Flashings, valleys, gutter valleys and other complements are equally important as they seal all the interstices and ensure proper water drainage.
Roofing materials vary enormously in performance and durability.
Copper is the highest performing material. Very resistant to climate changes, it does not corrode, does not require maintenance and has an unlimited lifetime.
Stainless steel has excellent corrosion resistance and does not require maintenance. It has a low elasticity co-efficient; its colour does not fade over time; and it has a long lifetime.
Lead is malleable and very resistant to corrosion. Its surface oxidises, protecting it and ensuring its longevity but it is toxic and doesn't last forewer.
Achieving a watertight seal between the elements requires skilled installation. Solder is required, sealant is forbidden