SketchUp is a popular Building Information Modelling (BIM) software used by architects, engineers, and designers for designing and visualizing buildings. However, the software lacks a built-in tool for designing complex roof structures. This paper presents the development of a roof design plugin for SketchUp using Ruby scripting. The plugin, called "RoofDesigner", allows users to create complex roof designs with ease and accuracy. The plugin was verified using a case study of a residential building, and the results show that it can significantly improve the efficiency and accuracy of roof design in SketchUp.