One theory is that long ago in england, the roofs on houses were made of hay, and the cats and dogs would sleep on the roof. When it rained, the roofs got slippery and the cats and dogs would slide off. Hence, it was "raining cats and dogs." Another explanation is that heavy rains would wash dead animals down the streets. However, the most popular one comes from Northern European myths in which cats were a One theory is that long ago in england, the roofs on houses were made of hay, and the cats and dogs would sleep on the roof. When it rained, the roofs got slippery and the cats and dogs would slide off. Hence, it was "raining cats and dogs." Another explanation is that heavy rains would wash dead animals down the streets. However, the most popular one comes from Northern European myths in which cats were a symbol for rain and dogs for wind. Thus, a heavy rain storm with strong winds was decribed "It's cats and dogs out there.