Framing around ducting - alberta building code

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I'm dropping a ceiling in a bathroom to cover some ductwork, and am not sure how much space I need to keep between the duct and my framing.

Anyone in Alberta know what's required?

Also, I'll be running the boards on the flat to keep as much height as I can. My bathroom is 5' wide. How long can 2x4's on the flat run before they need to be supported in the middle? Supports are not an option, as the ducting prevents me from gettin up anything into the joists.

Thanks in advance.