Whatever your choice of roofing surface, you will probably need flashing. Flashing is a crucial part of all exterior work, both on the roof and siding. Flashing is metal (aluminum or copper, occasionally lead) or plastic film. It is applied in strips to areas where dissimilar materials adjoin, such as the intersection of the masonry chimney and the roofing shingles, where the siding abuts the window frames, and so on. Good flashing work is essential to keeping a structure watertight, as the most likely place for leakage to occur is where different materials meet.
Whatever the choice of roof materials, the coursing should be regular to the eye and parallel to roof edges. From one course to the next, the joints should be staggered to prevent leakage. Beware of a contractor who relies on tar for joints. Except with certain roofs where a membrane is used, tar is a lazy expedient that should not be used for a new roofing surface.
For most roofing, a material like building felt (a.k.a. tar paper) is rolled on before the shingles are nailed in place. With cedar shakes, however, lengths of furring strips (sometimes called “cedar breathers”) will be laid across the roof in order to allow the roof to breathe. In snowy areas, a membrane called a snow and ice shield may also be laid.