Residential roofs require lumber of a large enough dimension to fit the insulation required by building code. This requirement is based on location. Sometimes when a roof has a new course of shingles or metal put over it, thin strips of wood are attached to tack on the materials.
If building code states that insulation of R-Value 30 is needed for a residential roof, then a 2" x 8" board is required for the roof joists. In some cases, smaller boards might be allowed if the area has no snow or if foam insulation is used that can be applied in a smaller cavity.