Life expectancy of a roof

A home inspector will not comment on the life expectancy of a specific roof. This falls outside, industry standards of practice. However, one can note life expectancies of general roofing systems.

From: The National Association of Home Builders/Study of Life Expectancy of Home Components. (view)

"The life of a roof depends on local weather conditions, proper building and design, material quality, and adequate maintenance. Slate, copper, and clay/concrete roofs have the longest life expectancy – over 50 years. Roofs made of asphalt shingles last for about 20 years while roofs made of fiber cement shingles have a life expectancy of about 25 years, and roofs made of wood shakes can be expected to last for about 30 years".

I found the chart below but was unable to establish its original source.