Any nonconforming building or structure damaged more than sixty percent of its fair market value, exclusive of the foundations, at the time of damage by fire, flood, explosion, wind, earthquake, war, riot or other calamity or Act of God, shall not be restored or reconstructed and used as before such happening; but if less than sixty percent is damaged above the foundation, it may be restored, reconstructed or used as before, provided that it be done within six months of such happening.