We've had some plans drawn up for Building Regulations approval for a
extension/refurbishment.

Our architect has specified the concrete slab at 150mm throughout. Our
groundwork contractor reckons that it can be poured at 100mm (this adds
up to a saving of a few hundred quid).

Does anyone know what the pros and cons are and what building control
are likely to say? Is the thinner thickness more likely to fail? I
presume it adds thermal mass if we want UFH.

"Jim" wrote in message
...
We've had some plans drawn up for Building Regulations approval for a
extension/refurbishment.

Our architect has specified the concrete slab at 150mm throughout. Our
groundwork contractor reckons that it can be poured at 100mm (this adds
up to a saving of a few hundred quid).

Does anyone know what the pros and cons are and what building control
are likely to say? Is the thinner thickness more likely to fail? I
presume it adds thermal mass if we want UFH.


It is normal to have a 50mm cement screed over 100mm concrete so maybe this
is how your architect arrived at 150mm.

mark

"mark" wrote:
"Jim" wrote in message
...
We've had some plans drawn up for Building Regulations approval for a
extension/refurbishment.

Our architect has specified the concrete slab at 150mm throughout. Our
groundwork contractor reckons that it can be poured at 100mm (this adds
up to a saving of a few hundred quid).

Does anyone know what the pros and cons are and what building control
are likely to say? Is the thinner thickness more likely to fail? I
presume it adds thermal mass if we want UFH.

It is normal to have a 50mm cement screed over 100mm concrete so maybe this
is how your architect arrived at 150mm.

Nope, the architect has specified a 75mm screed (inc UFH) over 150mm
concrete!

Jim wrote:
We've had some plans drawn up for Building Regulations approval for a
extension/refurbishment.

Our architect has specified the concrete slab at 150mm throughout. Our
groundwork contractor reckons that it can be poured at 100mm (this
adds up to a saving of a few hundred quid).

Does anyone know what the pros and cons are and what building control
are likely to say? Is the thinner thickness more likely to fail? I
presume it adds thermal mass if we want UFH.

I don't know who's done your plans but your groundwork contractor is
correct, 100mm of concrete is ample for a domestic floor and will pass any
building control scrutiny, with or without any additional screed.

you will need either 75mm of kingspan (or equivalent) insulation, or, if you
decide to go the cheaper route, 125mm of polystrene - this will require more
excavation so this /could/ prove a false economy, but where there is a lot
of infill required, it works out quite a bit cheaper.

--
Phil L
RSRL Tipster Of The Year 2008

"Jim" wrote in message
...
We've had some plans drawn up for Building Regulations approval for a
extension/refurbishment.

Our architect has specified the concrete slab at 150mm throughout. Our
groundwork contractor reckons that it can be poured at 100mm (this adds
up to a saving of a few hundred quid).

Does anyone know what the pros and cons are and what building control
are likely to say? Is the thinner thickness more likely to fail? I
presume it adds thermal mass if we want UFH.

Who do you trust more, the architect or the contractor?
The bco will go with the architect if he has any sense.

dennis@home wrote:
"Jim" wrote in message
...
We've had some plans drawn up for Building Regulations approval for a
extension/refurbishment.

Our architect has specified the concrete slab at 150mm throughout.
Our groundwork contractor reckons that it can be poured at 100mm
(this adds up to a saving of a few hundred quid).

Does anyone know what the pros and cons are and what building control
are likely to say? Is the thinner thickness more likely to fail? I
presume it adds thermal mass if we want UFH.

Who do you trust more, the architect or the contractor?

The contractor because the architect has never done the job, he's only read
about it.

The bco will go with the architect if he has any sense.

Thankfully, they almost always go with the contractor, especially in cases
like this, where it shows that the architect hasn't a clue and has gone
overboard as an arse covering excercise.

He's specified a 225mm floor! - the concrete in the foundations only has to
be that deep and it's holding up an entire building along with it's floors
and roof - what's going to be on a floor? - at the very heaviest end of the
scale, possibly a bathful of water, which if it were upstairs, would be on
18mm chipboard.

--
Phil L
RSRL Tipster Of The Year 2008

On 26 Feb, 22:25, "Phil L" wrote:
Jim wrote:
We've had some plans drawn up for Building Regulations approval for a
extension/refurbishment.

Our architect has specified the concrete slab at 150mm throughout. Our
groundwork contractor reckons that it can be poured at 100mm (this
adds up to a saving of a few hundred quid).

