HI
I'm sure the main house isnt on a raft as wouldn't a raft incorporate the floor slab? I now my main house is on block and beam so would that suggest strip or trench fill footings?
How much roughly am I looking at for a raft? Depth was roughly 2m for good ground.
Thanks
Mike
Hi Mike, just joined this forum but I work in building control.. You are right in saying that if you have a b+b floor then you can exclude the possibility of the existing house being on a raft, as a raft is the foundation and floor slab all in one, but as said above, there is the chance you may have a piled ring beam.
Your question is quite hard to answer without knowing the ground conditions where you live, as this would make a difference on the design. Firstly I would find out why the council would prefer a raft? I mean, I could say I prefer a raft or piling on every job as a belt and braces approach just to be on the safe side, even when its not necessary! rafts are generally used when there is either risk of subsidence (eg. in known mining areas) or the ground has a low bearing capacity.
If you are having to go down to 2m to find decent ground then it might be that is actually more economical to do a raft than strip foundations anyway.
If you was to do a raft it wouldn't go down to 2m.. The whole point of using a raft is that it spreads the load over the whole area of the extension so saves digging down so far. I would say in general you would be looking at a 300-400mm slab with a thickened toe of between 450-550mm.
The next question to ask the council would be, so they want structural engineers details/calculations for the raft? Generally I would say, yes they would. (which obviously has cost involved)
With regard to thee cost, its hard to say as it depends on the design you need. As with a lot of things in building, each build has its own requirement so it varys greatly and depends where you are in the county.. For example where I am in London a muck away grab lorry is around £300-350 each but I know some areas of the country its half that.
But what you would need to price up is:
1. Muck away costs - 1 lorry would guess
2. Concrete costs - work out area in cubic metres (widthxdepthxlength) and find out how much you can get a concrete lorry per cubic metre of conc.
3. Re-inforcement (if reaquired by design)
4. Labour costs (including machine hire etc.)
Depends how much you can do yourself etc.
Hope that helps you a bit!
A
