# Forum Home Renovation Concreting  How To Pour Footings Accurate to a mm ?

## abrogard

This is a nit-picking fussy sort of question I guess. That's probably because I'm a fussy nit-picking sort of guy.  
I like to think things out before I do them especially if I've never done it before. 
Now I've never poured a footing before that has to have the top of it at a certain height BELOW the ground. 
And the nitpicking details of just how I'm going to do that keep worrying and irritating me. 
They have to be 380mm below surface level. That will allow the precut specified length columns to stand on them at exactly the right height to enable the walls to clear the slab when it's poured. 
I get a string line stretched over the hole.  (which is 650 deep)  
I pour concrete in the hole, roughly about 200mm. 
Then when the concrete is solid enough I put a housebrick in there so's I can push it down into the wet concrete until the depth is exactly 380mm. 
I get tape and measure from string line to brick.  Too flimsy. 
I cut a piece of wood the right length and use it to measure from string line to brick. Quicker, more solid, not waving around so much. Can stand it on the brick and eyeball the string line. Only one end I need to look at. 
Still a bit floppy and vague, can't hold the string line to measure, can only eyeball it. But anyway... 
Depth insufficient. Push the brick down into the mud a bit. Measure again. 
Too deep. Try to pull the brick up a bit. Measure again.... 
Seems about right.  
Job done. 
Next hole. 
Is that really the way it has to be done? 
And because in my inexperience I put the bricks in when the concrete was too soft they slowly sink down 5 mm and one column is standing in mid air when I come to erect the thing.?? 
Or I try the brick thing too late and they won't go down? 
This nitpicking trivial question is about a trivial build, I'll add. It's only a little shed and I suppose it is possible that it can be constructed just as successfully if the columns are at heights differing by up to 20mm, say,  crowbar a bit up here and a bit up there, get the wall sheets on to stiffen it all up,  finish pouring the footings to get it all set in concrete. 
So my shed will go up no matter what. 
But my nitpicking mind wants to know: is this really the way to do a proper job? 
ab   :Smilie:

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## rrobor

If you are doing footings for a brick wall, you leave it low by the depthof the mud to set the bricks in. As its under the soil, you can have play with the depth of mud you use. Footings are sloshed in on the near enough is good enough basis.

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## cherub65

are they steel columns with base plates?

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## ScroozAdmin

Not sure what you mean with the brick tbh and measuring down off a string line is never accurate as it would need to be perfectly positioned across all holes and its unlikely to be/stay level. 
If you want to be super accurate with foundation concrete you'll need A) a self levelling rotating laser level  *or* B) an optical level (bit like a small theodolite). You set wooden stakes in the hole at various points around the site and measure off the top of the stake until you are set at the required level. You then pour your concrete to the top of the stake and your done. 
This gear is fairly pro stuff and expensive, although you can hire it, an effective DIY method is to A) set your first stake to the height you want B) use a good straight edge or spirit level to measure off to the next etc.etc. Use a piece of wood to get you out of the hole and above ground level for transferring. Its not the best way as you can easily accumulate and transfer small errors around the site, but unless your setting bolts or critical structural steel then I wouldn't get too stressy, you can lose a fair bit with packing and shimming steel or in the first few courses if its brickwork (nearly all level adjustments are done in the foundations where possible so you are visually level as you come out of the ground). Unsquare or unlevel setting out above ground is a major problem as you'll be constantly chasing the error around the rest of the job. 
A water level is also an excellent and mostly overlooked tool for levelling and worth investigating (you can make one out of a hose). Hope that helps  :2thumbsup:

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## Tools

Your columns shouldn't be sitting hard down to the footing. They should be nom 20mm up and sitting on steel packers set to the correct height, then the base plate should be grouted. Apart from the height issue with sitting the column direct on the footing, you are unlikely to get the column to have full bearing on the footing. 
Tools

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## Bedford

What we do is to pour a 200mm thick base in the hole, about 75mm lower than the post. 
When this is set, and with a big washer and nut welded to the bottom of the post, we insert a piece of allthread and adjust the height. As said above it then needs to be grouted to be fully load bearing and then just fill the rest with concrete.

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## Gaza

would pack baseplate with grout as said above, also you could have some formwork in the hole with a nail not driven all tha way home at the height the concerete should stop at. 
i would pour 50mm low no less than that.