Does anyone know what the pros and cons are and what building control
are likely to say? Is the thinner thickness more likely to fail? I
presume it adds thermal mass if we want UFH.

I don't know who's done your plans but your groundwork contractor is
correct, 100mm of concrete is ample for a domestic floor and will pass any
building control scrutiny, with or without any additional screed.

you will need either 75mm of kingspan (or equivalent) insulation, or, if you
decide to go the cheaper route, 125mm of polystrene - this will require more
excavation so this /could/ prove a false economy, but where there is a lot
of infill required, it works out quite a bit cheaper.

--
Phil L
RSRL Tipster Of The Year 2008

And no, it doesn't add to the thermal mass as it's below the
insulation. The only thermal mass you need to consider is the screed
which ideally you want as thin as reasonable, normally that seems to
mean about 70mm.
[My screed goes down on Sunday so my UFCH pipes are now all fitted and
ready to go].

"Phil L" wrote in message
.. .
dennis@home wrote:
"Jim" wrote in message
...
We've had some plans drawn up for Building Regulations approval for a
extension/refurbishment.

Our architect has specified the concrete slab at 150mm throughout.
Our groundwork contractor reckons that it can be poured at 100mm
(this adds up to a saving of a few hundred quid).

Does anyone know what the pros and cons are and what building control
are likely to say? Is the thinner thickness more likely to fail? I
presume it adds thermal mass if we want UFH.

Who do you trust more, the architect or the contractor?

The contractor because the architect has never done the job, he's only
read about it.

The bco will go with the architect if he has any sense.

Thankfully, they almost always go with the contractor, especially in cases
like this, where it shows that the architect hasn't a clue and has gone
overboard as an arse covering excercise.

He's specified a 225mm floor! - the concrete in the foundations only has
to be that deep and it's holding up an entire building along with it's
floors and roof - what's going to be on a floor? - at the very heaviest
end of the scale, possibly a bathful of water, which if it were upstairs,
would be on 18mm chipboard.

As the ground conditions are unreported I don't think its safe to advise
anyone about this.

I can tell you this, my foundations are far thicker.
100 mm isn't enough if there is any steel in it as the 50 mm covering isn't
enough to prevent corrosion.
You need at least 150 mm to ensure the steel is deep enough.

On Thu, 26 Feb 2009 22:25:51 -0000 Dennis@home wrote :
Who do you trust more, the architect or the contractor?
The bco will go with the architect if he has any sense.

Tbe BCO is only entitled to ask for the minimum that will satisfy the
relevant regs. 100mm is the norm for domestic unreinforced slabs.

Proper attention to what is under the slab is the important thing here.

--
Tony Bryer, 'Software to build on' from Greentram
www.superbeam.co.uk www.superbeam.com www.greentram.com

"Jim" wrote in message
...
We've had some plans drawn up for Building Regulations approval for a
extension/refurbishment.

Our architect has specified the concrete slab at 150mm throughout. Our
groundwork contractor reckons that it can be poured at 100mm (this adds
up to a saving of a few hundred quid).

Does anyone know what the pros and cons are and what building control
are likely to say? Is the thinner thickness more likely to fail? I
presume it adds thermal mass if we want UFH.


Perhaps it is on clay and has to have extra thickness to allow for ground
heave?

mark

mark wrote:
"Jim" wrote in message
...
We've had some plans drawn up for Building Regulations approval for a
extension/refurbishment.

Our architect has specified the concrete slab at 150mm throughout. Our
groundwork contractor reckons that it can be poured at 100mm (this adds
up to a saving of a few hundred quid).

Does anyone know what the pros and cons are and what building control
are likely to say? Is the thinner thickness more likely to fail? I
presume it adds thermal mass if we want UFH.


Perhaps it is on clay and has to have extra thickness to allow for ground
heave?

It's more normal to use a suspended concrete floor in these cases..


mark

The Natural Philosopher coughed up some electrons that declared:

mark wrote:

Perhaps it is on clay and has to have extra thickness to allow for ground
heave?

It's more normal to use a suspended concrete floor in these cases..


Although I'd have to add that I've got 4" slab floors direct (practically)
on clay, and no issues with heave.

Although the clay round my house is always damp/wet due to the lay of the
land.

Cheers

Tim

"dennis@home" wrote:
He's specified a 225mm floor! - the concrete in the foundations only has
to be that deep and it's holding up an entire building along with it's
floors and roof - what's going to be on a floor? - at the very heaviest
end of the scale, possibly a bathful of water, which if it were upstairs,
would be on 18mm chipboard.

