From NachiWiki

Most metal roofing products consist of steel or aluminum, although some consist of copper and other metals. Steel is invariably galvanized by the application of a zinc or zinc/aluminum coating, which greatly reduces the rate of corrosion. Metal roofing is available as traditional seam and batten, tiles, shingles, and shakes. Products also come in a variety of styles and colors. Metal roofs with solid sheathing control noise from rain, hail, and bad weather just as well as any other roofing material. Metal roofing can also help eliminate ice damming at the eves. And in wildfire-prone areas, metal roofing helps protect buildings from fire should burning embers land on the roof. Metal roofing costs more than asphalt, but it typically lasts 2 to 3 times longer than asphalt or wood shingles.

Grounding

In the metal roofing trade, their national organizations have, over the years, commissioned several independent research projects and experiments regarding the topic of grounding. It was found that metal roof structures are "no more likely to be struck by lightning than non-metal roof structures" At this time, the trade organizations do not take a position for or against grounding. Grounding or not grounding is up to the individual contractor and client. There has been some effort by some code, insurance companies and safety people to force grounding but, based on the research and and historical information, the metal roofing industry has been able to send these efforts to ground (pun intended). In addition, based on the research and historic data, the industry has taken the stand that if it is required to ground metal roofs, all roofs should be grounded.

This article was originally based a post by Thomas Valosin on the NACHI message board.