I tried to find pictures of the way these tinaco bases were made but maybe peculiar to this area or my builders. Anyway, they build a ring of bricks on the roof surface about 3 high, stucco it and fill with sand. The problem with that is water seeps into the sand from rain or an overflowing tinaco (broken float valve). My lower tinaco was causing water to come out between the original roof and the flat tiles above it so I knew it had to be seeping thru the sand. The last thing you want it trapped moisture on cement.
My solution was to seal the space between the tank and the base with impermeabilizante and fiber material. About 2 inches up the tank, the flat area and down the side of the base. I eventually did the whole roof of the lower tinaco and just started the process on my upper one. It's all part of getting ready for the rainy season and luckily a broken float valve alerted me to it in the dry season.
The upper tan tinaco base sealed
Lower tinaco with whole roof sealed
Upper tinaco - just an interesting foto