Slab for new shed

8 May 2007
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United Kingdom
Building on a previous thread - I have an existing large flat concrete slab (used to be the floor of a now demolished garage) slightly under grade of adjacent garden.
I am putting a new shed/workshop there - and am considering casting a new raised slab (100mm or so) on which the shed can sit. I'll then surround the whole thing with gravel to improve appearance.
So - some questions:

- DPM necessary for new slab? Or should I not bother, and instead use bearers on top of new slab on which the shed can sit?
- I have a load of old concrete edging - around 1m x 80mm x 40mm. Can I put these to some use in the new slab? Either sit them inside the formwork to reduce the amount of poured concrete, or alternatively use them on top of the slab as the bearers on which the shed can sit?

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So - to follow up on this original thread (that nobody responded to :)). I've decided best thing would be to cast a new slab - 75-100mm off the existing concrete.

So - a question. Should I install a DPM between the existing concrete and the slab which is yet to be cast? Or, given I'm getting a wooden shed, should I put a dpm on top of the new slab before the shed is put in place. The shed doesn't sit on a frame of any height - so I'd be concerned about damp getting in between DPM and shed (although maybe silicone around the bottom edge would sort this?)

Any comments/suggestions gratefully received.

I am a DIYer.
I did put a DPM under my concrete for shed - I am sure not needed but felt it might stop some water coming up.
Although at the time also seemed a bit pointless as it rains on the same concrete.
But my thought was that if is used the concrete as a floor in future might be better.
So my answer is - not sure.

should I put a dpm on top of the new slab before the shed is put in place
This I think is a bad idea.
You will never get it flat.
Any water or condensation will pool on the DPM under shed.
And eventually it might rot leaving a mess of plastic bits or wind might move it.

In my mind you want -
- Slight slope on concrete as a whole so rain water flows away.
- if shed smaller than concrete you want gutters to reduce water dropping from roof onto concrete
- slight drop on all edges of concrete.
- possibly a slight gap under shed so some airflow.
- Possibly use Damp Proof course under any bearing points where wood shed sits on concrete
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Thanks for this - I've ended up casting the slab this week with DPM underneath. Haha - first time trying to do something like this (it's about 5m x 4m) so it was an interesting experience when the van turned up and dumped the concrete within about 10 minutes. But I'm getting there.

I'm hoping I've sized it so there's a slight overhang from the shed - so in theory water will run off. I'll be fitting guttering etc anyway to deal with West of Scotland rain!

thanks for the input - much appreciated.


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