At the expense of possibly being named in a lawsuit should you fall, I will refrain from 'saying' what I think you should do. ABSOLUTELY, whenever working up on the roof, I CAN say that you should take all the appropriate safety precautions.
Now, what does that mean??? It means you could buy/rent/borrow the necessary equipment, or, you could get your own 'ideas' based on what others do. Here are a couple of links.