best substrate to fix stone on beam/block outside terrace

[jonnyc]

Just seen a job to quote for where there is a beam block base with an extra layer of blocks bedded in sand cement on top of beam/block base to bring levels up.
the stone will be 50mm thick sawn york .
this area is approx 35 sq mts = 7 x 5 mts.
i had thought of suggesting groundwork contractor lay minimum thickness 75mm compacted scalpings to required fall , sitting on a geotex membrane . I would then solid bed yorkstone 600 x random x 50 mm thick on top of scalpings on 50 mm sand /cement bed . this method would let moisture escape down if it gets through stone.
or had thought of unbonded screed min 75mm laid on membrane, set to fall. I would then lay ditra mat and fix stone with flex adhesive.
any views on which would be better or another idea!
there will also be a stone edge piece around perimeter approx 300 mm wide.
at present the suggestion is that this stone is bedded partly on to retaining brick wall and other part on the scalpings behind or screed with ditra .
in either case i am worried that edge stone may crack if partly fixed on to brick which is on solid base and partly on to scalps/or screed which are not bonded,
any advice appreciated.

 

[Spud]

use a dpm over the brick wall jonny it will keep the stone unbonded and stop any rising moisture from the wall and allow 65mm for your screed bed depth