Results of research on the diversity of wood-rot fungi found in buildings and outdoor wooden engineering structures in Poland are presented in this article. A total of 47 species and genera of wood-rot fungi from Basidiomycota (19 brown rot fungi, 28 white rot fungi) and 1 genus from Ascomycota (1 fungus that does not cause wood decay) were found in damaged buildings. The greatest number of wood-rot fungi was reported on outdoor wooden engineering structures (33 species), followed by unoccupied residential buildings (30 species). The lowest diversity was found in occupied residential buildings (20 species). A total of 34 species and genera of fungi were found in all examined structures, out of which 17 species caused brown rot, 16 caused white rot, and 1 did not cause wood decay.