For rain water collection. you can use an awning with a generous slope to avoid pooling the water and holding too much weight. you also want a diverter valve so the initial collection of water is used to flush away the collected dust, spores and assorted airborne contaminants that collected since last rain fall.
Leave the valve set for dumping into a down spout and only change the diverter valve to feed the tank when you are sure its running clean and clear. Cruising boaters who practice collecting rainwater find they have to treat the stored water with chlorine to prevent the tank getting gungy with microbial growth,
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