Here’s how you can stack objects on each other so they take into account the collisions of other objects.
First, it’s got to know about this useful shortcut:
Press Y - to snap the selected object to the mouse position and also create a trace to the collision which puts things on top properly.
Another method would be:
Select an object.
Go to Mode and select Snap to static collision.
You should now be able to place objects on the surface of other collisions.
You can see the difference between both snap-modes on the video below:
