Original F-scale:
F1 Tornado Damage (Online Tornado FAQ)

This wood-frame house was pushed bodily off its concrete block foundation by the Spencer SD tornado of 30 May 1998 (a tornado which later did marginal F4 damage in the town of Spencer). Here, the house had no bottom anchoring at all. It was simply resting on its foundation by gravity alone; so it was easy for relatively weak winds near the edge of the tornado to slide the house aside with minor structural damage. It experienced partial roof removal, only on the windward (near) side; therefore, this damage site was rated F1.