Let's say you have a projection (top view) of the roof, and the roof is supposed to make at an angle. In this case, you will help elementary mathematics and more specifically geometry. Divide the entire area of the roof into individual geometric shapes. In most cases the roof "break" into triangles or trapezoids, at least – on rectangles and parallelepipeds. Calculate square of each element separately by appropriate formulas.
Since the roof is uneven and the slope of each shape relative to the earth, every of these figures multiplied by the cosine of the angle of inclination. After that, all the obtained results are added. The result is the required value the area of the roof.
Remember – calculate the area of the roof should not be at the edges of the existing structure, but to the eaves. In addition, whatever the roofing material, it is usually put lapped to prevent water from leaking past the roofing. That is, trimming, intersection, and increasing the area of the material here is unavoidable. Therefore, in material it is best to take a small margin of 10-20%. Thus, in the case of more or less simple form of the roof , you can do 10%, but if there are many junctions and corners, it is better to increase reserves to 15-20%.