I've moved into a new-build house that already has some concrete paving slabs/flagstones laid in the back garden that I'd like to replace with slate paving. After lifting some of the existing slabs and checking out the base, it appears to be a 6 inch thick layer of (what would have been) dry mix.
The new slate slabs are quite a bit thinner than the existing slabs, although they do vary in thickness.
My question is, do I need to excavate the existing base and lay a new foundation for the slate, or could I just take up the current slabs and lay down a thin foundation over the existing base?
For clarity, the existing bed is now a solid mass of what seems to be sharp sand and cement. I'd say it's not concrete but has definitely solidified and requires a pickaxe and some fairly hefty blows to break it up.