The application of shingles; or the procedure of applying shingles or laying parallel felts so that one longitudinal edge of each felt overlaps and the other longitudinal edge of the adjacent shingle or felts underlaps. Felts are commonly shingled from a downslope portion of the roof to the upslope portion of the roof area so that runoff water flows over rather than against each felt lap. Felts are also applied in shingle fashion on relatively low slopes.