# Forum Home Renovation Sub Flooring  How to stop concrete stumps from sinking?

## matto 1

Hi all, 
Just a question im just about to start our extension out the back which is weatherboard on bearers and joists. Im doing it as an owner builder and getting the old man to help me, we are going to put in all the concrete stumps ourselves,but the old man is freaking out that they will sink. I have been told that if you have the right concrete slurry you can just push the stumps in the hole then wiggle them around to get your desired height, and they should just stay there. Just wondering if anyone has any better ideas so that i can calm the old man down, i realise that if there out a couple a mm i can always pack under the bearer but would prefer to get them all the same level if i can. Your thoughts would be appreciated. 
Cheers Matto

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## dazzler

What does the building plan show?

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## matto 1

Shows everything, but doesnt tell me how to put the stumps in.

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## dazzler

> Shows everything, but doesnt tell me how to put the stumps in.

  I would dig the holes to the specs on the plan, put some blue metal in the hole for the post to sit on, brace to make them level and the correct height.  Call the council inspector to inspect (this would normally be an inspection point), then pour the concrete and when its set you can remove the braces and everything will be square and the correct height. 
You would normally need to pack a few anyway as the the bearers are rarely dead straight.

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## Tools

> I would dig the holes to the specs on the plan, put some blue metal in the hole for the post to sit on, brace to make them level and the correct height.  Call the council inspector to inspect (this would normally be an inspection point), then pour the concrete and when its set you can remove the braces and everything will be square and the correct height. 
> You would normally need to pack a few anyway as the the bearers are rarely dead straight.

  Stumps need 150mm of concrete under them for bearing.  
Tools

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## kombiman

You want to brace is as it sets.  Yes it should just sit there but if it moves you are screwed.  I have put plenty of posts in a thick mix when fencing and steel house stumps I screw to the bearer first; easier as it in replacing rather than building new. 
When the builder built my extension on 6 mtr steel stumps he screwed them to the laminated beam and held it and in place with props and bracing then poured the concrete.   
Most concrete stumps (I've only played with high ones in Qld) are really really really heavy so once it has gone too far down you have it sitting too low, in a thick mix pondering how the [insert choice of statement here] you are going to get it to the correct height.  Packing a new build is a not the right answer  :Biggrin:   http://www.renovateforum.com/showpos...63&postcount=4 
That stump weighed 300kg when dumped.

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## president_ltd

in response to the original question: 
yes, if your mix isn't sloppy, you can place a concrete stump in the slurry and it will stay in place.  as to how long the concrete stump can be before the process no longer works - not sure - but i did used this for up to ~1200mm concrete stumps which i guess weighed in at around 40-45kg each. 
for our reno, we did them a couple of different ways.  for new stumps - as above. 
but for replacing some exsting rotten (redwood) stumps with concrete ones, the bearer around where the stump was supported using jacks, the existing beared removed and hole for it dug out a bit, a new concrete stump was 'hung' from the bearer then the cement slurry put in the hole - and finally once set enough, jacks removed. 
i'd imagine you could do either way - or a combination of both.  if you lay out the new bearer and support it in enough places so it isn't sagging, no reason why you couldn't use the 'hang over the holes' method. 
make sure you know what you're doing re requirements for building inspector inspections etc.

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## Planned LScape

Yes as said you will need a conc pad underneath to form a stable base for the stumps. 
Don't fill the stump holes up all the way with concrete, as the concrete stump itself can break at ground level if it expands at different rates to the concrete in the hole. We usually put conc in the bottom, and push it in enough to stabilise the stump. Don't use rapid set either, this can cause breakage too

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## gypsychippy

Hey Matto, 
As already mentioned it is dependent on the length of the proposed concrete columns. "president ltd"'s suggestion of 1200mm max. is a good rule of thumb. Generally you will need a min. 600m of column cast into your bored pier although you will need a min. of 900mm continuous bored pier depth for tie down (resistance to uplift). Therefore based on this I would generally bore 1200mm deep holes and pour a 300mm thick pad set to height within the pier using an automatic laser level. The following day simply place the columns into the pier & onto to base (either by hand / block & tackle / crane), Temp. fix timber braces to columns using G-cramps & fix to braces timber pegs in ground to position columns (obviously working to string lines) prior to concrete pour. Yes you end up using 300mm extra per column & there is 2 concrete pours, but it wont be much fun if the concrete goes off half way through the process of placement. Carefull planning & preparation is the secret to any successful building project (well many parts of life) & never pour more than you can handle, it's always quicker & economical to get it right the first time. Hope this helps. Peace.

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