"Zipadee Doodar" wrote in message
...
I inted to lay decking over a tatty concrete area at side of house. What
is
the minimum depth of joist to use?

If you are laying on concrete all you need is sufficient depth to allow air
circulation and water drainage.

Would I get away with something like 2 x 2 inch joists
laid on thin roof slates to raise and level them over the concrete

The slates will almost certainly fail. Use pads of mortar - it's very easy
to mix in a wheelbarrow.