After measuring thousands of roof plans, over a period of 20 years ranging from apartments, tract houses custom homes here are some good tips.

   To accurately measure a roof plan, one of first things to verify is the scale the plan is drawn with. If you only have a roof plan and no elevation or foundation plans, then you will have no way to verify the scale. You will have to obtain additional plans or proceed with the scale given and make a note, such as "Scale not verified". To verify scale, look at the foundation plan and note of the size of the larger areas of the foundation, then compare those to roof plan using the scale given.

   The 2 most popular scales used are 1/8" & 1/4". I have seen roof plans labeled 1/4" = 1 ft, but in fact were drawn with 3/16". To measure the roof, start off by squaring every section of the roof off. Make light lines on the plan with a pencil. Count how many different squares or areas you have. Write this number down. Measure each rectangle. 

The roof below will have 5 different calculations. Compare this your calculations to the number you wrote down when you started. Always look at the elevation plans, sometimes they will show parts that are left off the roof plan, such as patios or bay windows. It is best to have a estimate sheet that is pre-labeled with headings such as Hips, Rakes or Gables, Valleys, Eaves, and  Ridge, that way you can tell at glance that nothing has been skipped. See sample estimate sheet here.