Floor slab in my new extension

[Pilot2002]

Hi. My new extension steps down from the current floor level of the existing house to a lower level. My question is - once I have worked out how far below the existing floor level the new floor level should be, how much more will I then have to dig out to allow for the construction of the floor slab. I think I'm right in saying that the slab comprises, compacted hardcore, sand binding, insulation, DPM, concrete slab and screed. Is there set thicknesses for each of these parts? Many thanks

 

[Deluks]

Hardcore 100-150mm
Sand depends on how jaggy your hardcore is but no more than 50mm, best to top off with MOT type1 to fill the gaps before laying on the sand. (Or you can do it all in MOT)
Insulation min 90mm.
DPM under the insulation AND a layer on top of the inso to act as a separation layer, although this can be regular medium gauge polythene.
Concrete 100mm
Screed min 65mm

You can cheat a bit and omit the screed: Hardcore>sand>dpm>insulation>polythene>concrete. But the concrete will need to be well finished or topped off with self levelling compound.

 

[Pilot2002]

That's really helpful thanks Deluks. Circa 400mm - thats quite a bit to dig out! Cheers. Ben

 

[Deluks]

Only dug 300 for my kitchen. I used bricks smashed up to golfball size and old roof tiles, topped off with a jumbo bag of MOT, was solid as, so very little sand required. Only 100mm there. Used 80mm insulation and about 110 concrete. If you've ever considered UFH, now's the time to put the pipe in. I cable tied speedfit pipe to concrete mesh and buried it in the slab for future connection. Only cost about £60 extra in materials.