DUAL SHIFT AT WORK

Hi. I would just like to ask for a legal advice regarding my father's situation at work. Before, he works at night shift until the morning. He was transferred for a morning schedule starting at 10AM then he was promoted as a supervisor but not during the morning instead during his previous work in the evenings. So he comes to work for the morning shift where he is a regular employee and also comes to work at night as a supervisor and when his people are absent he is the one to do the job. It seems like he is assigned to a morning and evening job whereas I believe he should only be assigned to either one. I am just concerned about his health and feels like he is underpaid doing both shift. I am not familiar with the Labor Code so it would be a pleasure if you could help me out on this one. Thank you!

He should be paid overtime for the other half of the work.

So it is just fine to do dual shifts? If I'm not mistaken he caters 8 hours for the morning shift and another 8 hours for the night shift. A total of 16-hour workday. Which I think is too much. If I'll be having further questions I will raise them here again. For the meantime, thanks a lot for your help ☺