Why do we use top curb angle for roof to shell attachment?
1. It helps to maintain the rigidity of the shell.
2. It provides a space where you weld the roof.
Why don't we directly weld the roof to the shell?
The top shell course is the thinnest (say 6 mm) while the roof could also be of the same thickness. Impossible to make a fillet weld with the roof plate resting on the shell all the while maintaining the desired leg length of the fillet (in this case 6 mm).