3 4 5 rule
or, you can stand on the ground and measure the distance from centre to edge, count the bricks up and measure the length for a more accurate area. no good if its dashed though.
I find 3/4/5 is close as most of the roofs in this country are about 30degrees ish. then just measure the length.
remember and allow for the edges though, 450mm all round I use.