A roofing system and roofing shingles for use therein are disclosed. The roofing system comprises a multiplicity of courses of roofing shingles having a headlap portion, a buttlap portion, a lateral leading edge, and a lateral trailing edge, wherein the headlap portion has a width that is less than a width of the buttlap portion defining a reduced-width headlap, wherein the lateral trailing edge of a shingle in a course partially overlaps and partially abuts the lateral leading edge of an adjacent previously installed shingle to form an interlocking edge between adjacent shingles in the course, and wherein the lateral edge includes a trailing edge projection having a weather edge, a rear edge, and a variable breadth that increases toward the rear edge of the trailing edge projection.