TPO and metal roofing are two options frequently used as commercial flat roof solutions. They are both very good but they are not equal in any way, and it is important to know their particularities before making a choice.
Thermoplastic polyolefin roofing consists of a membrane that can last about 20 years, being not just affordable but also very cost effective. It is easy to apply and maintain, as well as resistant to ozone, chemical and ultraviolet exposure. TPO membrane is also considered a green roofing option, just like metal roofing.
On the other hand, metal roofing is a significantly more expensive option, but it will have a minimum service life of 50 years. It is also better when it comes to keeping water away. However, improper design, especially in a snowy area, can lead to problems that may end up with a roof collapse. Besides, a metal roof may require gutters, which represent another maintenance item that can be avoided if you opt for a TPO roof.
According to a skilled Castle Rock commercial roofing contractor I know, if you retrofit your commercial roof to metal, you will need an engineer and a carpenter for consultation and construction. For installing a TPO membrane, all you need is an experienced TPO roofing contractor.
So, ultimately, your choice will depend on your budget and on the longevity, you desire for your roof.