Mice in a roof space? What have you got up there? A hay loft? Or is it in between floors, so the ceiling is below the floor upstairs?
Only ask, because I'm wondering how you know it's mice. In this farm, the mice aren't great ones for climbing; the roof is a daft place to nest because there's no food nearby, so they stay near the ground, scuttling under the floor boards. Even rats, which do climb, seldom or never make it to the upstairs celing and roof. Bats, on the other hand, get in and make a heck of a noise scratching about upstairs.
Poison is the easiest answer (not for bats, obviously which are protected anyway!). We poison the rats and don't get invasions of flies in the area, and get no smell.