Water in the roof? and not the leaky type.

While doing some sketching, I really started to debate options with the roof. Most solar passive design homes have the typical flat pitched roof to maximize the window area in the southern direction. To rebuild a roof one can easily being looking into 20000 dollars or more. Not including the cost of removing materials. This is quite an issue with renovating this house. This is what lead me to researching into water trombe walls. I found a report which was written for the university of Oregon. I had never heard of water as use for thermal mass in construction, but when you look at cities such as Toronto which is built beside a lake, it drastically moderates the temperature. Now what if I were to change the orientation of the walls to be a ceiling? Then with the use of fans, circulation could occur. Again the issue with that would be supporting water or concrete. Anyone have any ideas or opinions?