Alternative Method of Building Extension

Tomjim
The point of building foundations is to transfer the load of your building to a satisfactory bearing strata, i.e. a layer of soil, rock or whatever which has the bearing strength to support your building without compressing and thus causing deflection and damage to the structure.

You seem to be describing some type of pile system, which is usually carried out with concrete piles driven into the ground at each corner of the building with beams spanning between to accept the loading from the building and transfer it to the piles, and onwards to the ground. Piles are expensive and should only be considered for very poor ground or other unusual ground conditions which render traditional foundations unacceptable.

You will be digging up the tarmac in any case to form a solid base for a concrete floor.

My advice is to appoint a structural engineer to design your foundations. Your extension is going to be tied into the existing house in some way, and by not putting in proper foundations you risk not only structural damage to the extension, but the extension pulling bits of the existing house out with it when it sails away.

The building regulations, with which you have to comply, have a section relating to Structure, which is referred to as TGD A, link below:

http://www.environ.ie/en/Publications/DevelopmentandHousing/BuildingStandards/FileDownLoad,1639,en.pdf

Traditional strip foundations, with a careful detail at the interface with the existing foundations, is usually the most cost effective way to construct domestic foundations. Hire an engineer with professional indemnity insurance, who will sign off on the foundations, and take it from there.

Tomjim said:

I have an existing Dormer Bungalow. The wife is now looking to build a small back kitchen circa 8ft * 12 ft which can be accessed from outside as well as inside. The purpose of this room will be to have a shower/ toilet which can be accessed from outside, a washing machine/ area to dry clothes, storage area, place to put clothes which are used for gardening etc.

The difficulty is all around the house is tarmaced, so this would cause a lot of work digging up tarmac etc to put down foundations.

I am wondering would it be possible to leave tarmac as is, not put in foundations, instead, dig 4 holes and put down 4 RSJs in each corner of new structure and block walls into each RSJ.

Would this work

Click to expand...

Yes, you could construct a lightweight structure on steel columns on ground beams and leave most of the tarmac in place - except it probably wouldn't last or weather very well and could detach from the house.

Steel posts can corrode, they are great conductors of heat and you'd still have to plumb in the services and take the connections through an uninsulated air gap into the extension.

Friction piles need careful calculations and end bearing piles require careful design and placement of the base - both are usually in concrete in my experience.

The tarmac you wish to "preserve" would be pock-marked with holes and would tend to settle under the weight of construction traffic and become badly eroded and marked.

The construction method you propose seems more suitable to a temporary detached pavilion than one of the permanent, more highly serviced parts of the house requiring a direct connection to it.

The seduction of lightweight prefabricated construction often palls when the reality of weathering and insulation in an Irish climate manifests itself.

And yes I have thought of this very method for a front extension/conversion to our own garage - detailing problems as noted above shelved it.

That's not to say its impossible, but you'd really need to know what you're doing with the substructures.


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.

onq has given a comprehensive experts opinion which answers your questions, but I thought I would add the following,

A builder with proper equipment will probably be able to rip up your tarmac in a few hours without breaking a sweat. I can't see any potential savings in avoiding this step.

Hi huskerdu

Please don't be offended if I make a slight correction.
I offered a "competent opinion" I hope, not an "experts opinion".

I'm vary of overstating either my competence or the extent to which a reader can rely on the advice offered on AAM, for reasons stated in my sig.

AAM cannot replace RL appointments and advisors "on the ground" in building matters.

Thanks for the vote of confidence though

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.

No offense taken. I totally understand your point about being careful about giving advice based on the limited information posted.

I was merely pointing out that my advice was based on no professional knowledge what so ever, just another punter who has used builders.

I stand by what I say - I cant see ripping up the tarmac being a big cost,when building an extension.

Agreed, and BTW, professional or not, I'm one of those that puts a lot of store in listening to punters like your goodself.
In the absence of a rigorous regime of empirical testing experience of punters actually doing things is like gold dust.



ONQ.