what depth support for decking?

I inted to lay decking over a tatty concrete area at side of house. What is the minimum depth of joist to use? I want to keep the increase in floor height to a minimum. 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 like this....
====================== deck board [ ] 16in [ ] 16in [ ] 2 inch joists == == == slate / tile leveller /////////////////////////////////////////////////// concrete base
or is this a recipe for a quick (though short) descent?
TIA ZD
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Zipadee Doodar wrote:

Its fine. I reckon I could have got away with 2x2" I see also that the pre-cut decking kits that B&Q and Wickes sell use 70x70mm frames
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