# Raft foundation & extension



## scuby (2 May 2011)

my house is on a raft foundation, and i'm thinking of putting in a  foundation now to make a raised patio area, and maybe in a few years put  in a conservatory/single floor extension.

I was hoping to do it in such a way, so i could put in the foundation  now and then finish it off now, to be like a raised patio area outside  my back kitchen door, and if we do go to build a conser/extension, all i  would then have to do is build from the floor up, and not have to go  and dig up an ordinary patio area, and waste time and money putting that  in first time around.

So, should i put a raft foundation in to match the house, as i presume  if we ever went to sell the buyers engineer might not like that ?

and if i went with a raft, is it much more expensive ?

anyone other alternatives ideas ? tbh i think i would rather go with a raft for this, but juct want to check opinions...

house is a semi D
thanks


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## onq (2 May 2011)

It seemed at one time that raft foundations were outlawed under the building regulations.
To my reading of the regulations,  everything loadbearing was supposed to have a proper foundation footing.
Yet more people seem to be going for this option on cost grounds and engineers have happily continued to design them.

I have never done a raft foundation - the unique combination of Irish sub-soil conditions have never tempted me to design a building with such a footing.
Could I suggest you address your questions to an engineer with experience of designing such foundations, and more specifically, extending buildings with raft foundations.
So much depends on soil conditions and site preparation that I think you really need to take expert advice for your particular building, ang specifically not rely on general advice from AAM.

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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## scuby (3 May 2011)

thanks for reply.

Just to clarify :

-engineer will be coming out to have a look
-posted here to see what the rest of the AAM posters thought, and if they  have any alernative idea, suggestions, so i could ask engineer
-Was not going to be deciding on advive of posters alone
House is 5 years old, so i presume raft building in not outdated yet


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## onq (3 May 2011)

On the contrary scuby.
They seem popular on the grounds of cost reduction for self-builders.

I have seen several posts expressing interest in them here on AAM and others from posters claiming to have done them, both here and on Boards.ie in the Planning Forum. 
You could ask the mainly architectural technician population over on Boards.ie what their experience or view may be, but remember, they aren't structural engineers or architects.

There does not seem to be a lot of case law suing technicians for offering poor advice, even where the technicians were acting as architects or engineers - so far!
Thus their advice may not be honed to as fine an edge as a qualified professional in the interest of avoiding legal action (see my sig file at the end of my posts).

Having said that there are some obvious issues.

Walls with foundations typically have strip footings a minimum of 2ft /600mm down to protect the house from frost heave or settlement.
The 2ft/600mm depth is supposedly below the level at which the ground freezes in winter, even last winter, thus avoiding frost heave in wet substrata.

The 2ft/600mm depth also ensures that you are on "firm bearing strata" and well below the level of roots and vegetation.
This is dependant on and subject to what is discovered in the Trial Hole Inspection by your site assessor/engineer.

Sites should not be assessed solely for the disposal of effluent to septic tank arrangements or surface water to soakaways.
This is because the correct design of the foundations is dependant on the bearing strength of the soil.

Reinforced raft foundations may be intended to be used to support relatively lightweight buildings on poor soils.
The edge beam is intended to consolidate the soil and prevent erosion beneath the slab edge and may help spread the load.
The building positioned and the slab designed to "balance" the centre of gravity of the building to avoid the slab settling to one side.
The raft may be "thickened" and have reinforcement under the location of supportin elements - loads should usually be spread not concentrated.

I pass on the below link for what its worth.
I do not endorse or approve the advice therein, nor have I read it.
It appears to be a link from Blackwell Publishing, a reputable publishing house, but it may be forged.
However the sections I have simmed through seem to offer a reasonable overview of the issues involved.
This cannot be a substitute for taking professional advice, but it may help you understand any advice given and reasons for it -

With all that caveat in mind, here is the link - http://www.blackwellpublishing.com/cooke/docs/samplechapter.pdf

From all the foregoing comments you can hopefully see that simply adding a bit to one side or other of a raft  foundation is not straightforward.

There are other considerations  besides cost when it comes to considering what form of foundation should be employed.
For example, if the site is partly bog, are there proposed or current excavations or piling for wind farms nearby?
Remember the landslides landslips allegedly caused by these activities.

What might happen to a raft founded dwelling in such a scenario - will it "float off"?
The house might need piling to bedrock to ensure some sort of stability, but even that might not be enough.
The site itself might need protection from landslip, with a wall rising from bedrock designed to divert the flow of bog material.
That's heavy engineering work and points to the site selection as the most critical part of the building process as all else flows from that.

So what I'm trying to say is that departing from standard foundation detailing should only be done after a thorough site investigation by an engineer and not merely for cost-saving reasons.
The design of the house must be limited to come within the performance parameters of the raft foundation design.
The raft itself must be properly specified to spread the load and protect the substrata.

Any work to an existing raft should take all the above (and probably a lot more, like subsoils water and drainage) into account.
In fact the first thing an engineer should do is assess the existing foundations.

I trust this advice is of some use.


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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## onq (3 May 2011)

BTW,

I forgot to say that I hadn't taken on board the fact that your house is a Semi-D.

Everything I said before about centring the building to avoid settlement looms larger where another building might be affected.

Assuming there is a 215mm concrete block party wall between the dwellings I'm not sure yours is a "true" raft - there are several variations.

All the more reason to get an engineer on board as you have been advised above and on boards recently.



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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