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## abrogard

I'd like to thank everyone for responding to me with all that helpful stuff. Thanks to you all. 
I'm really glad I asked. Felt silly doing it but it was well worth while as it turns out. 
I'll tell you what's going on: It is a small  shed, as I said, a 9m x 6m prefab from a company called Stratco.  
Steel. All steel. Ten steel columns, that C section stuff, corrugate roof, colourbond sidings. 
All the brick business is part of their instructions for building the footings. I was totally confused by this 'brick' business and it took me quite a while to get out of them what it was all about.  
What it was all about turned out to be as best I can make out a method of getting the height, or depth rather, accurate. You are apparently intended to pour round about 200mm of concrete first and that won't be spot on for depth - it'd be hard to do that. Not like pouring above ground to formwork where you screed it off at the exact height. 
To get it exact you lay a brick on the wet concrete and push it down into it to exactly the depth that suits you. 
Sorry it takes me all those words to explain it. 
The columns are steel and are 500mm longer than the sides and are meant to have 20mm space above the slab between wall and slab. Gives you 480 mm to play with. 
100mm for the slab gives you 380mm. 
That was my problem. In my inexperience. How to pour a footing that would be precisely 380mm below ground level.  Using a string line. They say that. Use a string line. 
The answer I've got from these posts is that I deliberately make it low and pack it up with steel.  I can forget the brick trick. I don't need it. 
The columns have no plate on the bottom. They are not meant for bolting to a slab. They are meant for reaching down into the ground to the footing. 
Only thing I'm unsure of now is the grout you all mention.  What is the composition of this? Apparently it is better for the job than just pouring concrete around. You say grout the bottom of the columns and then pour concrete. 
I assume I'd need to let the grout cure before I pour the finish to the footing then. So how long would that take? 
You all make very clear that I shouldn't expect to have my footings ready to perfectly fit the depth required.  Makes it clear that what I'm talking about is a problem faced by everyone and well known and handled in various ways but they virtually all use a 'make the footing low and pack up' method. 
Beauty. Thanks. 
Water levelling. I had great faith in this where I have no faith in a string line and no faith in carrying a level around a site via a bubble level, either. 
Imagine how startled I am to find that it's not as easy as I'd thought.  You can easily get false levels from a water (hose type) level. 
I've got a clear plastic hose with clear tap water in it and it's hard to see what is going on sometimes and what I've found, I think, is that on the rough ground of the site parts of the hose get an airlock in there. And you get two columns of water, one at each side. And somehow they find their own different levels. 
Sounds like a far-fetched story but I can vouch that I screwed around for numerous attempts and got numerous different readings and eventually gave it up and accepted the string level as a working level for the time being.  At least it went all around the site and came back to within 10mm of where it left. 
Before I set it in concrete (ha ha) I do intend to water level it again, slowly and carefully. Either that or hire a dumpy level.  And I might try colouring the water so's I can see at a glance that I've got a filled hose all the way without airlocks. I like the idea of water levelling. Makes me feel like an ancient Egyptian laying out the pyramids...... 
thanks again, guys.. 
 ab   :Smilie:

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## rrobor

OK I think I got how you do your shed as its similar to how my shed was done. Do not pour  footings in,  Build the shell of your shed and fit it into the footing holes, then near the footing holes wedge up the frame to the correct height, once you are happy as to level and square pour your footings. My advice is it should be bolted down.  The pressure on that in a gale could be quite something,  I can see no sense in having a footing without a tie to it

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## abrogard

I'm inclined to agree about tying down to the footings, but it is funny no one else seems to mention it. 
Stratco don't call for any such thing and their specs and construction instructions are pretty detailed (detailed enough to have all that confusing 'brick' thing in them). 
But on rethinking I remember trying to get steel posts out of the ground that were set in concrete. It sure grips them. There's no way the steel post would come up out of the concrete. 
I've bought a dumpy level. How I'm going to explain to the wife I don't know.  I thought the hell with it.  
I did another water levelling and discovered the way to go is use a hose with main pressure to fill the water pipe, drive out all bubbles/airlocks. 
So I've string levelled and water levelled and when the dumpy arrives I'll dumpy level. Perhaps I'll get flat site. 
I figure the dumpy will perhaps be useful in levelling the surface of the site.  I've had to build it up 200mm at one end and I've used 10 tonnes of road base.  It looks pretty flat but it isn't.  
The dumpy can quickly identify where the lumps are. 
I think I should start another thread about how to level a surface.  Another one of my dumb questions. The answer seems pretty obvious - use some sort of screed board technique - but what seems obvious to me often turns out to be quite different in practice.  
So the answer to my question, 'how to pour footing accurate to a mm?' is 'Don't. Pour deliberately low and pack up.' 
And you need to grout the packing before finally pouring.   
I learned a bit about grout googling around and it seems to  be just cement and water but I couldn't find the proportions anywhere and instead I found there's grout you can buy in a powder. I suppose that's the stuff to use. I'll check it out at the hardware store.     :Smilie:

