I have recently purchased a conservatory but have been told I need a concrete raft for it to sit on. Can any one help me and advise which is the best way to go about this please.
building a concrete raft
- Thread starter stuhod
- Start date
Ps and Gs rigid; raft centre of gravity not coincident with that of load, due to shape. Ground ****e, bearing pressure greatest at far end, cons'y rotates away from house, even though peak bearing pressure may only be in the order of 10kN/m2. It happens.
The ground has been made up although it has stood for 10years. We can't get down to clay for foundations to build walls on. The steps that were in place over that period have moved about an inch from the house wall.
Therefore I was told to put in a raft, what do others think. If I need one how do I go about constructing one. Thanks
Therefore I was told to put in a raft, what do others think. If I need one how do I go about constructing one. Thanks
Personally? I wouldn't. 4 piles and an rc slab/beam will make sure that it stays there and the cost won't be humongous either.
The art of a raft is to try and get the respective centres of gravity of the raft and the building loading to be as near to coincident as possible. But that relies on assumptions as to how the loading is made up and will be in actuality. In other words, a best guesstimate (like most engineering calculations), but, essentially, it would mean making the raft extend out beyond the end of the conservatory parallel to the house wall. That means putting a step in it, to get it below the ground level outside, making the placing of rebar more awkward. Have you got drains under where the conservatory is going? If so, more complications.
4 piles and a beam/slab arrangement much more straightforward and no more costly, imo. With the added benefit of knowing that that isn't going to go anywhere.
As you're on made ground, is the house piled?
The art of a raft is to try and get the respective centres of gravity of the raft and the building loading to be as near to coincident as possible. But that relies on assumptions as to how the loading is made up and will be in actuality. In other words, a best guesstimate (like most engineering calculations), but, essentially, it would mean making the raft extend out beyond the end of the conservatory parallel to the house wall. That means putting a step in it, to get it below the ground level outside, making the placing of rebar more awkward. Have you got drains under where the conservatory is going? If so, more complications.
4 piles and a beam/slab arrangement much more straightforward and no more costly, imo. With the added benefit of knowing that that isn't going to go anywhere.
As you're on made ground, is the house piled?
The house is built on traditional concrete foundations on good clay reinforced with steel. It is only the rear of the house which has been made up approx. 5ft in depth which concerned me. Sorry don't understand what is meant by 4 piles and a rc slab/beam. Is it possible to explain to this DIY man. 
Thanks
ThanksOnly 5'?! You could either pull 450 trench to good stuff and use trenchfill foundation, or pad bases to that depth with high level reinforced foundation spanning between these. Both simpler and cheaper than a raft; and they won't move!
Thanks for all the info and help. cheers
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