If I understand you right you cant.the weld is always harder than the soft steel of roof. If its cracking on the edges of the weld ,weld the cracks.
the heat settings on that a little welder may be a little low. a cold weld will also crack like that..Then again so would to much heat...would you like me to explain how I set the heat and wire speed just right? I'm pretty sure those 140's use both gas or flux wire,your using gas ,right?