Thanks Jacob for your response.
I was able to find the exact required area (slab underside area) following the logic in the image here.
I could apply my script for other rooms too. But a problem occurs when there are more than 4 beams intersecting each other creating an enclosed space between them.
Here while finding the area of a particular surface (in the shape of # sign), the Surface.ByPerimeterPoints node gives a warning because the wires making the edges intersect at some point (perhaps due to the jump between 27 - 28). Geometry Working Range seems good.
Is there a way I could work it out as I need this to work for many rooms?
Once again, I was trying to find the underside area, whatever is visible from below, for rooms without any ceilings.
Any other workflow would be appreciated. Thanks again for bearing with my bad image-editing skills.