Comparison floor area and roof area

I am pretty new in this GIS topic and I would really appreciate if someone can help me with this problem.

I am doing a research for university that shall show the available roof area for installing photovoltaic moduls in comparison to the respective floor area in QGIS.

I have two vector layers so far. One contains the floor area of the houses, the second one the appropriate roof areas. All I want is the amount of roof area in percent (or at least in m²) of their respective floor area. Important is, that only the roof areas that sit in the floor areas shall be involved. As you can see in the picture below, some of the roof areas are bigger then the floor areas. I thought this problem would be very easy to solve, even for a newbie like me, but i could not handle this yet.

So you can better imagine the problem, a screenshot of how it looks like: