Interesting the comments on Dons use of multiple roof covering materials.
I offer up this example of one of my builds where I used not 2 but a total of 5 different coverings : tar paper, raised seam metal roofing, corregated iron roofing, individually cut and applied wood shingle roofing.
I forgot to list the striped green and white real cloth awnings above the shipping doors on the rear wall.
The build was my O scale scratch blowup from HO too O scale of fine scale miniatures #295. I called it Q C TWEEDLE AND WAS MY BUILD #436. Here is a link to that build on this site in scratch building. Many of the comments i received not on this site were about the use of the multiple roof coverings and how much they liked the variety.
You do not have to have a monocromatic appearing roof system. The person I idolised the most , George Sellios of Fine Scale Miniatures did some incredibal roof treatments with patching and actual burned thru roofs exposing the rafters underneathe. I set real fire to one roof for the ultimate moelling realness and you could smell the fire burned area too.
Tom Yorke also did some great roof treatments on his buildings. He is generally now mostly known as the MASTER OF DECAY and has built a couple of buildings totally wrecked and wonderfull , visually.