# Forum Home Renovation Structural Renovation  Thickness for small suspended slab???

## Make it work

Hi all,
I am setting up to pour a suspended slab for a landing at the back door. It will be about 1mtr square supported fully along one edge and on the corners on at least a full brick at the other end. I also intend using reo and at least 25mpa concrete. 
So my question is, will a thickness of 75mm be OK or is this too thick or too thin?  
Cheers
Alan M

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## bricks

75mm is way too thin.
Plus you need to have reo and stressing to hold the slab together after you take the form work away.
If I were you I'd just buy a concrete pre-fab slab/paver ( yes you can get them made and delivered), Place it on top.  
Or fill the hole underneath with sand or rubble and pour a slab on top of that.

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## pawnhead

It's only a metre square and the clear span would be even less than that so 75mm would probably do if you're using a sheet mesh reo, but for an extra few bucks I'd make it 100mm.
To save buying a full sheet of reo @ 2.4m x 6m, I'd just buy a length of 12mm rebar and cut lengths @ 940mm. (you could get the reo place to cut them for you). This gives you 30mm of concrete cover at the edges so they don't get wet and rust, but make sure that they overlap your supporting base by at least 50mm. Lay them in a square grid pattern (both ways) 150mm apart(200mm would be OK, but play it safe for a few bucks more) , tie every second intersection with tie wire (or a bit of string) and support the grid on 4x25mm bar chairs, so that it's closer to the bottom where the tensile stress is located in a simply supported suspended slab.  :2thumbsup:  
Disclaimer: I'm a builder, not an engineer, but this is just how I'd do it, and from my experience, if anything it's overkill.  :Wink:

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## ian

You will need a piece of formwork, which wont be recoverable, plus supports under the slab to support the wet concrete.  
In this situation I'd use 15 or 18mm fibre cement sheet for the formwork. 
You haven't said what you are going to place on the slab, but if it's tiles I'd be inclined to forget the slab and use two or three thicknesses of 18mm FC sheet glued and screwed together.  
ian
use be I'd be in

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## Make it work

Thanks for the replies, 
The top of the landing and the top of the steps will be tiled but I do prefer concrete.
I may be able to make it thicker than 75mm but I don't have much to play with. I did think I may be able to put a lintal in accross the less supported edges and I suppose I can always drop the corry formwork down below the supporting brickwork. 
OK, off to lay some more bricks. will post pics later. 
Cheers 
Alan M

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## pawnhead

Well I haven't really seen a slab that thin unless it was a garden path on ground, but then I haven't seen a slab that small designed before. But I did pour a slab that was engineer designed once that tapered down to 75mm. It was a cantilevered slab and it was 100mm thick at the supporting wall, then it overhung the wall by 600mm, and the bottom tapered up to 75mm for a sleek thin look at the edge. It had 12mm reo bars @ 200mm centres, top and bottom, that met at the end of the cantilever.
I'd say that 75mm would be OK for such a small span, but I'd go to whatever trouble was needed to make it 100mm since neither of us are qualified to make that sort of decision in the first place. Especially if there's a chance of anyone falling if it breaks.

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## Make it work

Good advice John, I think the steel is the answer, but I will bump it up as close to 100mm without going too high. 
Cheers 
Alan M

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## bricks

just thinking about your problem,  
Why dont you build the supporting walls and the columns that hold the concrete up, then get a base made from bondeck or even a section of thick calv sheet or something.  
You'll still have to put reo in but theres less chance of the slab dropping out from underneath?

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## Terrian

SL102 reo or better for a suspended slab, consult an engineer....

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## pawnhead

> consult an engineer....

  Start with an architect. He'll be able to help you get it through council with your development application, followed by your building application, advertisment fees, inspection fees, footpath damage levy, etc, etc.
With any luck, you may be able to pour as soon as next May/June if you've got enough money left.  :Wink:

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## Terrian

> Start with an architect. He'll be able to help you get it through council with your development application, followed by your building application, advertisment fees, inspection fees, footpath damage levy, etc, etc.
> With any luck, you may be able to pour as soon as next May/June if you've got enough money left.

  that, my friend, is what I call going just a little overboard  :Smilie:  
I would hate to see the guy (?) pour this suspended slab, only to have it fail in a few months time :Frown:

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## DvdHntr

> that, my friend, is what I call going just a little overboard  
> I would hate to see the guy (?) pour this suspended slab, only to have it fail in a few months time

  Minimum 100mm thick and the 12 rebars sound like the best option. I would use bondek/condek. Refer to their online guides.

