Original F-scale:
F3 Tornado Damage (Online Tornado FAQ)

All but a few parts of the outer and inner walls were toppled or removed from this house in Moore, OK, on 3 May 1999. For a well-built home, any removal of inner walls constitutes EF3 damage; so this site was rated high-end EF-3. The same tornado caused F5 damage in several locations elsewhere in its path.