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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?

Edit

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.

  • When you say dry mix is this compacted gravel? – Ed Beal Nov 21 '16 at 1:50
  • When you say 'dry mix' do you mean concrete mix with sand and gravel laid dry but which hardened into a solid mass over time? Surely if this is the case you would not have to remove it, but just put a bed of exterior mortar of the correct depth and set the new slates into that. – Jim Stewart Nov 21 '16 at 1:53

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