Floor slab - minimum thickness for modern construction?

[Tim S]

Hi,

Fairly sure I know this - but just want to get my facts straight:

Whats the minimum acceptable depth for a bit of concrete poured onto the earth (with a DPM), for a room thats about 1m x 2.5m?

I thought it was about 100mm.

Having removed some shuttering today, I discovered the builders had a managed to get as thin as 50mm.

And they "forgot" to blind the ground with sand - the DPM sheet is sitting on pointy rocks. The reason I know this is I asked for a channel to be left for me to fit a channel drain, aka:

'm pretty sure I'm going to be telling them to get back pronto and do it again (for free) but I just wanted to be sure I wasn't being unreasonable.

Sigh.

They did OK with most of the fiddly bits, because I was specify like a government department, and checking regularly.

I relaxed with the floor slab, because "builders know how to do floor slabs" All I said was do it to building regs and leave 100mm above for me to add insulation and screed with UFH. Sounded simple enough...

Makes me feel totally justified in DIYing 90% of the project - rather wishing I'd made it 95%!

And these are not "rip off dudes" - they're known to me and generally pleasant and helpful. Just seems that anything to do with floors given them the wobbles.

Cheers

Tim

[ALex]

From the ground up

100mmm consolidated hardcore sand blinding damp proof membrane 100 Insulation 100mm concrete 75mm screed or floating floor

that would pass most building control assuming you have applied for this.

[Tim S]

ALex coughed up some electrons that declared:

Righty ho - thanks for that :)

Oh yes. Got a big fat BNA active full of random and diverse things...

To be honest, the BCO doesn't seem much bothered, beyond the drains being watertight, some Part P wibbles and basic fire and structural integrity. His view on Part L was "do your best, but we cut you plenty of slack on older buildings; just make it better than it was before".

Existing floor is 100mm ish of concrete on damp earth and 25-50mm screed at random.

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.

Cheers

Tim

[abuse]

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

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

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.

[Tim S]

abuse@localhost coughed up some electrons that declared:

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.

I agree

Sounds like a plan - thanks Andrew.

Cheers

Tim

[Tim S]

Tim S coughed up some electrons that declared:

Sorry - brain spasm

[Tim S]

Quick follow up (exceptionally bottom quoted *if* you really need the context)

I had some other questions for the BCO which were outstanding anyway, so I asked him round - he's coming tomorrow and said he's happy to have a look at the floor too. Might as well get my £321's worth!

I proposed a solution to him based on suggestions here, to kick the ball off. I'm sure he'll tweak it, but it's a tiny floor area, so I'm more than happy to do what he says :)

I'll post a summary of his acceptable solution later.

Better buy some biscuits ;->

(The other questions are to do with roof insulation/ventilation and damp - none of which are easy to explain, so I figured it was easier to show him now I've uncovered stuff.

Cheers

Tim

Tim S coughed up some electrons that declared: