extending existing concrete slab/foundations



hi all, just interrupting myself reading through every single post on here with a genuine enquiry...

there is an existing single storey extension on my home that i would like to extend.. permission pending. it spans the rear width of the back of the house but it is too narrow, the ceiling too low and whether i were to extend or not i would still dismantle and raise the roof height as both myself and partner are too tall under it, so i figure i should build a new wall a couple of metres out at the same time and take the time to create a bit more sideways space too.

i have a vague idea that to join this proposed foundation to the existing foundation that i either need to key it in somehow or leave some sort of expansion/contraction joint(?) and understand that this is compliant with current building regulations(?)

also can someone put me straight that i wont have to pull up the old slab either and that i can carry out the same keying in or expansion/contraction joint when i pour the new slab next to it?

what are the pros and cons of fixing a foundation and slab as opposed to using joints?

the subsoil is a bit clay though as high as a metre down gets pretty dry and the the existing foundations seem to go only a little further than that.

no large trees directly influencing the building though i have it in my head the contraction/expansion joint thing is the way to go and then i only have to watch the interior floor surface crack forewarning the impending collapse of the roof! )

cheers all..
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2 Feb 2006
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United Kingdom
There is no need for ex-con joints at all regards foundations.

Dig down to existing, and either butt against, wrap around or underpin, depending upon the depth of the existing concrete.

As for the floor slab; It depends how you go about achieving the thermal numbers etc. Is the existing floor slab at the same height as the main house floor? Is it insulated? Does it have polythene underneath?


hi noseall, existing extension screed and slab sit at the same level as the main house and i can see a damp proofing membrane in the external wall so id assume it went under the slab.

havnt thought about how to make a seamless joint in this membrane..

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