Lead is what seals the joins in a roof, the places where two surfaces meet and water wants to get in. That means flashings where the roof meets a chimney or wall, valleys where two pitches run together, soakers between tiles and brickwork, and covering to bay roofs, dormers and parapets. It's cut and dressed by hand on site to fit the exact shape of the roof. Done properly it moves with the building instead of cracking, and it outlasts almost everything else up there.