"john west++++" wrote:

Hello john

jw| hi, we have a small wooden garden shed on a cast concrete
jw| base. the shed is pretty rotten so we want to replace with a
jw| larger breeze block construction shed ( that's if breeze
jw| block is the cheapest kind of shed to build, that's not
jw| wood. [guess breeze blocks will work out cheaper than
jw| the pre fabricated kind of concrete sheds you can buy
jw| anyway?])

Concrete blocks, not breeze blocks. Might seem a minor quibble, but it
could save you some embarrassment down the builders yard.

Might be worth hunting around for faced blocks. Much more expensive
though, and most peeps are happy with rendering over the ugly.

Make for great sheds though.

jw| my question is; since i am going to need a larger concrete
jw| base, should i smash up the existing one or is there a
jw| satisfactory way to 'key in' a piece of new larger concrete
jw| base onto to the existing one. many thanks for any advice.

You can key in, but you need to do the math as to whether the existing
is deep enough for whatever height you're planning on, in which case
you might not need any further concrete. If not, then trench footings
are the way forward, either alongside or break out a trench and go
that way.

However... Damp proofing. Concrete sheds don't breathe like wood so
damp is more of a problem. It would pay, I think, to rip out all
existing concrete and start afresh, putting down a DPM when you lay
the new pad/raft. Adequate ventilation also needed (grilles, vents,
etc)

--
Simon Avery, Dartmoor, UK
uk.d-i-y FAQ: http://www.diyfaq.org.uk/