Original F-scale:
F4 Tornado Damage (Online Tornado FAQ)

A tornado in Moore, OK on 3 May 1999 demolished this house (foreground) down to a short pile of debris on and around the foundation, with no walls standing. In order for this scene to be rated F5, the debris must have been swept away, leaving behind evidence that the house was well-attached to its slab. [The brick house in the left background suffered F3 damage, with a mixture of inner and outer walls removed.] This tornado caused an immense amount of F4 damage on its path through the southern portion of the Oklahoma City metropolitan area, and several locales of F5 damage.