The system we built to catch rain water on deck is quite simple and VERY effective—that is, until we removed our toe rail. In a nice heavy rain we could collect about a gallon a minute. It was amazing. A few drops of bleach and a little post tank filter and we could drink it.
We may modify the deck before we paint in to regain the advantages of this system. Below is a sketch diagram of the system and a few photos of the components.