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.
