abuse@localhost coughed up some electrons that declared:

On Sun, 08 Mar 2009 00:41:34 +0000, a certain chimpanzee, Tim S
randomly hit a keyboard and produced:

I was going to have (and thought I had, until I looked)

100mm concrete
50mm celotex (OK , that's probably worth more in jablite-mm)
50mm screed with UFH water pipes
tiles.

In that order, so there wasn't too much thermal mass in the UFH.

Even I know you need sand blinding under a DPM, or it's going to get
holed.

It's a small and independent section of floor, taking little load. But as
lax as my mental specification was, I thought that 50mm of concrete on
earth and no sand was rather taking the mick - and quite liable to
cracking if the base wasn't 100% stable.

Whe you say, "on earth", do you mean that literally?

Yes, literally - with respect to the original floor. In the areas that were
dug out, there seems to be little evidence of hardcore, though there may
have been a token amount once which has just got full of fines.

There should be a
base below any ground-supported concrete slab that's level, well
compacted and inert to prevent any voids forming under the concrete.
The easiest and most reliable way of achieving that is to have a base
of hardcore that can be compacted. If not, I'd want to inspect any
'earth' below the slab _very_ carefully for any organic matter, how
well it's compacted, etc.

I agree


The base is then usually blinded to prevent any stones penetrating the
membrane. The _minimum_ thickness of concrete over this should be
100mm.

If you're laying insulation over your concrete, I'd suggest at least a
65mm screed, preferably reinforced. Maybe even thicker if you have
heating pipes in it. BTW, the more thermal mass in the slab the
better. This will even out any temperature fluctuations. Me, I would
stick the insulation under the concrete.

Sounds like a plan - thanks Andrew.

Cheers

Tim