I plan on installing a patio using natural (irregular) shaped flagstone with irish moss between the stones as filler. I originally planned on using 5" of compacted stone dust (or 3/4" "A" Gravel) as a base and 2" of concrete sand to lay the flagstones on.

I noticed on several other websites and posts that I should not use stone dust (or "A" gravel") as a base as it will not allow drainage. I live in Southern Ontario so the ground freezes and it goes below 0C (32F) in winter.

Any thoughts whether it is okay to use stone dust as a base, and if not, what should I be using?