A method of manufacturing a building product includes, forming a slurry of gypsum free cementitious material into successive fiber cement layers, and forming an interlayer in between the fiber cement layers, wherein the interlayer comprises a bond promoter of substantially gypsum free, acrylic based resin and water, and dewatering and curing the slurry while the bond promoter is between the fiber cement layers, such that the bond promoter bonds the fiber cement layers to one another with an increased interlaminar bond strength.