The renovation of the CenturyLink office building at 600 New Century included renovation of 4 floors of office space while they remained occupied. Work in the basement included HVAC upgrades over telephone switching equipment. Such work we refer to as Critical Environment Construction^TM and it requires very special skills. An active call center remained in operation while work was conducted on the same floor. New HVAC units were installed on the roof of the building requiring helicopter lifting of units to the roof. A completely new roof was also installed.