Extending Existing Concrete Shed Base

23 May 2006
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United Kingdom
I have an existing 6’ x 4’ garden shed sitting on a concrete base. I want to replace the existing shed with a 8’ x 6’ shed and existing concrete base isn’t big enough. I don’t want to dig up and lay a new larger base, so what’s going to be the easiest way to extend the base. My thinking was just to extend it using slabs around the edge of the existing base at the same level. Does this seem reasonable or am I likely to get problems with the slabs sinking at a different rate from the concrete base?
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Shouldn't really be a problem since the shed is a light structure. Just make sure yo dig deep enough and include about a 100mm layer of compacted hardcore base, and 25mm or so of compacted and levelled drymix mortar (3 or 4 parts sharp sand to 1 cement) for the slabs to be laid on.

Alternatively, you could put shuttering around the edge, lay the hardcore as suggested and pour wet concrete in - may prove quicker and cheaper than paving slabs. I tend to advise on putting the shed on tanalised timbers (2-by-1 or preferably 2-by-2), over which a floor can be laid when the shed is constructed - keeps everything dry!
I did this to my shed base last year.

I needed to increase it from an 8 x 6 ft to a 10 x 8 ft. I placed shuttering arount two sides, brushed a mix of PVA/water onto the existing edge for the new concrete to bond, and poured my concrete in.

The newly laid concrete is actually much harder and stronger than the original base it butts up to, which was begining to crumble along those edges anyway.

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