Help with base for garden shed

15 Jun 2014
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United Kingdom
My old garden shed was rotten and beyond repair. It was built on a slab base, but was never level. The slabs were laid on that red sand/grit and had sunk/moved. The old shed was only sitting on the slabs at the periphery. Anyway, now to dry and do a better job.
Have have lifted the slabs and cleared all that red sand/grit. The soil itself is so soft, it is like wet clay. This is the lowest part of the garden, so all water seems to drain toward this point. Unfortunately, relocating the shed is not an option.
So how can I get a decent base for my new shed. I was thinking of maybe 50mm Type 1 compacted with a small wacker plate. Then 50mm of recycled glass paving sand from B&Q, and then 25mm sand/cement dry mix or slab crete. Then laying the slabs on top of that.
The shed is a 10' x 8' ft so no massive load on it. Any help/advice will be greatly appreciated.
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Why not use a concrete slab?
If it's a 10' X 8' wooden shed, I'd have thought 50mm Type 1, tamped to refusal, then 75mm concrete slab.
Cast you slab so that any rain from the sides of the shed can't run under the sides of the shed.
The most usual reason for sheds getting rotten is that they have not underfloor ventilation, so the bottom timbers rot. It is best to stand them off the deck with either tanalised timbers or bricks. Now your deck, if the earth is really like a soft clay there are only two ways of coping with this, one is to build on a raft of concrete which which spread the weight evenly over the whole area or to dig down to the sub soil. For the raft I would use a 4" thick concrete, for the sub soil I would sink in 18" blocks with mortar between them to get the height just inboard of your sheds base in 9 places, then sit your 6" X 2" timbers on them (3 X 10'), then sit the shed on these bearers. With this method you will need some form of weed control, I would use the slabs for this purpose.
With your proposal:- the 50 mm of glass sand and the 25mm of mortar and the slabs really do not add very much to the strength or load distribution .

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