How to measure vertical (straight-down) area of an object?

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?

You could use Sandbox > Drape

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I would probably have made sure the roof was safely grouped, then drawn rectangles at eaves level. Then, just like @mihai.s, I would have removed any internal lines to the surface created, selected the surface and noted the area from Entity Info. When done, I would delete the new shape created.

Thanks for this mihai.s. This would be a good solution although in my case the roof is not a simple shape but a series of groups – so the drape tool doesn’t work (in fact it just causes SketchUp to crash!t)

Move a copy of your complex roof with separate groups aside or copy and paste in a new file.

Then explode all groups and subgroups, if any until you have only raw geometry left in the model.

Proceed as Mihai explained, draping the model on an horizontal rectangular face large enough to contain everything.

Remove unnecessary edges from the inside of the result to only keep the external perimeter.

Use the Text Tool on the final surface. The default text that will be proposed is the surface. If the units are not to your liking, go to Model Info --> Units and choose what you need.

Another possibility is to add all the inclined surfaces of the different parts of the roof. Then, knowing the angle of this roof, you can get the horizontal projection using trigonometry. For example, if the total area is A, the horizontal projection is A * cos(angle), “cos” is the cosine function and “angle” is the angle of the roof. If you want that angle you can measure it with SketchUp or use the arctan function with the pitch, “arctan” is the arctangent. Of course, if the different parts of the roof have different slopes, you are in for a lot of calculations. In such a case, Mihai method is the fastest.

There can also be groups and use the Drape command. Or you can use an extension.

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Manual method if you are can’t make drape work.