Zone doesn't calculate areas properly under the roof

[Anonymous]

Hello everybody!
I am trying to calculate some room surfaces using zones.
As these rooms are situated under the roof, and partially their height is under 1m, in order to calculate the using area I have to substract that "unused" area from the total. It is more clear in the image 1: in the zones options I set the condition: the area corresponding to the height less that 1 m must be substracted from the room area 100%, respectively, if the height is less that 2 m, then it should be substracted 50%. It look logic, the only problem is, when i draw the zone nothing happens..no area is substracted..the roof is not considered...What do I do wrong?
Any help will be appreciated.
Thanx

[Anonymous]

image 2

[Erwin Edel]

You need to trim the zones with roofs. Right click when you have zone selected.

[Anonymous]

Hello and thank you for your reply!

Your advice is very useful, but I found a better solution for my case: I substracted the surfaces needed with the normal hatch. I just needed to check the hatch to be substracted from the zone, to set the amount I needed (50%, 100%) and after that the zone had to be updated.

Thank you again.
PS: sorry, my archicad is in french..