I was asked again today how to do a takeoff of a 2d roof drawing so that the measurement you get is accurate in the 3D roof.
It's a simple mathematical table, and you can download and print it out here.
Update: Chris from Southern Store Fixtures has requested a clarification.
Now the blue area (bottom drawing - click to enlarge) shows a flat square footage of 981.8 square feet. But this is in 2 dimensions. Since we know the slope of the roof (6:12), we look it up on the table and find 1.12. Multiply 981.8 * 1.12 to get 1099.6 square feet, which is the true 3D area of this roof.
Interestingly, it doesn't matter how many ways the roof slopes, or how complex it is - if all planes slope at the same pitch, you can apply the same factor to all of them. If the roof has multiple different pitches, just measure each part separately and apply the appropriate factor. (And one other note - roofs are usually measured in squares - 1 square is 100 square feet, so divide by 100!) Hope that helps!