# Anybody built using Raft Foundation?



## Delfagio1 (1 Mar 2010)

Hey all,

Anybody here ever built a house using a raft foundation?
If anyone has I would love to hear from you to get some information.

Thanks

Steven


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## Delfagio1 (2 Mar 2010)

Anyone ???? Ha ha


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## Capt. Beaky (2 Mar 2010)

All you want to know can be culled from here.
http://www.scribd.com/doc/14585530/STAAD-PRO-tutorial

What are the soil/water table conditions?


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## onq (2 Mar 2010)

Capt. Beaky said:


> All you want to know can be culled from here.
> http://www.scribd.com/doc/14585530/STAAD-PRO-tutorial
> 
> What are the soil/water table conditions?



Captain Beaky, that leads to steel design website.

Presumably the foundations will be of concrete.

And there are issues with raft fundations.

ONQ.


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## Delfagio1 (2 Mar 2010)

Thanks Capt Beaky

Yea the soil conditions are good and the water table is very deep as there is a river at the botom of the field which is approx 15m-20m below the height of our site.


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## Eng Car 1 (2 Mar 2010)

Delfagio1 are you trying to design this raft yourself or are you just asking in general about raft design?

I suggest that if you need a foundation design, you do not attempt to find one over the internet. You should seek out and appoint a Competent Chartered Strucural Engineer to assess and design a suitable foundation for the ground conditions and structural loadings for your proposed building.  Also try to go on recommendations and make sure that they have PI insurance.


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## Capt. Beaky (2 Mar 2010)

Delfagio1. If you go off page on the earlier link, you'll find out a little info. I did a raft foundation, where a terraced house was demolished, the basement wasn't more than 600mm above the water table and the houses at both sides had stepped brick founds. It's pretty basic to do but you've got to get an engineer to do the figures. But going on what you said in your opener, you should have no problem


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## Delfagio1 (2 Mar 2010)

Hey Eng Car 1,

No not trying to design raft foundation myself, just get info on it. If anyone had pictures of doing one. Does the radon barrier go underneath the slab with the radon pipes and sump box etc? 

I will get an engineer to dsign it but I will construct it and build / pour it myself.

Thanks also Capt Beaky, 

I went looking through it and found some info


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## sallins (2 Mar 2010)

If your soil conditions are good at normal depths as you seem to indicate, traditional spread foundations will likely work out cheaper than a raft foundation.Suggest you first seek advice from a Structural Engineer or from a Chartered Engineer specialising in structures.


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## onq (2 Mar 2010)

A standard raft foundation can be considered for a simple house plan of one storey, subject to engineers design.
This kind of foundation doesn't seem to be covered in Part A of the building regulations - in Part A all loadbearing walls are required to be suported by a strip foundation.

Loadbearing walls in a two storey house can include internal walls or loadbearing studs to support first floor joists or slabs, giving a line load to be transmitted to the ground.
Houses may also incorporate point loads where there are columns or concentrated loads for example at the ends of beams, at and around stairs opes, and at masonry chimney breasts which can occur on internal or external walls.
As long as the point loads, concentrated loads and line loads have been assessed and the design amended to cater for them, a slab foundation designed by a competent structural engineer may be appropriate in the case of simple, symmetrically planned houses.

The main benefit appears to be cost, but the applicability of this kind of construction is limited to very simple or very large and complex structures, because in the mid rage, where most houses exist, the preference is for well designed strip or pad foundations, or piles in extreme cases with ground beams.

In addition, unless your engineer has properly investigated and prepared the ground beforehand and designed the foundation type appropriate to the conditions, merely installing a standard spec raft foundation with some extra rebar can be a disaster waiting to happen.
Rafts can crack due to uneven settlement due to ground conditions and can even "tip" in extreme cases, where for example the ground on one site of the house is more supportive than on the other side.
Where the raft, installed by someone on a "job" basis is a generic raft, but the house isn't evenly "balanced and centred", the raft can be eccentrically loaded which can lead to cracking or differential settlement.

This problem can be exacerbated where a raft is used as a cost-effective foundation to an extension and instead of biting the bullet and tying in the new work at foundation level with a strip similar to the existing dwelling, the raft is chosen for cost and may well end up being costly, sould the raft settle and the extension rotate around the interface with the existing house.

In addition specifying a raft now for a bungalow, without internal loadbearing walls will make it difficult to allow for the later conversion of the attic.
The floor loads will require to be supporte mid-span and that means line or point loads being transmitted through studs that are not design as loadbearing elements.
If you strengthen them or replace them with loadbearing studs or loadbearing blockwork the loads they transmit must be suported by the slab, which has not been designed for this.

So the lesson to be learnt is - build cost effectively now and fail to allow for later vertical extension and you may come to regret it.
In no case proceed to site without proper site investigation, preparation and foundation design by an engineer.

FWIW

ONQ.

[broken link removed]

All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.


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