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## pawnhead

Welcome to the forums DvdHntr. I see you're taking an interest in engineering matters with your posts so far. It's always good to have another well informed contributor on board.
What are your qualifications if you don't mind my asking?

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## Make it work

Here is the pic I promised. Now with the brickwork done it is easy to see where the slab must go.
I was going to put another course of bricks on but based on the advice I got from all you learned folks I will now make the slab thicker.
Before any one points it out, the sliding door is yet to be moved to its new position.

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## pawnhead

That's nothing. It's only spanning 700 at its widest. I'd just buy a sheet of MDF for formwork and set it up 100mm below your finished height, and let the concrete drop down to the brickwork just around the edges. Install it in two pieces so you can get it out when you strip. A few rubbish 4x2 joists under the perimeter and one in the guts, sitting on timber props, or brick stacks. MDF edgeboards, tied back the through bracing, with tie wire back to the deck.  
You've got a bit of work going on there I see. If you've got enough of that trench mesh left over you could use that.

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## DvdHntr

> What are your qualifications if you don't mind my asking?

  Structural Engineer, MIEAust 
Was looking for something else and came across the board. When I saw some of the posts I thought I may be able to help. Also, I am doing a renovation and when I come to do some of the trickier parts I thought I could get some advice.

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## Make it work

> Welcome to the forums DvdHntr.

  Welcome from me too, I am newish here too and have found it very helpful and enjoyable. 
Make sure to explore other parts of the site, the list is long and varied but all good. 
I'm sure you will have good input and advice.

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## Pulpo

Alan 
Nice work on the brickwork. 
I would also agree that 100mm would be my choice. 
This looks rather very straight forward. 
I usually use f72 or f82 mesh meaning 7mm/ 8mm thick and 200mm square spacing, but an entire sheet would be a waste, and some 12mm reo would be a good choice. 
As for the formwork, ideally form ply is the best option for the base and would have some temporary support in the middle. 
Some timber or form ply for the sides which also needs to be supported. 
It becomes a bloody mess when part of the form falls apart at the pouring stage. 
Personally I would not use compressed fibro cement sheeting for form work [too expensive] nor would I use mdf, maybe in a pinch. 
Good luck 
Pulpo

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## pawnhead

> nor would I use mdf, maybe in a pinch.

  For a single use it's fine and it could even be reused a few times depending on how wet it's been. It's less than half the price and if you used a sheet of formply it would only be good for edge boards after cutting it up for that slab. Probably put a couple of joists through the guts, but that's no big deal.

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## Make it work

I have some 19mm yellow tongue flooring offcuts that will be big enough and I have enough reo. All that is stopping me is that I am waiting till the frame of the house is done, so the new slab won't be in the way of anything. 
I will post more pics once I have made the formwork. 
How long should I allow for the new slab to cure before I remove the temporary supports? 
Thanks to all so far.

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## bricks

> How long should I allow for the new slab to cure before I remove the temporary supports? 
> Thanks to all so far.

  
Just asking why you absolutely need to have a void under that slab? 
Surely the best option would be to put plastic water barrier ( forte-con) around the inside of the brickwork and back fill with sand? 
As for cureing time, the number that usually gets thrown around is 28 days, you could cover the top of the slab with plastic after you pour, This helps to stop the top drying out faster than the bottom and prevents cracking in the surface, ( it cures evenly).  
And make sure you insulate the concrete slab from the side of your house. 
Nice brickwork by the way.

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## Make it work

> Just asking why you absolutely need to have a void under that slab? 
> Surely the best option would be to put plastic water barrier ( forte-con) around the inside of the brickwork and back fill with sand?   Before I demolished them, the old back steps were filled with sand and bricks and ANTS. Little buggers, there were millions of them and we could never kill the nest, also with what I now know about protecting against termites, it was a good point of undetected entry for them.
> I left a void that will have an opening door to be able to inspect for nasties and spray with the annual pest control. 
> As for cureing time, the number that usually gets thrown around is 28 days, you could cover the top of the slab with plastic after you pour, This helps to stop the top drying out faster than the bottom and prevents cracking in the surface, ( it cures evenly).  
> And make sure you insulate the concrete slab from the side of your house.  I put 10mm expansion joint between the brickwork of the step and the house and plan to do the same or leave a gap between the slab and the framework, is this what you mean? 
> Nice brickwork by the way.  Thanks I'm slow but getting good at it now.

