When you are talking about a serious survival water system this instructable is one of the best I have seen. It is a great system because of its effectiveness but also because of its simplicity. Many systems require a steep learning curve and that is what puts many people off. This is also a system that looks as thought it could be easily modified to your homes unique specs.
The collection of rainwater into a safe container is one of the best ways to get fresh water. This water would require very littler filtering and possible no filtering at all. Of course, there is the problem of the asphalt roof tiles that could potentially contaminate that source. Still, in a true collapse situation a system like this could make the daily task of sourcing safe water a lot easier.