Is fibreglass an appropriate material for a slanting roof?

Fibreglass shingles are made for slopes of 4 inches rise /12 inches of run, and higher,if your roof is steep enough Yes this is what you want,also where are you,what part of the country are you,some sections are better off with a slightly different type of shingle,example:asphalt base with copper flecks to stop mold/mildew forming in high moisture areas.

Your local lumber yard or roofing supply house would have yhe necessary info.

Hope this helps,
Ken