I have prepared a model of a house, and would like to know the *vertical* roof area in order to calculate its ability to collect rainwater. When calculating a building’s ability to collect rain, we assume that all rain falls vertically. Therefore, the pitch of the roof doesn’t matter.

This is not the same as measuring the *actual* area of the roof, since the roof is sloped. I simply want to know the area as it appears in parallel projection, i.e. straight down. The building isn’t a simple rectangle so trying to calculate this by hand is quite a lengthy process of summing together all the separate pieces of the roof, which are at different heights.

Is there a quicker way to achieve this? One idea that springs to mind is that I could create a large rectangle (bigger than the whole house) and then use the house as a ‘cookie cutter’ to cut out a piece from the rectangle. The area removed from the rectangle would represent the area of the roof area. But how to do this? Or is there another way I haven’t thought of?