In contrast built in place walls.
How to bring walls up to slanted roof.
One method of achieving the slant in your roof is to have one of the walls higher than the other.
Start by exploding the wall group component or you can move the plane inside the group component your choice.
The answer is to shim it in place and nail it to the ceiling adjacent or adjoining walls and the floor allowing the nails to run between the floor and plate.