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## ScroozAdmin

dumpy level is definately the way to go, you'll be using for everything, shelves will never have been as straight  :Biggrin:  
just to clarify this post, as I think there may be some confusion, you only use grout if you have baseplates attached to your columns, the grout is used to provide a full bearing surface for the underside of the plate as the top surface of poured concrete is never perfectly flat and in 90% of the cases shimming or packing will have been used under the plates to level the structure.  
If (as I suspect you are) simply plonking a column end into a hole and mass filling it then theres no need for any grouting, hence there's no ties mentioned either. Shout up if you want some info on site levelling with a dumpy.  :2thumbsup:

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## abrogard

Ah, okay, I was working off 'Bedford' and 'Tools' posts. Yes, I'm just sticking 'C' section columns down into the hole.  They both talk about grouting what appear to be column ends stuck down a hole. 
 My whole question has been about how I get the footing in the hole to  be at exactly the right height/depth (whichever way you look at it). 
 The answer I'm reading into all this is that I deliberately make them lower than they should be and then pack it up exactly with metal packing of some sort either before putting the columns in or when I do put them in. 
 The answer suggested by the providers of the pre-cut shed is to put a housebrick on top of the wet concrete and push it down into the concrete to get to the exact height.  
 That method didn't satisfy me because I couldn't see how I could easily and accurately measure down from a stringline to this brick. 
 But if I've got dumpy maybe I should go back to that idea.  
 I'm confused. Making too much of it all I guess. Just do it. 
 I certainly would like some info on site levelling with a dumpy if I can get some.  What's 'shout up' mean? Something special like PM'ing or just mention it like I just did?  
 ab    :Smilie:

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## ScroozAdmin

rrobor's idea is probably easiest, build the frame up, make sure its square and level and then concrete it up. Or get some baseplates welded on your C sections and go with the shimming and grouting plan. 
Using a dumpy is very simple, you'll probably get a load of instructions with it but generally the idea is to set it on its tripod at a point where you can see the whole site (you don't want to move it mid-measure) and adjust it until its perfectly level (using the circular spirit bubble guide). You now have a level plane across the whole site. 
So you get your measuring staff and place it on whatever you want to measure, let's say a wooden foundation stake, and read off the measurement e.g. 1870mm. Now go to stake 2 and do the same, if it were 1970mm then you know stake 2 is 100mm lower than 1, to get your foundation level you would pull up stake 2 100mm and you are perfectly level. 
So let's say you want to set out footings at 300mm thick The first stake you choose is your reference point or datum and for foundations should be at the highest point of the site. You knock the stake in so it has 300mm exposed and take a measurement (mark the stake as D) say the measurement was 2150mm, you now go to your other stakes and push in or pull up until they all read 2150mm, some will need 300mm concrete and others 450, 390, 330 etc etc but the top surface will be perfectly level. 
Pretty easy really once you get going. If you decide to build the frame up first then simply put the base of the staff on the base of the column and measure off, shim the frame until you have the same reading on all columns and your ready to pour footings for the levellest shed in town  :Biggrin:  
Hope that helps, surprised how cheap these are now, a few years ago they'd be a good couple of grand a pop, I guess lasers have killed the market somewhat.

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## abrogard

rrobor's idea?  Like don't pour anything at all in the holes? Have the frame floating in the air in the holes, chocked up somewhere on the girts or something, and then pour the whole footing? 
 What about I pour about 200mm and let it set and try to chock up each footing to the totally accurate height and then stand the frame on that - chocking the frame on the girts or whatever wherever it is maybe necessary - and then finalise the pour? 
 Or with my super dooper dumpy should I have total confidence and aim to pour the footings to the exact required height before I stand the frame in the holes. Aiming to have the frame immediately stand exactly where it should be? 
 Practically I can't exactly see how you can pour a slab in a hole in the ground to an exact measurement.  You'd have to be putting the final dollops of concrete in a cupful at a time or something, wouldn't you? 
 And doesn't the concrete kinda 'slump' down a bit during the setting process? 
Looks to me like I should deliberately pour short and then chock up or do the 'brick into wet concrete' thing as a way of easily getting the precise height. 
 I just wonder how easy/hard it'll be to judge when the concrete is hard enough to support the brick. 
 Then if they bugger me up by sending columns too long I can easily pound up the brick and go down more without having to chop bits off the column - housebricks being easy to smash.   
 Thanks for the clues about levelling.  That's all good about the foundations. But what about levelling across the block?  
 Currently I'm planning to take some quick rough levels to isolate high spots and I'll dig them out, shovel the tops off them. 
 Then I'll have a string line down each side of the block and I'll have a sort of 'screed board'  6 metres wide which I'll guide with the string lines and slowly walk it down the block levelling the stuff before it as I go using a hoe. 
 Seems very fussy but with only a 100mm slab planned a tiny bump of height of only  2inches - 50mm - will  thin my slab to 50mm.  Too thin. Drop something heavy on that and it'll crack it I reckon. 
 Does that seem reasonable? Or should I make it a thicker slab? Or is there an easier way of levelling?  
 Seems to me I'm very probably making a big fuss about nothing here, as I usually do before doing anything i've not done before.  thanks, everyone, for your patience with me. 
 ab   :Smilie:

