I worked this out as we are/were thinking about adding a coiled tubing to our roof for solar hot water. In order to use it you will need to first figure out how much water you want. Once you know how much water you want and what the inside diameter of the tubing is, you can use this calculator to determine the length of tubing needed. You will need to convert from cubic inches to gallons. Dont forget to check how much weight you are adding to your roof (gallons to pounds).
The spread sheet will allow you to figure the size and number of coils needed to achieve you goal. Change the numbers in the box on the left to suit your needs.