How do I stop my attic stud walls projecting through the collared ceiling?

When you draw attic stud walls and there is a collared ceiling, they may project through the collared ceiling up to the roof. 

If you intend to build some or all of your stud walls to the roof line this is ok, but if you want any of your stud walls to stop at the height of the collared ceiling then you need to toggle this option in the walls Properties. This will alter the cost of the wall and also the 3D image.

The video below shows how this works.