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## Bedford

> Then if they bugger me up by sending columns too long I can easily pound up the brick and go down more without having to chop bits off the column - housebricks being easy to smash.

  I'd be cutting the column if it was too long, smashing a brick out could give you a variance of 3 inches that you then have to consider. :Smilie:

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## Pulse

Don't make a small job bigger than it needs to be... analysis paralysis can be the downfall of a DIY. 
Pour 200mm- 300mm in the bottom of each hole, use your dumpy to measure the height below your finished floor level. Cut the ends of the stumps to suit the measurements such that the shed is level. 
Level roughly by digging out about 30mm below where the concrete is to start. Then add 30mm of sand and level and compact it. Plastic sheet, reo, then pour your nice even, level slab and fill in the rest of the holes as part of that pour. 
Cheers
Pulse

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## Bedford

:2thumbsup:  :2thumbsup:  :Smilie:

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## abrogard

Bedford:  yep, I really meant 'easy to work' - I could angle grind the top off it in seconds.  
But, mate, your post got me in because it sounds like you do this regularly and there's no substitute for that kind of experience. 
I just didn't quite understand the 'washer and nut'.  I understand the nut, I can visualize that, and then threaded rod going through it. But the washer? Where's it? 
And the grouting. Some doubt has recently cast on whether I'd need to grout like that or not. Would you insist on grouting - i.e. with proprietary grout - or do you just mean use some sloppy concrete down there? 
Myself, my light little shed, I guess I could just stand the columns right on the footing.  Or would you suggest it'd be well worth my while to go to the trouble of this welding job?   
Pulse:  I love it 'analysis paralysis' - that would be exactly what I get, exactly. 
Thanks for the clues.  Cut the stumps is easier than fussy measuring down there, okay. 
Level the ground with a layer of sand. Suddenly I understand something. The sand will level easy, won't it, but my road base won't.   Sand or, I guess, cracker dust would do.  
 So it looks like I'm finally set to go, having got all the clues. 
 And virtually any other bloke would have got the job done by now.  
 But that's it: 'analysis paralysis', that's what I've got. 
 We couldn't get ABC 1 or 2 on our tv and I've spent weeks worrying and fussing about it. Googled here and there. Talked to this and that person. Got quotes for antenna replacements, estimates of what antenna would be required - all kinds of stuff. 
 Today I just went up the ladder to the roof and twisted the antenna around a bit so's it pointed the same way as the neighbour's.  Came down, switched on the tv, told it to search for channels, and found ABC 1 and 2 and a few more besides.   
 Job took 10 minutes, max.  
 Look out the back window of the bus, that's me, running behind, trying to catch up with not a hope in hell......... 
 regards, 
 ab   :Smilie:

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## Bedford

> I just didn't quite understand the 'washer and nut'.  I understand the nut, I can visualize that, and then threaded rod going through it. But the washer? Where's it?

   The washer is to stop the nut disapearing up the post ie the post sits on the washer which sits on the nut, and by screwing the bolt /allthread you can raise / lower the post.  

> And the grouting. Some doubt has recently cast on whether I'd need to grout like that or not. Would you insist on grouting - i.e. with proprietary grout - or do you just mean use some sloppy concrete down there?

  Just make sure you have some concrete filling the gap below the post and above your original pad, this is to make sure the weight is transfered directly to the pad and not just sitting on the bolt. The bolt is just a way of adjusting the height, the overall weight is transfered to the pad when you fill the gap between the post and the pad. 
This is how we would do it, but if your using a C section post it's just as easy to pack under the post with washers (to get correct height) then just fill the hole with concrete, tampening down around the post.   

> Myself, my light little shed, I guess I could just stand the columns right on the footing.  Or would you suggest it'd be well worth my while to go to the trouble of this welding job?

   Just stand, pack with washers, make sure your vertical (may need a temporary prop) chuck in the concrete in and slap the roof on......it's gunna be winter soon! :Biggrin:

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## abrogard

Got it. 
 Thanks all.   :2thumbsup:

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## Bedford

I'm working on a pic for you, may take a while.....better with concrete than computors :Wink:

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## ScroozAdmin

> rrobor's idea?  Like don't pour anything at all in the holes? Have the frame floating in the air in the holes, chocked up somewhere on the girts or something, and then pour the whole footing?

  yes that's what I mean and pour so the concrete's up the post by a foot or so as this will act as a tie down also. None of the ideas on here are technically wrong so its a case of picking the method you prefer and going for it.  :2thumbsup:    

> Thanks for the clues about levelling.  That's all good about the foundations. But what about levelling across the block?

  Not sure how you'll be pouring your slab but a normal method of levelling one is to level the formwork so you can screed off it, running a straight edge across it before you pour will show you any high spots in the base.

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## abrogard

What I'm trying to do it follow the Stratco instructions exactly. Both because it might be necessary to make things fit and for the sake of training myself to do that.  So if i seem a bit fussy here and there that might be the reason. 
 About the levelling: I won't be digging it out to where I want it. I've had to build up the block because it fell away about 250mm at the far end. So I put a few tonnes of road base on there. And that was a couple of weeks ago and it has compacted down well. 
 But it makes it a bit of a chore levelling it out.  I've put a plank across and levelled it with spirit level and used a hoe to loosen up the surface and shovel the excess away. 
 Generally what I had was too much at the shallow end by up to 40mm. That'd make a big mess of a 100mm slab. 
 Now I seem to have it fairly level I'll get some cracker dust in or sand - what's the best? any opinions? - and lay about 30mm on top for doing some fine levelling. 
 I like pictures. I like that idea. So I'll include a pic here of the shed site as it looks today.  The wheelbarrow gives an idea of size. But it's just a 6 x 9 really. Nearly the smallest real shed you can have. If I can learn to do this properly I can back myself to do something bigger.  
 What about plastic under the concrete?  I always do that under a slab.  I expect it is just as useful under (and around the sides of) footings? 
 I think of the daftest things to worry myself with. Like packing up with washers.  Sounds good until I think - what washers? I don't have any washers.  And 20mm - 40mm packed up? that'd take a lot of washers.  
 So I'm thinking of pouring the footing to the exact height if I can.  And that makes me nervous so then I think again of Stratco's housebrick.  If it won't sink down into the concrete it'd be a good way of getting a height accurate to a millimetre, wouldn't it? 
 Haven't got any housebricks. Might mix up some concrete and make some little slabs about 200mm x 200mm x 70mm, say, and use them. 
 This getting a height business seems to complicated I can't believe it. 
 Even with my dumpy (which arrived the other day and I've used to check the levels generally) I'm going to have to take a level off a peg in the ground somewhere, first. Then I'll have to put something solid over a footing hole - like maybe a wooden frame like a soccer goal maybe , or a door frame,  so's I got something solid to measure from.  Measuring from the dirty bottom of the hole with dirt falling in at any time wouldn't be real good. 
 So put a frame over every hole.  Then I can use the level to find the height of that frame above the slab level.  Then figure out how far down the hole to find the top of the footing I'm pouring. Then get in the hole and maybe stick a piece of wood in the side of the hole to make the height or drive a peg down into it, the top of which will mark the height. 
 Then pour to that height - a finicky business with concrete. Or deliberately pour low and plan to  pack up with washers (?).   Or wait for the concrete to harden a bit and put something like a housebrick on top of it and push it down into the concrete to get the right height. 
 Cripes. What a kerfuffle just for a few foundation holes for a tiny backyard shed. 
 So the best scheme really does seem to be build the shed frame floating in the air above the footings. Packed up on the bottom wall girts (or the wall itself?)  And then pour the footings to lock everything into place. 
 But not very satisfactory that, really. I don't like something temporarily 'floating'.  I like things locked down solid as we go. 
 Look at that. Every time I post here I write another book.  How would you like to have me on site with you? Drive you mad, wouldn't I? 
 Imagine what it is like having a mind like mine... nothing's ever simple.... 
 Ah well, back to it.... 
 Thanks everyone, again. 
 regards, 
 ab   :Smilie:

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## autogenous

Sorry I didnt read the whole thread... _
Footings are sloshed in on the near enough is good enough basis._ 
No no no no no  :Biggrin:  
You could do what the Egyptians did by digging the trench and filling it with water then placing height pegs but theres better. 
Dig your trench then with a long clear plastic hose filled with water 3 inches from either end establish an initial datum height then use the two ends from the datum peg bang in the next and so on.  Water finds its own level. 
This makes it look more complex than it is.  You simply want the clear hose only where the water is at the height of the datum peg and bang the new peg into the height of the water at the other end of the hose. 
You need 2 things 
1. Approx 5 metres of clear tubing
2. Water filled into the tube till it fills to approx 5 inches either end  Link:    Making and Using a Water Level

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## autogenous

Its a little different than what I thought 
 					Originally Posted by *abrogard*   _rrobor's idea? Like don't pour anything at all in the holes? Have the frame floating in the air in the holes, chocked up somewhere on the girts or something, and then pour the whole footing?
------------------_ 
     yes that's what I mean and pour so the concrete's up the post by a foot or so as this will act as a tie down also. None of the ideas on here are technically wrong so its a case of picking the method you prefer and going for it.  :2thumbsup: 
------------------ 
You can build the frame with the posts on bricks in the footing hole.  Tweak the floor height by jacking the frame and putting packers such as cement fibre sheet between the bricks and underside of posts (4-6mm) to level. 
Then fill the holes once the floor structure is level and square with concrete. 20:20:80 or what ever is specified by engineers detail etc if it is needed. 
And what ever they said  :2thumbsup:

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## abrogard

I'm beginning to get a grasp on how I'm going to do it. The shed arrives Wednesday week so I want to get the footings poured this week. 
 Got no specs for the concrete that I recall. I guess 20:20:80 would be fine. 
 But the footings I'll be mixing and pouring myself. They're just 10 different column footings - not like trenchwork footings for a wall.  That's part of the hassle with it. 
 What sort of concrete I make at home I don't know. I think I might do a home slump test and measure my quantities carefully and see what I've got. 
 I use 1:5, cement to premix.  And there's, by volume, 1 water added to that. 
 What it is physically, on the ground, is five 4 litre icecream plastic containers to 1 of cement and 1 of water.  
 And my mixer takes two such loads.  And that makes one barrow load.  
 And it takes millions of barrow loads to do anything. 
 Roughly speaking I think my ancient old mixer was designed to make 1 cubic foot of concrete. Could that be right? 
  It is a tiny amount.  But more than you want to mix by hand all day.  
  So you can see I've looked into it a bit but there's a long way to go before I thoroughly understand what's going one, what I'm doing. 
  There's volumes specified.
  Then there's weights specified.
  Then there's dry ingredients against wet concrete.
  Then there's allowance for wastage.
  There's bag volumes for cement. 
 And all of this mixed up between metric and imperial. 
 So I'm still finding my way. 
 It's very interesting though. I like it.  
 WATER LEVELLING: 
 Yes, that's great.  That's the trick I never thought of: use a reservoir! 
 And a thinner tube.  
 I was using a 13mm (half inch) ID tube and no reservoir.  You walk around the site and the water spills out of the other end.  The tube drops into a footings hole and climbs over a mound of dirt and you've got airlocks in it. Which I learned to get rid of by forcing water through it. 
 And you've really got to get down to it. Tape the tube to an upright and lie down in the dirt to make a mark exactly where the water is. 
 Move somewhere and - like the story said (the link you provided) - water spills and the levels are different.  So your second set of levels are not the same as the first and you've got to go around measuring with a tape. 
 I finished up with levels that today I found suspect. I used my dumpy today to check and all the levelling with hoe and shovel I've been doing was wasted. I've taken too much off.  Because of relying on the string line, which sagged. 
 And, maybe, because the water levels were not right. 
 Or maybe my dumpy is not right.  To set it up you have to check that the bubble remains in the centre when you turn it through 180 degrees. Well, it doesn't, not exactly. It stays inside the black circle but not in the exact centre of it.  I decided to let that go, lest I get involved in hours of screwing around trying to reset it.  But maybe it it too far out.  Bit hard to  believe, though. You'd have to have a fair old angular difference from the horizontal to make 20mm over a 6metre distance - measured from 20 metres away, wouldn't you? 
Well I make it an angle of 2minutes 17.51seconds for an error of 20mm over 30 metres.  But I've no way of knowing if the kind of error represented by a bubble wandering around within the black inner circle. 
So I remain interested in water levelling.  I truly agree, it is the most accurate.  When it is done properly. 
Thanks to you I've got the clues on how to do it properly: use a reservoir, use thin tube.  I'll do just that. 
 ( Just reminded me of shooting. Full bore shooters shoot to 1 minute of angle and that is said to be represented by 1 inch at 100 yards.  So if a human shooter can achieve that kind of control I'd expect a machine to be heaps better. ) 
 God, I talk. I hope I'm not wasting everyone's time and patience. I guess you're all old enough to look after yourselves. 
 ab   :Smilie:     
 ab   :Smilie:

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## cherub65

> Or maybe my dumpy is not right. To set it up you have to check that the bubble remains in the centre when you turn it through 180 degrees. Well, it doesn't, not exactly. It stays inside the black circle but not in the exact centre of it.

