Use glass wine bottles with inner bottom cut out for smaller version of the same design. When you evaporate and condense water it becomes pure and is completely safe to drink. Nasty chemicals and bugs do not evaporate with the water.
How to calculate how much water you can harvest from your roof: Collection area (total square footage of your roof) x x (the number assigned to the collection efficiency of a tile roof) x rainfall (in actual or averaged inches) = gallons/year.
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