Architectural interior layouts can sometimes force roof framing to have odd gussets and low slope areas. Roof slopes below 2:12 (2" of decline over 12" level) cannot be shingled as per the manufacturer's specifications. These low slope details need to be roofed with a solid, single system, which is most commonly a rubberized membrane. Rubberized membranes have a lifetime of 20 - 25 years. When installing a 35 - 50 year shingle, these details will have to be re-roofed before the shingle roof needs to be replaced. Traditionally on slate roofs, these details are roofed with flat lock copper and soldered to match the longevity of slate. With new technology in shingle manufacturing, giving modern shingle an extended life,
copper, low-slope roofing is reborn as a necessity for the integrity of a true "Lifetime Shingle" roofing system.
Here is some "before and after" pictures of a roof in Media, PA with poor drainage details.