In the 1500's houses had thatched roofs-thick straw-piled high, with no wood underneath. It was the only place for animals to get warm, so all the cats and other small animals (mice, bugs) lived in the roof. When it rained it became slippery and sometimes the animals would slip and off the roof. Hence the saying "It's raining cats and dogs."
Admin: Is there a word-count limit on posting a discovery??!! I am pretty sure I typed the whole thing somehow it got cut out?!