A building maintenance unit (BMU) is an automatic, remote-controlled, or mechanical device, usually suspended from the roof, which moves systematically over some surface of a structure while carrying human window washers or mechanical robots to maintain or clean the covered surfaces.A building maintenance unit (BMU) is an automatic, remote-controlled, or mechanical device, usually suspended from the roof, which moves systematically over some surface of a structure while carrying human window washers or mechanical robots to maintain or clean the covered surfaces.