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:

http://photos.dionic.net/v/public/bu..._0009.jpg.html

I'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

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.

ALex coughed up some electrons that declared:

From the ground up

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

Righty ho - thanks for that

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

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

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? 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.
--
Hugo Nebula
"If no one on the Internet wants a piece of this,
just how far from the pack have you strayed?"

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

Tim S coughed up some electrons that declared:


Sounds like a plan - thanks Andrew.
Sorry - brain spasm

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:

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:

http://photos.dionic.net/v/public/bu..._0009.jpg.html

I'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