  Bubble just has to stay in circle not center, Use method below to check your level.
can get false readings if staff or tape is not held vertical also. 
1. Drive two pegs, A & B, into the ground six metres or more apart 
2. Set Dumpy up at C 
3. Shoot a level at  A and B 
4. Relocate Dumpy to D and shoot a level at A and B  
D -------------- A -------------- C ------------------ B

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## abrogard

Thanks for that. I discovered something googling around and looking at the tiny little 'user manual' that came with my dumpy.  I discovered what 'automatic' means. I had wondered about that from the beginning. 
 I've owned a theodolite before but never a dumpy. It means 'automatically levelling', doesn't it?  Theodolites don't have that. Well mine didn't. 
 And there's a little switch under the thing that is the 'compensator lock'. 
 And the 'compensator' is the automatic levelling part of it.  And when that's working I only need the bubble within the circle because the compensator can find dead level from there. 
 Well I'll be......   Whatever will they think of next. 
 So that's great. 
 I see it has a compensator range plus or minus 15 seconds. And a bubble sensitivity of only 8 seconds. 
 That deals with the problem I had when I googled somewhere and read that dumpy level bubbles were only accurate to 6 seconds. I thought, cripes, I'd be better off with a rifle. 
 So today I'll go out and check that the compensator is switched on and things are operating properly. 
 It says the 'Standard deviation for 1km double run levelling' is 1.0mm. Well I'm not sure what that means but it could mean that it is capable of levelling out to 1km and then back with an error of only 1.0mm.  Wouldn't that be wonderful? 
 I'll use your method to check. 
 thanks again, 
 regards, 
 ab   :Smilie:

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## abrogard

Well a bit of an update for anyone new to the game who might read this thread at some time. What I've learned so far: 
 I've learned it is bloody hard to accurately level and/or measure footing heights down in a hole. 
 That 'hole' bit is important. There's quite a difference working in a hole in the ground and working in a footings trench. I'm working in holes in the ground. 
 I have used my dumpy level. 
 I found the easiest and best way seemed to  be to pound a stake down into the hole - in a corner somewhere so's it wouldn't affect the integrity of the specified concrete footing slab - down to the depth required. 
 I got those heights with the dumpy and they were all supposed to be the same. 
 But: difficulties in measuring.  The stakes don't go down perfectly straight. So you get up to about five mil difference on different parts of the head of the stake. 
 Then pour the concrete to that height. 
 But: difficulties pouring and levelling down in that hole. With 20mm aggregate.  And you shouldn't (I'm supposing, because it says 'roughen the surface) smooth it out.  So you've got this lumpy surface from natural roughness and then perhaps variations in height because I haven't flattened it out properly, not being able to properly screed down there in a 350mm hole. 
 So at the end of that exercise I've got 10 holes with concrete in them and I find the depths vary by up to 25mm.  Why?  Bad measuring by me. Bad pounding in of the stakes. Bad levelling of the concrete down there. 
 So right there I learned the most important thing, like I'd already been told a number of times earlier in this thread:  Pour deliberately low !  At least 20mm, probably up to 50mm. 
 And then I've run around them all again with the dumpy, checking those readings and what I get is up to a 5mm variation on any one hole between the reading the first time I did it and the reading the second time. 
 Why?  My errors in reading the dumpy. Variations in where the staff is placed in the hole. 
 So I learn from that there's a kinda limit to the accuracy I can aim for while I'm on an unfinished concrete surface. 
 So now I'm going to go around and try to pack up each hole to where it should be, using maybe small sheets of fibro as I think someone suggested, perhaps stick them in solid with a bit of mortar or if I  can find some fine gravel chips make up a concrete mix. 
 I'd hope to be able to get them all the same within 5mm. 
 And my only little worry now - me and my endless bloody nit-picking neurotic worries about trivial detail - is just how much variation is tolerated in a shed wall? Any? Or none at all?   
 Or does it just not matter because the wall, bolted, pop rivetted and screwed together on the ground, fastened securely to its columns, will be a totally rigid thing and will sit straight and level along its length anyway? 
 And the wall at this side and the wall at that side can tolerate maybe up to 10mm difference in height? 
 and the back wall and the front wall with roller doors can accommodate this variation in height? 
 The shed should arrive on Wednesday and soon thereafter I'll discover the answer to all these little questions. 
 Main thing I've learned so far? Dumpy level is the way to go without a doubt.  I could do another site preparation ten times as quick knowing what I know now.  Belt the holes down quicktime, happily going low. Plant stakes in them - well I think I'd use reo rod fair in the centre of the hole next time - deliberately low 20mm.  Level that off the reo rod.  Measure. Adjust the differences. 
 Yes, my dumpy was very affordable and I'm delighted in it. Once I realised there was that 'automatic' feature, that self-levelling thing. My old theodolite didn't have that. 
 It is wonderful. I put it on the staff and take a reading and then turn a footscrew and see the reading raise or lower because of it - AND THEN see it come back to where it was !!  Like magic.  Gives you enormous confidence in the validity of your level, your line of sight.  
 Which confidence for me was utterly lacking in my reading of line levels and spirit levels and water levels. 
 But so simple, really. Why hasn't it always been done? Or has it and I just didn't know? 
 Done with prisms, apparently.  I don't know how. But I can see that one could do it with a simple pendulum, dampened, couldn't you? On gymbals. 
'T' shaped thing. Sight across the bar at the top, weight hangs down the upright.  
 But mine is an el-cheapo. Came inadequately packed (from China, of course) and the staff (included in the price) had the innermost (destined to be the uppermost piece when extended) piece bashed over at the top where it naturally protruded a couple of inches. Ah well, that's no great problem. 
 And the 'compensator', they call it, the actual self levelling thing, has a button underneath that allows locking it for travelling and unlocking it for use. And it doesn't work. I can't lock it up again. Ah well, no matter.  Maybe it would work if I just pushed it hard enough. Or maybe you are meant to hold the thing a certain way, at a certain angle, to get the prisms to hang out of the way and then push it. I'll try contact the manufacturers and ask them. 
 But I'm happy with it, delighted with it.  32 x magnification.  I set it up some 20metres away from the site, outside the storage shed, and read the staff from there and can read easily - easily - to 1mm.  In a fraction of a second. Try that with any kind of bubble.  
  ab   :Smilie:  
p.s.  The image is the right way up, too - another thing my old theodo didn't have.

