Sorry for my lack of clarity, I was talking about both types of roofs, since the classic Somerville triple decker has a flat roof, and the classic Philadelphia

Sorry for my lack of clarity, I was talking about both types of roofs, since the classic Somerville triple decker has a flat roof, and the classic Philadelphia a pitched roof with shingles. I was wondering about the former since I've never seen one as such, aside from some one story commercial ones when I lived in a higher adjacent apartment. And thanks for the comments, you've saved me some research when it comes time to replace my roof.