Shrinking a polygon -- calculation logic

[RobinCris]

I am working on a project of modeling building for greenBuilding.

for the simulation, I need to put the vertices for the outside of the flat external wall, the the centeral of the wall between flats.

For this, I am doing great and I also calculated the area.

But I also need the NET floor area by reducing the walls thickness.

Not all the wall have the same thickness but I do know this

1. the Gross area
2. the thickness of each wall
3. the length of each wall
4. the vertices for the bigger polygon

How can I substract the walls from the Gross to get the NET floor area

 

[berkeman]

for the simulation, I need to put the vertices for the outside of the flat external wall, the the centeral of the wall between flats.

For this, I am doing great and I also calculated the area.

How can I substract the walls from the Gross to get the NET floor area

How about just making new walls with the external vertices where the inside vertices of the first walls are? That should give you the internal area that you want, no?

Do you have any plots of your work to help us visualize what you are doing?