# Forum Home Renovation Sub Flooring  concrete the stumps?

## mako

Gday. Building a new deck, under 1m off ground in fairly sandy soil. Should i concrete the stumps in?..50/50 with soil or straight cement or just pack the dirt back in? and how deep should they be in the ground. the deck will be 4.5 metres by 3.5 metres. Cheers.

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## Bear

:confused: As was explained to me recently by a restumper. The only time they concrete in posts is if they use concrete posts. Apparantly, they are finding now with the wooden stumps that are cocreted in that moisture is becoming a big problem. I think also has to do with shrinkage and the fact that concrete is pourous. Hope that helps 
Regards 
Bear

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## Tools

Timber stumps are put in with a sole plate under them,and then just back filled and rammed with soil. 
Tools

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## Shaun04

And if youre going to concrete them in the timber has to be a H1 durability timber and also CCA treated.

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## Tools

Shaun,I assume you mean H4.H1 is not suitable for outside use,let alone in the ground.If you are in Victoria,CCA treated timber is only permitted for use as garden edging,power poles and such.It is no longer allowed to be used for decking. 
Tools

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## Shaun04

Arrr yeah H4 it is, got my numbers mixed up.

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## Bleedin Thumb

If I were putting in timber posts in a sandy soil I would pour a concrete pad in the base of the hole & let it set ( or a sole plate as tools suggests)then place my post in and backfill. Just in case the soil has a low bearing capacity. If your deck is free standing it doesnt hurt to backfill with rapid set concrete placed in dry and let the soils own moisture set it thus avoiding the post swelling and then shrinking.
Ive been constructing with treated pine for over 20 years and almost always backfill with concrete (wet) and never had a problem even though I'm aware of the theory may be because I always finish my concete below soil level so soil drops into the gap as it appears?? who knows>

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## ausdesign

Just to clarify things.
The sole plate (required) size, whether it be timber or concrete is determined from the area & load the stump is supporting.
Concrete stumps 99.99% of the time have concrete poured around them because no one wants to be trimming concrete stumps to the right height or digging a hole to the exact mm in depth to suit the stump.
Concrete backfill is used when subfloor bracing requirements need to be met otherwise compacted soil is used.
Water penetrating between a timber stump & concrete backfill should not be an issue if the concrete is domed above ground level & the subfloor area is graded to prevent ponding of water - all good building practice. 
Mako if the deck area is free standing then you may need to look at bracing the stumps but if, as in most cases, it is extended from the house then backfilling with soil should be fine.

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## Bleedin Thumb

> Just to clarify things. 
> Water penetrating between a timber stump & concrete backfill should not be an issue if the concrete is domed above ground level & the subfloor area is graded to prevent ponding of water - all good building practice. 
> .

  I was always under the impression that the issue here was that the timber stump absorbed water from the wet concrete and swells. Eventually it dries out and shrinks (after the concrete has set) causing a gap between the stump and the surrounding concrete. How will this be alleviated by leaving the concrete proud of the ground?

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## woodsprite

Local builder always uses a decent sole plate and backfills with soil mixed with about 4 shovels full of cement, then lets it all 'set' with time. Seems to work OK and is dead easy.
Jeff

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