In 3D you have to pull your roof by a segment and select this option in the context menu :
Then input the distance...
It's the same operation than in 2D but in both you have to verify your georeferencing.
If you do not check the georeferencing, the shadow projection will be wrong.
Can you illustrate your problem if I did not give you the answer you wish ?
Christophe - FRANCE
Archicad Designer and Teacher
Archicad 15 to 23 FRA FULL
"Quality is never an accident; it's always the result of an intelligent effort " - John Ruskin