As the ground conditions are unreported I don't think its safe to advise
anyone about this.

I can tell you this, my foundations are far thicker.
100 mm isn't enough if there is any steel in it as the 50 mm covering isn't
enough to prevent corrosion.
You need at least 150 mm to ensure the steel is deep enough.

I should add, the same groundworks contractor did a set of foundations
(for a timber framed barn) for us last year, which is about 100 yards
away from the current proposed extension - so he should be adequately
aware of the ground conditions! It's fairly well drained gravelly soil.

"Jim" wrote in message
...
We've had some plans drawn up for Building Regulations approval for a
extension/refurbishment.

Our architect has specified the concrete slab at 150mm throughout. Our
groundwork contractor reckons that it can be poured at 100mm (this adds
up to a saving of a few hundred quid).

Does anyone know what the pros and cons are and what building control
are likely to say? Is the thinner thickness more likely to fail? I
presume it adds thermal mass if we want UFH.


minimum thickness of a poured slab is 150mm, unless you can prove via
engineering calcs that an alternative solution will work.

and minimum 50mm cover over any reinforcing used.

"mark" wrote in message
et...

"Jim" wrote in message
...
We've had some plans drawn up for Building Regulations approval for a
extension/refurbishment.

Our architect has specified the concrete slab at 150mm throughout. Our
groundwork contractor reckons that it can be poured at 100mm (this adds
up to a saving of a few hundred quid).

Does anyone know what the pros and cons are and what building control
are likely to say? Is the thinner thickness more likely to fail? I
presume it adds thermal mass if we want UFH.


It is normal to have a 50mm cement screed over 100mm concrete so maybe
this is how your architect arrived at 150mm.

mark


This is incorrect .... slab thickness is one thing, and minimum standard
thickness is 150mm

As mentioned you can use thinner if you can prove that it will supply
necessary strength (reinforced or high strength mix)
BCO will not like less than 150mm.

Screed on top is of no structural strength and is leveling layer.

You could use power floated concrete and not bother with a screed. .....
depends on what is going on top of it.

Rick Hughes wrote:
"Jim" wrote in message
...
We've had some plans drawn up for Building Regulations
approval for a extension/refurbishment.

Our architect has specified the concrete slab at 150mm
throughout. Our groundwork contractor reckons that it can be
poured at 100mm (this adds up to a saving of a few hundred
quid). Does anyone know what the pros and cons are and what building
control are likely to say? Is the thinner thickness more
likely to fail? I presume it adds thermal mass if we want UFH.


minimum thickness of a poured slab is 150mm, unless you can
prove via engineering calcs that an alternative solution will
work.
and minimum 50mm cover over any reinforcing used.

I'd say don't do it anyway - there are other ways to save a few hundred
squids without skimping on the fundamental components.

Rick Hughes wrote:
"Jim" wrote in message
...
We've had some plans drawn up for Building Regulations approval for a
extension/refurbishment.

Our architect has specified the concrete slab at 150mm throughout.
Our groundwork contractor reckons that it can be poured at 100mm
(this adds up to a saving of a few hundred quid).

Does anyone know what the pros and cons are and what building control
are likely to say? Is the thinner thickness more likely to fail? I
presume it adds thermal mass if we want UFH.


minimum thickness of a poured slab is 150mm, unless you can prove via
engineering calcs that an alternative solution will work.

no it is not - the minimum is 100mm and in twenty years of building, and at
least 200 floor pours, I have only once known it to be anything above this.


and minimum 50mm cover over any reinforcing used.

which is why it's 100mm - the only reason it is so much is so that
reinforcing, where it's required, (and rarely is this the case with a
domestic floor) is in the middle of a 100mm slab.

only once since 1984, have I seen plans which had a slab drawn at 150mm and
they were for a cellar of a pub - needless to say, we set it out for 100mm
and the BCO agreed, upon inspection prior to pouring, that even100mm was
overkill...so that's what we poured.

Bearing in mind that almost all concrete products produced for driving HGV's
over are 50 - 60mm, a hundred millimetres for simply walking on is
ridiculous in the extreme.

--
Phil L
RSRL Tipster Of The Year 2008

"Phil L" wrote in message
. ..
Rick Hughes wrote:
"Jim" wrote in message
...
We've had some plans drawn up for Building Regulations approval for a
extension/refurbishment.

Our architect has specified the concrete slab at 150mm throughout.
Our groundwork contractor reckons that it can be poured at 100mm
(this adds up to a saving of a few hundred quid).

