig down to 'suitable load bearing strata' etc. The go-to default depth of 1m below ground level is considered a catch all for most extensions, which it often is. The reality is that you keep going until you find good virgin ground that is bereft of any fertile (dark soft soil) matter. If you did down and you are still seeing brick ends, tie wires, coke cans and scaffold clips - go deeper.

however the house foundation is on a 6 inch slab which you hit around 50 cm down

Oh...?
You mean your house is built upon a raft foundation?

 

[Jake2021]

Yes house is on a raft, it probably isn’t over kill for what blockwork there was for the porch v glazing there wasn’t much weight lol
But at least it won’t sink lol

My ground here is far from perfect unless you like making cups and plates lots of clay and a high water level ‍

 

[George_engineer]

It could just be the edge of a strip footing if it's 500mm down. Really it depends when it was built, post 1970 I'd assume raft, between 1950 to 1970 then could just be a thinner strip footing.

 

[Jake2021]

Think it was built 30-40’s it a ex council house .
Best wishes
Jake

 

[^woody^]

If the house is on a raft, a raft which floats, then you don't normally want to add an extension that is on fixed foundations.

Further, the main house has been built on a raft for a reason, so the question then arises as to whether a traditional strip foundation is suitable for the ground conditions there.

 

[Jake2021]

Woody could it be built on a raft foundation due to the clay and high water level?

If it is on a raft foundation would a new extension also have to be on a raft?
Thanks
Jake

 

[noseall]

Woody could it be built on a raft foundation due to the clay and high water level?

High water table - yes.