Power shears are great at cutting through sheet metal. There is no other tool as effective and if you are installing a metal roof you will surely want these. Unfortunately they are expensive and will likely only be used for a few days. So I recommend either buying used or buying a professional brand that will work great and have a higher resell price, since you will likely be selling them.
A good option is the DEWALT DW890 18 Gauge Shears. These are the shears that I own and they work great.
If you don't want to spend so much on a tool that won't be used frequently, DeWalt also makes an attachment for an impact driver that converts it into a shear tool. I've personally never used it, but the reviews are very high and it typically retails for around only $60.
