The person who designs a roof system has to consider two main areas. They are the field of a roof and flashings. A field is the open part of a roof. Flashings are the edges around anything that interrupt the field. Flashings are installed around walls and penetrations. Penetrations are things such as vents, pipes, drains, skylights and anything else on a roof (see picture). The field and flashing materials may be different. They may be applied in different ways.

The field and flashings are two areas of a roof. A low-slope roof also has four parts. The four parts are the roof deck, vapor retarder, insulation and roof membrane. Together, these four parts are called an assembly.