In recent years there have been big advances in materials and design choices for flat roofs so weathertightness and energy performance of modern flat roofs has improved. This Good Building Guide focuses on one of the commonest examples of a new flat roofed building - an extension built onto an existing house. Here a flat roof may be the only practical option, and may also be the cheapest. This Guide advises on how to make a good job of a new felted roof extension. 6 pages.