Build up extension from patio?

[Deluks]

~ Silly question I know when put like that, but got you clicking didn't it

Digging up a couple of my patio slabs , I found they were on a bed of about 2" of sharp/very lean mix, which itself was sitting on a solid 3-4" concrete slab. This must have been the original patio when the house was built. Beneath that is earth, then a chalk bed which starts at about 600mm.

The patio had been here for 30+ years and hasn't budged, even with a load of concrete blocks stacked on it for a few months, (prolly about half a ton worth spread over 420mm square)

The concrete beneath is about 150-200mm below FFL in the house, with this in mind would it be passable to dpm/inso/screed over this to make a ground bearing floor slab in an extension?

I know it could be construed as bad practice, but would be a nice cheat if ever an extension was built (obviously strip founds would still be needed)

I don't know yet how far out the conc goes but it's rock hard, on solid stuff beneath and is flat as a pancake on the surface.

I basically want to know if anyone has done this, or heard of it being done? and passed by building.

I think this was asked recently, with inconclusive replies.

 

[noseall]

I have left an existing slab in place with no prob's from b.c., when the old slab was completely covering the new extension floor area.

However, i have also known them to strongly advise the removal of an existing slab when said slab only covered a portion of the proposed extension floor.

Their argument was that as only part of the new extension was in possession of the old slab then the remainder would settle at a differing (greater) rate, thus potentially leading to cracking of the new slab.

 

[Deluks]

'strongly advise'

Did you take said advice? Or did you stitch in with rebar and toe it in with a step?

 

[noseall]

Did you take said advice? Or did you stitch in with rebar and toe it in with a step?

We hoofed it out, only because it was my first extension, having gone self employed and bco's were there to be feared.

 

[Deluks]

I expect they fear you now, they probably know you as the mad axeman!

 

[^woody^]

I expect they fear you now, they probably know you as the mad axeman!

No, they know he gets his info from here . lol

There should be no problem with putting your new floor on top of this slab. The slab will be perfectly settled over time, and will spread load better than any new sub-base