Raft foundation

Hi,

Can someone explain to me what exactly a raft foundation is and exactly what sequence of steps are needed to build one ;

I hear terms like "level and fill", "shutter", "lay the membrane", "tie steel", "pour slab" but I am not sure what they mean.

Fill with what ?
What is membrane for ?
What is a shutter ?
etc

Thanks

Raft foundations are generally used in poor soil conditrions, where an alternative strip foundation would place too much load on the sub-soil. In terms of the terminology used,

'Level and fill' probably refers to the ground, i.e. that is is prepared prior to the raft

"shutter" - think mould, to form the concrete. A timber frame is created into which the concrete is poured.

"lay the membrane" - this is the water proof membrane, made from polythene sheeting. It is also known in the business as DPM (Damp Proof Membrane). This goes between the the concrete and the ground underneath.

"tie steel" - most rafts, certainly for houses, would involve very little tieing of steel as pre-fabricated meshes are generally used. If individual steel reinforcement bars are used, they are laid crossing each other and tied with a small wire in the middle to for a grid.

"pour slab" - this is the placing of the concrete, usually from a concrete truck (pre-mixed)

One thing you omitted is insulation - ideally, a perimeter strip of 1 metre should be placed around the edge to prevent heat loss. It goes ontop of the DPM, under the concrete.

Hope this helps.

Thanks quinno

what exactly do you fill your foundation with besides concrete ?

Just concrete.

Read this post - it is informative.

Marion

From the original post, I'm a little worried that the OP is a novice house builder who is planning on building his own raft foundation, without the input of an engineer.

I strongly suggest if that is so, that he not do it. For one thing, how is he planning on certifying the work for drawing down mortgage payments etc.?

You ll definatly need an engineer with insurance. If your land is bad enough to need a raft its bad enough to need an engineer

membrane is probably the radon barrier.

Superman said:

From the original post, I'm a little worried that the OP is a novice house builder who is planning on building his own raft foundation, without the input of an engineer.

Click to expand...

I get that impression too. If that being the case then the OP has more money than sense

I have a raft built house that is flooding with water below ground level ( over 2000 litres pumped out to date ). My NHBC surveyor has informed me that it is because the ground slopes towards the property. I have now removed that problem by lifting paving flags etc yet the issue persists. I think there may be a build issue but i can't get anyone to take responsibility. Any ideas on why i keep flooding. PS the water only comes into the property when it rains.

Jeff said:

I have a raft built house that is flooding with water below ground level ( over 2000 litres pumped out to date ). My NHBC surveyor has informed me that it is because the ground slopes towards the property. I have now removed that problem by lifting paving flags etc yet the issue persists. I think there may be a build issue but i can't get anyone to take responsibility. Any ideas on why i keep flooding. PS the water only comes into the property when it rains.

Click to expand...

exavate out aprox 4m from house in all directions and go below foundation level, insert french drians, with a layer of 1-2 " stone fill and then pea gravel...probably the simplest and cheapest..other methods available

ahve run of for drain away from house to a lower level.

Defo get in a good builder and a civil engineer to advise....each site is individual and soil tests and land counturs as well as water table need o be looked at.