The first thing to check is what scale the roof is drawn in. The roof below is drawn in 1/8" scale This means that, if using a regular tape measure that 1 inch would equal 8 feet. In this drawing a 1/8" scale ruler is being used. The next thing to determine is the roof pitch see (How to determine roof pitch.) The easiest and most accurate method to measuring a roof plan is to square everything off. This means that you want to divide the roof into sections. Notice the bold dotted line this dotted line separates the main roof from the gable pop-out on the front of house. With this method only 2 calculations are needed. Measure all the hips, rakes, and valleys. Add for pitch. Measure all the eaves and top ridge and you have a complete takeoff.