Summary: In your project's sketch scene, click "Draw" on the upper tool bar. Then sketch the roof by left-clicking to set a point, dragging the mouse to trace the roof line, and left-clicking again to set the end point. Right clicking on an existing point will allow you to move that point and any associated lines.
Watch the video below to see the process:
Note: We've made some improvements to the Web App's sketch scene. See the update here.
Watch the video below to see the updated web app sketch scene: