1) Roofs already collect a large surface area and funnel into a single point via roof guttering - so tapping the flow from the gutter run off would enable a viable volume of water turn a motor in the drain pipe as it descended and generate power.
2) Piezo electric generating from the roof, though was looking at a form of artificial grass format that would capture the wind as well as the rain hitting it, generating the vibrations to the base which would be a piezo element.
The first idea, maybe has some traction, though the volume of water (yearly rainfall) with roof surface area and the napkin maths didn't show a huge gain and bit of a limited market due to rainfall you would need to generate anything noticeable.
Second idea, kinda got mooted as roofs solar panels did better with the real-estate, and whilst for non south facing or some area's it may well been useful. Then the area's were solar is not as good due to latitude, well, then you have snow. So I kinda self-killed that avenue off as well.
Figured that tapping the drainage flow of water from a whole area combined would work better and for that need a organised drainage system all connected and yes you do get those. Though they are also the sewage system and with that, the cleanish flow of rainwater would be less common once you factor in the rest of the mix.
But I'm all for micro based generation, like a wall of small wind turbines instead of one large one and more aesthetic and palatable to add to existing buildings and area's.