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## rrobor

You know you are getting yourself tied in a knot over a simple job. It doesnt matter if you are a hair out.
 The posts are on soil and that moves with moisture. The guy who concretes the floor aint gonna use a dumpy, 
he is going to have a few pegs and do it to the pegs and by eye. My 3 bay garage went against a fence.
 They built the frame dug where the holes were to be, tinned the wall against the fence and then walked 
and dropped the frame into the holes. One guy started the mixer the other chalked up the garage and squared it. 
Took him about half an hour with a bit of string across the corners and a big spirit level and as many bricks and 
bits of wood he could find.
I have a footing 5 feet deep for a brick front, that over the space of 20 years has developed a slight bow of 
probably 2 or 3 mm so dont expect your footings to remain perfect.

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## abrogard

Yep, rrobor, I dig what you're saying. It's true, I know.  
 Get this thing built the best, most accurate way I can and then I'll know. Then I'll be in a position to set off on a job like this from scratch and do it all off the cuff, no problems. 
 But first I gotta find my way.   :Smilie:

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## Bedford

Hi abrogard, it's probably too late but I think I've got the pic for you.

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## abrogard

Yes, okay mate, thanks, I get the idea. 
  Yes, it is up. But not finished.  Needs roof, roller doors and floorslab but, yes, it is up on its footings finally.  I learned a lot. Would do it better, quicker and easier next time. Not entirely happy with this time but it'll do the job. 
 Just to end the thread I'll quote what I see as the answer to my question - how to pour footings accurate to a mm ? 
  1. Don't.  Pour them deliberately low.   
      An easy way to do that is to dig the hole and then  put a peg down in the hole reading off it with a dumpy, driving it down (or pulling it back up) to get the height you want. Pour to the height of that peg. 
     Now read where you've poured the concrete to.  Easier if you've poured flat and smooth. 
  2. Now find some suitable packing and pack up the bottom of the holes (the surface of the concrete) to the actual height you want.  
  I used bits of fibro.  You can use bolts and washers and whatever.  
  3. Build on it. 
  4. Now measure how your building is standing. If necessary do your final adjustments.  We left them out, near enough was good enough. We had 10mm in height difference across the site, across the shed (shame, shame, red face). 
  5. Pour your final footing.   
  Now it is set in concrete and a fair sort of a job to change it.   :Smilie:  
  I'll attach a pic.   
  Thanks for everyone for their help. And happy building.   :Smilie:

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