Has anyone heard of concrete walls with water stored in a cavity between them and in a cavity in the roof?

Question

The cavity in the roof catches rain water that can be used to water a garden

Answer 1

im not sure

Answer 2

Yes and no. Many homes have what is known as a "cistern". This is a holding tank old school concrete new school plastic or resin  compounds.  

These are typically in the basement or out side above or under ground depending on the weather.

To put a cavity in the roof would be a problem if it leaked.

Texas has used these for years because of lack of water, but new york still has a form of this on lots of roof tops in wood.

And there you go.

Answer 3

It will help keep the house cool.

It will act as a buffer, when heat is gained to the room it will be absorbed into the wall.

Cooler surfaces absorb heat..