Building a block shed on a concrete slab, is this okay?

28 Jun 2005
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United Kingdom
Hi I have a 10 x 8 feet concrete slab built on very tough to dig soil. It has 4.5" of compacted Mot1 base and 4.5" of concrete with a rebar grid in it. I want to build a block wall shed (not sure what blocks I would need to use) laid around the edge of the slab up to a height of about 7 feet and then a timber felt roof on top.

I want to clad the walls with timber to match adjacent wood shed so that it looks like a timber shed on the outside but is block built inside.

Can I use the existing base as it is?

What blocks would/could I use outside for lightness etc?
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Sounds like the base will be more than adequate and you could use Thermalite blocks which are much lighter than concrete or breeze and the bonus is they can be cut with a handsaw.
I’m not a builder so I could be talking out of my arris but would you need to lay a damp proof membrane down first, build the blocks onto that and then give it a 2” screed finish? Maybe depends on what you intend to use it for I suppose…..
What tends to happen with this design is damp getting in around the base. If you were casting the slab/base for this purpose then you could trowel the edge with a curve while setting to help stop this happening. I would start with a couple of courses of engineering bricks with some waterproofer in the cement.
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Helps to know the intended use. Garden shed? Miscellaneous storage including papers (has to stay bone dry)? Firewood or liquid fuel storage?
Insulation needed?
Rodent proofing?
Security level (valuables, vs. open bags of potting soil)?
A vapor barrier seems a good idea, especially if there isn't one under the slab.

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