As mentioned, consider snow load. We had 120" snow here west of Boston a few years ago. Very little snow until January, then 5 consecutive weekends of around 2 feet. Not fun outside in deep snow with a roof rake knocking snow off the roof.
We had steel put on in 2011. I think it was. That or 2010. Over top of one layer of shingles that were put on the roof when the house was built in 1996. Should have been done a couple years earlier - '30 year' shingles didn't last half that. They just strapped over the shingles with 1x3 then steel on that. No issues since. Seems to be a big difference of opinion here, re. needing strapping or not. Lots just lay the steel right on top of the shingles - and that's working pretty closely with the manufacturer, we have two of them in the province and the guys I know doing it and selling it have direct contact with them.