Block paving over patio foundation

[Tmparry]

Hi Guys just wondering if the foundation of patio slabs would go any way to saving some work on new block paving foundations? Would it need digging out or would a layer of sharp sand suffice?

[Highworth paving]

Assuming this is a patio and slabs are laid on a proper sub base you could lay on top. The problem is your finished block lever needs to be 150mm below your damp course on the house. If you use 30mm Sharp Sand and a 50mm Block your original patio needs to be 230mm down from damp to start with. Hope this helps
Regards
Craig

[seanandruby]

All is not lost ...

read this

[Tony McC]

It's always something of a gamble re-using an existing driveway sub-base - best to do a couple of trial holes to check depth, compaction and aggregate type - but patio sub-bases are notorious for being what we might call "short-changed".

Too many patio sub-bases are litle more than a blinding of chippings or cruher run material. I have so-called professionals telling me they never have any problems using just 50mm of Type 1 - how the hell do you get 50mm thickness of an aggregate that has a a upper size of 37.5mm, anyway?

I'd put in a couple of trial holes to see what we have. As long as there's at least 100mm of well-compacted Type 1 or Type 2 aggregate, you might be able to follow Sean's suggestions, but remember, because block paving has no bound bedding or jointing, it *must* have an adequate sub-base, or it "will* fail.

If in doubt: dig it out!