To make a gambrel roof, three separate 'sub-roofs' need to be created to make up to overall roof. These are two outer single-faced roofs, and one gable roof form in the middle (with lower slope angle than the outer two sub-roofs).
The sidewall (the walls attaching to the gable side) needs to be split in such a manner that each part attaches separately to each 'sub-roof'.
A. For each outer sub-roof,
Select the Roof icon from the Architecture ribbon. Once in the Roof Sketcher mode, either select the Pick tool and click on a room, to create a roof whose base takes the room's form; or, use the line tools to manually draw the roof base's outline.
Under Roof Edge Settings;
For ONLY the edge subtending a slope, define the desired Slope Angle (degrees) in the corresponding field and set the Roof Edge Visibility to 'Visible'.
For the other edges , define the Slope Angle (degrees) to 90, and set the Roof Edge Visibility to 'Invisible'.
Click on Apply to apply changes.
B. For the middle sub-roof,
Select the Roof icon from the Architecture ribbon. Once in the Roof Sketcher mode, either select the Pick tool and click on a room, to create a roof whose base takes the room's form; or, use the line tools to manually draw the roof base's outline.
Under Roof Edge Settings;
For the opposite edges subtending a slope, define the desired Slope Angle (degrees) in the corresponding field and set the Roof Edge Visibility to 'Visible'.
For the other two edges , define the Slope Angle (degrees) to 90, and set the Roof Edge Visibility to 'Invisible'.
Click on Apply to apply changes.
Once done, adjust the elevation of the roofs in such a way that they align with each other, by selecting the roof and putting in the desired value under the Elevation field in the left panel.
Then, to attach the walls to the roof, select Attach Wall tool in the Architecture ribbon, select the desired walls and then the corresponding sub-roof to which you wish to attach them.