The lightweight steel substructure for RIB-ROOF metal roofing systems creates roof slopes on existing supporting structures and modular building frames. It provides a possible foundation for refurbishment projects, modular construction, and functional roof assemblies. By forming precise roof slopes on existing structures and prefabricated modules, the system creates clear conditions for drainage, load transfer, and installation. Low dead weight, high load-bearing capacity, and a well-designed system geometry support efficient on-site installation.