re: Can I pour my concrete patio floor directly over dirt?

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I would put some visqueen under it just in case you ever decide to enclose it.

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Visqueen isn't a bad idea, but you can't just enclose an uncovered patio without proper footings in the slab. A patio is typically a 3.5" - 4" thick slab from edge to edge. You would need footings under any load-bearing posts or walls that could handle the ceiling & roof loads.

As ConstructionAg suggested, drill some dowels and use rebar (preferably with epoxy) in the existing slab. This will help prevent any movement and uneven shifting of the new slab.