When you create a gable roof, a false surface such as siding is created beneath the pitched ends of the gable roof. You can get rid of this false surface by raking your exterior walls so that they extend to the roof’s pitch.
To extend gable end walls to the roof:
Select the wall beneath the gable end. You can select additional gable end walls using Shift+click.
Right-click and select Properties, or select Edit > Modify Elements > Properties.
In the Walls dialog, select the Behavior tab.
To rake the external side of the wall, check the Exterior Wall Tops to Roof check box. If you want to offset the wall a specific distance from the roof surface, enter a value in the Offset below roof surface edit box.
To rake the internal side of the wall, check the Interior Wall Tops to Roof check box. If you want to offset the wall a specific distance from the roof surface, enter a value in the Offset below roof surface edit box.
Click OK. The walls are adjusted.