  Thanks for the info, I appreciate all the advice I can get.

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## brynk

greetings - here is something else for you to consider  :Biggrin:  
in situtations i have encountered where an un-reinforced load-bearing masonry wall is supporting a slab, if the poured slab is not prevented from keying in to the top course of the bricks by a sheet of gal / plastic or similar then cracks will appear in the brickwork from the side-wise forces applied by the concrete.  
the best method i have seen is a sheet of plastic laid up the entire outer face of the wall, over the top and a little to the inside - this will protect your very smartly done face-brickwork from getting concrete splatter all over it, as well as stopping the conc from tieing into the top of the wall itself. 
another option, if you were to use a pre-cast concrete panel then the problem may be avoided - then all you would need to do is lift it in place once it has cured for a suitable period of time, say, 28 days. another would be to put mortar along the top course & let it cure before forming & pouring your slab. 
r's brynk

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## thebuildingsurv

You would have to be pretty tough to lift a concrete panel ?

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## brynk

as they say, the devil's in the details  :Biggrin:  
(but then, someone once said god is in the details as well? does that mean god and the devil are both in the details? or can only one exist there at any given time? could it mean that god is the devil, taking on the suitable form depending on what side of the bed he got up on this morning? maybe it depends on the detail itself? if it's a mongrel detail then it could be the devil, and if it was a mongrel detail that worked out it could be god? what about the boring old run-of-the-mill details? do they also get graced by god &/ the devil? what if it was one of many of the same details? the first one works out but it was hard, then the rest of them come in just fine? and the biggest question of all, why don't they answer all of this in one of the aussie standards?) 
maybe pour it on a platform right next to the brick then slide it across on rails, jack it up, take the rails out then lower it? or cast in a lifting lug then hoist it into place with a clever arrangement of pulleys and string? archimedes would be happy to help!

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## pawnhead

> the best method i have seen is a sheet of plastic laid up the entire outer face of the wall, over the top and a little to the inside - this will protect your very smartly done face-brickwork from getting concrete splatter all over it, as well as stopping the conc from tieing into the top of the wall itself.

  You can get a specially made     slip joint for that. Two strips of galv sheet metal with grease between them.    

> as they say, the devil's in the details   (but then, someone once said god is in the details as well? does that mean god and the devil are both in the details? or can only one exist there at any given time? could it mean that god is the devil, taking on the suitable form depending on what side of the bed he got up on this morning? maybe it depends on the detail itself? if it's a mongrel detail then it could be the devil, and if it was a mongrel detail that worked out it could be god? what about the boring old run-of-the-mill details? do they also get graced by god &/ the devil? what if it was one of many of the same details? the first one works out but it was hard, then the rest of them come in just fine? and the biggest question of all, why don't they answer all of this in one of the aussie standards?)

  I see that you're cogitating about God there brynk. My mate Socrates asked this question;
"Is the pious loved by the gods because it is pious, or is it pious because it is loved by the gods?" Euthyphro dilemma 
And my old mate Al said this;
"I cannot imagine a God who rewards and punishes the objects of his creation and is but a reflection of human frailty." Albert Einstein

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## Make it work

> if the poured slab is not prevented from keying in to the top course of the bricks by a sheet of gal...

  I have put ant capping on the top course of bricks because it is up against the timber frame on 2 sides, so that serves this purpose too. 
The new door was delivered late last week and will be fitted this week. 
Then it's slab makin time... 
I will post more pics when I get to that stage. 
Thanks ALL...

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## Make it work

We have a FRAME, we have a DOOR, we have a SLAB and we have much RAIN. 
Why does it always bloody rain when I pour concrete? 
I ended up making it 170mm thick, the same height as the steps and I used 65 X 65 X 5 gal RHS on three sides to support the slab. I used 12mm rebar @ about 125 centres, welded in a grid and made a removable timber formwork which will come off early in march. 
Pics will follow when I remove the plastic covers.

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## Make it work

Pics as promised 
1. The formwork supporting the slab
2. The finished slab, waiting to be tiled (eventually)

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## sports fan

thats a solid set of stairs/ landing ! you could use that void as a bomb shelter

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## Make it work

Great idea! now where to get a shrink ray machine...

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