Laying riven flags onto a concrete base

[bigglesskyman]

Hi, I would like to lay some riven flags onto a solid concrete base ( base is 8" thick ), It's important not to allow water under the flags as the ground can freeze for 2/3 weeks at a time due the altitude we live at (1300 Ft).
The base is level , can anyone offer any advice on what to bed the slabs on .?
Thx .

[Tony McC]

3:1 moist-mix mortar, based on a grit sand, with a plasticiser to make it workable.

[bigglesskyman]

Hi ,
Many thanks for the quick reply much appreciated.
Just one further question, along similar lines I'm constructing a small patio this time the flags will need to be seated on top of a shale surface through which water constantly passes as the garden is on a hill side (the amount of water varying due to the season etc.)
Can you offer any advice on how to go about this task .
Thank again .
Mark

[Tony McC]

Same again. You could use a permeable bedding, which would involve using a 6-3mm clean grit with 200kg OPC per m³, but unless you're sure a permeable bed is required, a standard 3:1 mortar based on grit sand will be more than adequate for a patio.