I’m sorry that you have to change out a drain pan on your rooftop unit, But your in the wrong forum for that. We only address food service here. You need to go to a HVAC forum.Â
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Like to add, with rooftops, when you have a drain pain rust through, It’s usually time for a new unit. Unless there is a environmental cause like being in a corrosive atmosphere. R22 is now gone,  With the larger rooftops they sometimes are Built to be more serviceable. Hopefully that is your case. After all, any AC unit you can walk in, or should I say climb, is nothing to sneeze at.