Snapping on roof section

[Anonymous]

Hi,

I have the following problem: I have a tilted roof, and each level's floorplan section plane cuts through it, I've set it up so that it cuts at 0 meter above the current floor, which allows me to see how big my real floor area will be. Now here's the problem: I would like to snap to this roof section, Archicad sometimes does allow me to snap to it, sometimes doesn't...

Here you can see a visual representation of my problem: