Method for laying a sub-roofing sheet on pitched roofs with adjacent roof surfaces, wherein at least one longitudinal edge-side adhesive zone is provided on the underside and on the upper side of the sub-roofing sheet, wherein the underside facing inwards in the laid state and the upper side facing the weather side are of identical construction, wherein the sub-roofing sheet has a watervapor-permeable and diffusion-open functional layer and wherein the sub-roofing sheet is of diffusion-open construction, wherein the sub-roofing membrane has an S d value of less than or equal to 0.5 m, wherein the sub-roofing sheet has an underlayer comprising the underside and an upper layer comprising the upper side, wherein the underlayer is formed symmetrically to the upper layer, wherein the underlayer and the upper layer have a carrier layer, namely an identically formed non-woven layer, wherein the functional layer is formed as a microporous film and/or as a monolithic membrane layer, wherein the underside has at least substantially the same weight per unit area as the upper side, and wherein the weight per unit area of the upper layer formed as a non-woven layer and the underlayer formed as identical to the upper layer is between 40 and 100 g/m 2, wherein a first sheet row is cut off obliquely at an adjacent roof surface, the cutoff part of the first sheet row forming an adjacent sheet row, the adjacent sheet row being turned over so that the upper side of the first sheet row forms the underside of the adjacent sheet row, the adjacent sheet row being connected indirectly and/or directly to the first sheet row and/or being arranged on the adjacent roof surface. wherein at least one longitudinal edge-side adhesive zone is provided on the underside and on the upper side of the sub-roofing sheet, wherein the underside facing inwards in the laid state and the upper side facing the weather side are of identical construction, wherein the sub-roofing sheet has a watervapor-permeable and diffusion-open functional layer and wherein the sub-roofing sheet is of diffusion-open construction, wherein the sub-roofing membrane has an S d value of less than or equal to 0.5 m, wherein the sub-roofing sheet has an underlayer comprising the underside and an upper layer comprising the upper side, wherein the underlayer is formed symmetrically to the upper layer, wherein the underlayer and the upper layer have a carrier layer, namely an identically formed non-woven layer, wherein the functional layer is formed as a microporous film and/or as a monolithic membrane layer, wherein the underside has at least substantially the same weight per unit area as the upper side, and wherein the weight per unit area of the upper layer formed as a non-woven layer and the underlayer formed as identical to the upper layer is between 40 and 100 g/m 2, wherein a first sheet row is cut off obliquely at an adjacent roof surface, the cutoff part of the first sheet row forming an adjacent sheet row, the adjacent sheet row being turned over so that the upper side of the first sheet row forms the underside of the adjacent sheet row, the adjacent sheet row being connected indirectly and/or directly to the first sheet row and/or being arranged on the adjacent roof surface.