Shed base hints

A new shed is arriving in a couple of weeks. For reasons of re-use of existing materials, the 10x7 shed will be sited on a matrix of 9 3x2 paving slabs (already laid onto hardcore / dry mix) with 6in gaps infilled with gravel, weedkiller and weedproof membrane.
To keep the base raised from the damp on the ground I'm planning on using a series of tanalised bearers laid horizontally with a DPM or similar in between the bearers and the floor of the shed.
Any recommendations as to the dimensions of the bearers, or their spacing? I was thinking probably 6 lots of 7foot lengths of 3x2 (or more likely, 6 x 2.4m of 50x75mm) laid side on so that the step up isn't too large.
I've read the wiki on this (as previously suggested) but it is silent about dimensions (presumably as every question is different!)
Any thoughts?
Thanks!
Matt
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larkim wrote:

Well... 400mm spacing would be typical for floor joists in a house, so 400mm would seem to ve a safe bet. You might get away with wider. In effect 50mm wide joists at 400mm centres would give a 350mm air gap.
Your proposal gives 2' centres, around 600mm less 75mm bearer width = 525mm airgap.
Unless the shed floor is less bendy than floorboards, that sounds like it will bounce a fair bit. I'd stick another couple of bearers in (ie 8) which will give you nearer 400mm centres.
Just an uneducated guess mind!
Tim
--
Tim Watts

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On Thursday, 7 June 2012 12:17:27 UTC+1, Tim Watts wrote:

Thanks all (Tim, Dave, NT) for the hints / advice.
All sounds good & sensible. For evenness / neatness I'll probably do 9 bearers then (3 per slab) of 50mx50m timber. Probably over-kill, but its SWMBO's 40th birthday present (aren't I the romantic...) so overkill is good!
Membrane / weedkiller is "just in case", but I might leave it out; depends how deep the gravel ends up being without having to buy any more than I have lying around the garden.
Matt
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Any size would work, its all in compression and supported full length. The deeper it is, the more ventilation. 2x3 would be fine, but in the end you can do as you like re size. Re spacing I'd look at the shed's construction and see wher eit needs support. If you've got lots of offcuts, those would do nearly as well as continuous lengths. Its the sort of job I'd be tempted to use oak firewood for, sawn to size.
NT
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