Complex sentence: In 1948 architectural and engineering investigations deemed it unsafe for occupancy and President Truman, his family, and the entire staff of the White House were relocated across the street for almost three years while the mansion was gutted, expanded, and rebuilt.
Split into simpler sentences: In 1948 architectural and engineering investigations deemed it unsafe for occupancy and President Truman, his family, and the entire White House staff were relocated across the street. For over three years the mansion was gutted, expanded, and rebuilt.