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
Nope, the architect has specified a 75mm screed (inc UFH) over 150mm concrete!
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.
Who do you trust more, the architect or the contractor? The bco will go with the architect if he has any sense.
The contractor because the architect has never done the job, he's only read about it.
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.
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].
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.
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.
Perhaps it is on clay and has to have extra thickness to allow for ground heave?
mark
It's more normal to use a suspended concrete floor in these cases..
The Natural Philosopher coughed up some electrons that declared:
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
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.
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.
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.
I'd say don't do it anyway - there are other ways to save a few hundred squids without skimping on the fundamental components.
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.
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.
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.
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.
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