OK, I managed to get on the roof in INF_OUTPOST. Here's how I do it:
1. Select the XM (weapon 0).
2. Go to the far right of the map.
3. Jetpack your way up and over the spike. Land on the right side.
4. Wait for your jetpack to replenish. You will need a full clip, so reload if you must.
5. Get some runup by running/jumping. Jump off the end, ALWAYS holding LEFT.
6. Release your jetpack.
7. When your jetpack almost runs out, start firing down.
8. Once your jetpack has run out, RELEASE THE JETPACK BUTTON.
9. Once your ammo has run dry, start jetpacking again. Only use it half the time; the other half is idling.
10. Once your XM has reloaded, use the rest of the jetpack, and start firing downwards again.
11. As soon as you run out of jetpack again, RELEASE THE JETPACK BUTTON.
12. When you are about to crash into the floor, fire the jetpack. Keep it going.
13. When you reach the dome, land on it. You will now be on the roof. Enjoy!
Screenshot:
P.S. This game kicks ass.
EDIT: Sorry if it's in the wrong forum, I just saw "Mapping (ideas /) General Discussions".