The most durable, trouble-free roof in nearly any application is a standing seamed, double locked copper roof. Unlike shingle systems, including slate, there is no way for water to enter the eaves area because of ice dams. Unlike single locked (or face nailed) metal systems, there is no way for the metal pans (sheets) to come loose or allow water to penetrate. This means that they can be effective down to a very low pitch, as low as 6 inches in 12 feet (NOTE: Meaning 6 inches of rise in 12 feet of run, a.k.a. .5/12.)
Double locked, standing seam metal roofs are reliable over pitches as low as 6 inches in 12 feet.
The downside is that installation is labor intensive and requires experienced roofing sheet metal mechanics with the ability to precisely form the metal and to solder it wherever flat seams are necessary - - at obstructions, transitions and terminations (such as chimneys, valleys and roof edges). Once properly installed, such a roof is the most durable and reliable possible. Depending on the application, such roofs should last 60 - 90 years, apart from high traffic areas, such as valleys.
The copper specified is typically 16 oz. (meaning one pound of weight per square foot), either unfinished or ‘Freedom Gray” (a Revere Copper trademark), in which the surface is coated with a solderable tin zinc alloy. With exposure, either surface ages organically and coherently, though not with complete uniformity. In the hands an experienced roofing sheet metal artisan, this material is agile over any surface, producing reliable service for a lifetime.