Does anyone know what the pros and cons are and what building control
are likely to say? Is the thinner thickness more likely to fail? I
presume it adds thermal mass if we want UFH.


minimum thickness of a poured slab is 150mm, unless you can prove via
engineering calcs that an alternative solution will work.

no it is not - the minimum is 100mm and in twenty years of building, and
at least 200 floor pours, I have only once known it to be anything above
this.


and minimum 50mm cover over any reinforcing used.

which is why it's 100mm - the only reason it is so much is so that
reinforcing, where it's required, (and rarely is this the case with a
domestic floor) is in the middle of a 100mm slab.

only once since 1984, have I seen plans which had a slab drawn at 150mm
and they were for a cellar of a pub - needless to say, we set it out for
100mm and the BCO agreed, upon inspection prior to pouring, that even100mm
was overkill...so that's what we poured.

Bearing in mind that almost all concrete products produced for driving
HGV's over are 50 - 60mm, a hundred millimetres for simply walking on is
ridiculous in the extreme.

Here we are jumping to conclusions, there is a big difference between a raft
to support a building and a floor slab.
The OP didn't say which but I expect its a raft.

"dennis@home" wrote:
Here we are jumping to conclusions, there is a big difference between a raft
to support a building and a floor slab.
The OP didn't say which but I expect its a raft.

This is just the floor slab, there are separate trench foundations for
the walls.

Here's what the groundwork contractor wants to do:

"Excavate to remove topsoil to stockpile on site. Excavate 38m2 to reduce
levels. Further excavate 23 L.M. x 600mm wide x 1.2m deep foundation
trenches. Provide concrete to the same max 900mm deep. Provide 15m2 x
100mm dense concrete blockwork, 2 courses to the inner leaf and 1 course
to the outer leaf. Provide 38m2 x 100mm deep concrete slab to a tamp
finish on DPM on sand blinding on 150mm compacted stone."

FWIW, my 7th edition Chudley says (p199)

"Thickness [of concrete bed] for domestic work is usually 100 to 150mm"o

Chudley is meant to be current with building regs, so it seems unlikely
that 100mm isn't allowable in some circumstances.

In article , Jim
wrote:
Does anyone know what the pros and cons are and what building control
are likely to say? Is the thinner thickness more likely to fail? I
presume it adds thermal mass if we want UFH.

I first installed UFH in about 1993 and we are currently installing it
again, in another house.
Basically, if you have a layer of insulation in the floor, only
concrete/screed ABOVE the insulation will provide effective thermal mass.
I couldgo on a bit more about this, if you want me to.
Almost certainly modern regs. will require insulation.

--
From Glorious Gloucestershire, near Lydney, using :------------
_ _________________________________________
/ \._._ |_ _ _ /' Orpheus Internet Services
\_/| |_)| |(/_|_|_ / 'Internet for Everyone'
_______ | ___________./ http://www.orpheusinternet.co.uk

dennis@home wrote:

Here we are jumping to conclusions, there is a big difference between
a raft to support a building and a floor slab.
The OP didn't say which but I expect its a raft.

in which case the subject title would have been 'raft thickness' - two
totally different things, and 150mm is nowhere near enough WRT a raft

--
Phil L
RSRL Tipster Of The Year 2008

"Jim" wrote in message
...
"dennis@home" wrote:
Here we are jumping to conclusions, there is a big difference between a
raft
to support a building and a floor slab.
The OP didn't say which but I expect its a raft.

This is just the floor slab, there are separate trench foundations for
the walls.

Here's what the groundwork contractor wants to do:

"Excavate to remove topsoil to stockpile on site. Excavate 38m2 to reduce
levels. Further excavate 23 L.M. x 600mm wide x 1.2m deep foundation
trenches. Provide concrete to the same max 900mm deep. Provide 15m2 x
100mm dense concrete blockwork, 2 courses to the inner leaf and 1 course
to the outer leaf. Provide 38m2 x 100mm deep concrete slab to a tamp
finish on DPM on sand blinding on 150mm compacted stone."

FWIW, my 7th edition Chudley says (p199)

"Thickness [of concrete bed] for domestic work is usually 100 to 150mm"o

Chudley is meant to be current with building regs, so it seems unlikely
that 100mm isn't allowable in some circumstances.

Well I asked the head of building control the question while we were feeling
a tree..

He said that it isn't worth saving a few quid on something that should last
a hundred years.
It will only save you about 1.9 m3 of concrete or about £150.