# Forum Home Renovation Sub Flooring  Depth of stumps

## pulpfriction

Hi all, can anyone advise if there is a simple calculation or rule on how to determine what depths I will need to go into the ground with concrete stumps for decking/veranda we are planning on building. 
Many thanks in advance.....
W

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## SilentButDeadly

Depends on your soil type and uplift force/load (basically wind classification).  Common depth numbers are 600mm to 1200mm and anything from 300 to 600mm round.  Ask a local structural engineer...

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## sundancewfs

I'll second what SilentButDeadly said, but add, its always deeper than you want to dig  :Frown:

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## UteMad

and more of them than you want to dig and they always use more concrete than you want to give'm.. oops i sound like a whinger  heheheheee 
utemad

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## sundancewfs

To be a little more specific...
My planned deck wil need 34 stumps. the holes for them will have to be a minimum of 2 metres deep. (As specified by the building surveyor)
We have type "P" soil. 
On the other hand the neighbour across the street is digging his stump holes and hitting bedrock at about 200mm
So as you can see, it really does vary from place to place. It could even vary over the area of the deck.

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## dmx

I had mine, 28 post holes, 300 x 300 x 500 mm depth for 8000 x 4000 mm deck area (not including steps).
been approved and inspected by local council recently.

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## Cooky

Soil type P is an unclassified soil type. In simple terms you might say there's disturbed soil on the site, such as fill. Often most of the site is fine, sometimes the site is shocking. 
I would definately not go with what the surveyor is saying, cos he's thinking that the worst case scenario requires 2 metres down. Get the soil report - which you obviously have cos you know the soil classification, and look at the written recommendations and the bore logs - both should indicate roughly how deep the bores went until they got ok soil. A good mob will also provide a bearing pressure for that ok soil. This pressure is what the engineer uses if one is required to design the footings. 
There isn't a formula as to how deep you go, but rather you use the soil guys and their bore logs to tell you what soil they want you to reach (e.g. natural sandy clay, orange in colour, stiff) and roughly how deep you have to go to get it. Then each hole will slightly vary in getting to this clay.  
If you have shocking soil, I would use bigger veranda beams to span longer, and use less posts and footings. Might save a few bob. 
Cheers
Cooky

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## GrahamC

If you really don't want to pay for a geotech report than you best bet is to keep digging until you hit firm ground.

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