The floor is significantly heavier gauge steel than the roof. Probably at least 14ga. where the roof might be 18 or so. Always best to try and match whatever you are repairing so using a sheetmetal gauge is where I would begin.
Also check to see if the existing metal is galvanized. Some is, most isn't from what I have seen. You do NOT want to be welding galvanized without proper prep and/or gear. It can screw you up in just a few breaths. All the zinc needs to be ground back from the weld zone or wear a proper mask.
For anyone not familiar with this problem...