Just measure the length and width of each plane on the roof, including dormers. Then, multiply length x width to get the square footage of each plane. Simply add the square footage of each of the planes together to derive the total square footage of your roof.
How do you work out the m2 of a flat roof?
How to calculate a flat roof area in square metres. Measure the length and width of the structure from the outside for accuracy. Multiply the two figures to calculate the square meterage of the flat roof. E.G. 4 metres (length) multiplied by 3 metres (width) = 12 square metres in total.