Oversite for suspended timber

[selfbuildextension]

I am currently clearing the area for an extension, its roughly 8m wide x 5.2m out externally, engineer and architect are still working on regs drawings but I'm keen to clear as much as i can whilst the weather is good as I'm doing it all by hand and there is a raised patio of 80 slabs.

Site is sloping away from the house.

Would it be safe to assume 450mm depth required at the moment for the oversite? 50 hardcore, 100 concrete, 150 air gap, 150 joist minimum

I'm happy to have to dig further once I know whats needed but want to avoid going to far just now, is there any downside to clearing the oversite before digging trenches?

 

[Notch7]

s there any downside to clearing the oversite before digging trenches

There is if you are using a digger. Not so much a problem if a hand dig.

Surely the joist depth will be larger than 150 if timber?

Are you not choosing block n beam?

 

[selfbuildextension]

Surely the joist depth will be larger than 150 if timber?

Are you not choosing block n beam?

yeah hand dig, access is poor and the trenches are only a few cm from the boundary on two sides so the digger would be offset so can't see that working too well.

150 was just a guess, I presume it will be much bigger but i thought if i assume 150, then ill just need to scrape a bit more off rather than going too deep.

When the SE visited they said the plans would likely come back suggesting timber to match existing. Its a full plans application so I'm just trying to do what I can whilst waiting.

 

[tony1851]

You dont necessarily need 100mm of concrete. IIRC you can use 50mm of concrete, or just 50mm fine inert material eg sand/gravel on 1200g polythene. But if you are using 150 joists (and presumably sleeper walls?) you'd probably be best with 100mm conc., unless you form a separate strip footing for the sleeper.
Have you got at least 300mm between the oversite finish and the finished ground floor? - that's the sort of figure you need for a suspended timber floor to ensure acceptable ventilation.

 

[noseall]

is there any downside to clearing the oversite before digging trenches?

Yes massive if you are getting a digger on site and if the weather turns foul. I'd go ape.

We dig the trenches then dig the oversite (in stages) as we pull back.