# Concrete floor height



## gwr (24 Apr 2018)

So I've just had a builder on the phone with a price for concrete base and single skin block building. This will be over 30 square meters allowing an inner timber stud to take insulation and leave a finished floor area of 30 square meters.

What concerned me was they said the slab would be 3 to 4 inch depth and deeper to around 5 to 6 inch around perimeter with DPM under the slab but no steel in the slab. Is this enough?

They said the slab would protrude the blocks by around 2 inches ! Is this not asking for water ingress at the bottom block.

The price included dashing 3 sides and they said the bell cast would be around an inch above the slab! This doesn't seem high enough to me, I thought around 150 mm minimum.

They also advised with it being single skin to put 2 coats of synthaprufe on internal walls before putting up stud and insulation.

I have known of the builder from a young age and he has a decent name for himself I would like any thoughts on this please before I call him back to say yes or nah.

The price for digging out for materials, slab base,laying slab and building up to around 2250 is around 5K


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## dzj (24 Apr 2018)

Reinforcing mesh can't hurt.


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## banjerbill (24 Apr 2018)

Perhaps get another quote or two and see what they offer. 

Bill


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## gwr (24 Apr 2018)

Yeah I think mesh should be used when building in block of a slab but I'm no expert. I'm looking to get more quotes but want it started ASAP now. It's not so much the price putting me off but the detail they want to go with. Thanks for replys


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## MikeG. (24 Apr 2018)

Don't ask builders to tell you what they propose doing.......tell them what you want. 

I can't tell you about the slab thickness without knowing what is going on around it: soil type, proximity of trees, water table, and so on. I can tell you, though, that you absolutely must not have the slab extend out past the line of the blockwork. Your render shouldn't finish below the DPC, and that should be a minimum of 150mm above ground level......and the top of your slab should also be 50mm above ground level. I suggest laying 3 courses of bricks, putting your DPC on top of that, and having the render finish at that point.

Oh, and those suggesting reinforcing in a 100mm thick slab are wasting your money. Steel needs 50mm of cover, which would put it at the mid-point of the depth of the slab.........which is the line of neutrality (neither in compression nor tension), and the only place where steel would have no effect whatever. Which isn't to say that this slab doesn't need reinforcing...just that if it needs reinforcing it needs to be thicker.


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## MikeG. (24 Apr 2018)

dzj":11bvtq6o said:


> Reinforcing mesh can't hurt.



Yeah it can. It can hurt your wallet without achieving anything, and further, if it is placed without enough cover it can cause the failure of the concrete in due course.


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## dzj (24 Apr 2018)

Mike, does it also have to be 50mm from the bottom, or only from the outer surface?


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## gwr (24 Apr 2018)

Thanks for clearing that up for me Mike I suspected that was the case with the slab protruding the blocks I don't understand why he was wanting to do this. And also taking render down that close to the slab didn't make and sense to me.
Mabe I should say thanks but no thanks to this particular builder.

there is no trees around the site and some clay about 12/15 inches below surface I'm not sure about the water table.

If the main slab were 5 inches deepened to 7/8 inches 500mm all around perimeter would this be stable enough without steel for a single skin building or does this depend on water table? 

Any thoughts on the synthaprufe.


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## MikeG. (25 Apr 2018)

dzj":1wlfjzjg said:


> Mike, does it also have to be 50mm from the bottom, or only from the outer surface?



Again, depends on what forces are being designed against:

Rule 1..........it needs a minimum of 50mm of concrete between the steel and the outside of the concrete, in every direction (up, down, sideways).

Rule 2: steel goes where concrete will be in tension. So if the slab is being designed across weak ground which might give way under it, or to support a load which will cause it to try to bend downwards, then the steel goes at the bottom of the thickness (bearing Rule 1 in mind). If the slab is being designed to withstand upward forces (heave in clay, tree roots, and so on), then the steel goes at the top of the slab thickness (again, see Rule 1).


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## MikeG. (25 Apr 2018)

gwr":3hu7ijg5 said:


> ........there is no trees around the site.........



I don't want to sound too building inspector-ish, but is this really so? A hedge, an ornamental hawthorn, a 4' high garden willow......they all count. And what do you mean by "site"? I Trees across a road can influence your slab or foundation design. Ornamental plants in your neighbours garden can too. I hope you can understand why I can't just tell you "you'll be fine with a 6" slab" or whatever, for a site I haven't seen, or even seen photos of. Let me just say this, though, assuming a perimeter to your slab of around 20 metres, you need a slab *at least* 125 thick in even the very best circumstances.


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## dzj (25 Apr 2018)

MikeG.":1d3um94p said:


> Rule 1..........it needs a minimum of 50mm of concrete between the steel and the outside of the concrete, in every direction (up, down, sideways).
> 
> Rule 2: steel goes where concrete will be in tension. So if the slab is being designed across weak ground which might give way under it, or to support a load which will cause it to try to bend downwards, then the steel goes at the bottom of the thickness (bearing Rule 1 in mind). If the slab is being designed to withstand upward forces (heave in clay, tree roots, and so on), then the steel goes at the top of the slab thickness (again, see Rule 1).



Sorry to be pestering you , I can understand tree roots, but why are these upward forces generated on clay surfaces?


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## novocaine (25 Apr 2018)

dzj":fh7vnlmq said:


> Sorry to be pestering you , I can understand tree roots, but why are these upward forces generated on clay surfaces?



Water


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## MikeG. (25 Apr 2018)

dzj":3rpmo44n said:


> MikeG.":3rpmo44n said:
> 
> 
> > Rule 1..........it needs a minimum of 50mm of concrete between the steel and the outside of the concrete, in every direction (up, down, sideways).
> ...



Clay shrinks when it dries, and swells when it gets wet. So if you build on clay you need to account for both movements, and if you build when it is dry there is every chance that the first force your slab will be subjected to is upward. The other classic is cutting down a tree. Tress keep ground locally drier than it would be otherwise (because they consume the water), and when they are cut down the ground can often heave (swell) as a result of becoming wetter again.


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## gwr (27 Apr 2018)

Thanks for the advice I only plan on building once and want to get this right so all the info i can get is great.When you say at least 125mm in the best of conditions does this need steel or 150 for that matter?


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## gwr (22 Jun 2018)

In the end I decided to go with strip foundations 1 meter deep 600 wide with 300 depth of concrete. I didn't quite get the size I wanted, my mistake, i had marked the founds with spray paint and forgot the lines were for center of founds and not the outside hey ho. We managed to get the width but not the length so I've ended up 6.6 x 4.9 from the center of founds which is over 30m2 internal but I had planned on the inner skin being 100x50 stud with 50mm cavity. So I'm thinking of using 75x50 and smaller cavity what could i cut the cavity down to? Would 25mm be ok or any other ideas for insulation as that is the main reason for interior skin along with ease of fixings.


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## gwr (22 Jun 2018)

Hi The builder started laying the blocks today and I've just returned from a great week on the broads, I hadn't really noticed the difference in the ground level from front to back of shed which is around 300 mm or so. This is obviously too much floor height to loose so I can't keep the floor slab above the natural ground level at the back of shed. 

Is it ok to have the floor level around ground level at the front which would mean 300mm or so below ground level at the back. The slab will have a dpm under it and brought up to dpc height and tucked under dpc.


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