Roofs are essentially just 'shapes' with their dimensions added. To create a roof, select the 'Roof' button in your Scope of Damage.
In this screen, you would then select the shape you need to add. In this guide, we'll choose 'Trapezoid'.
You'll then enter in the dimensions of the shape you've selected.
Once the shape has been created, you can move onto the next shape. When all shapes of your roof have been entered you can add in the waste factors at the bottom of your roof entry screen.
Once complete you can scope your roof just like you would any area.