why waste time messing about with harvesting rain water, you price your jobs on access to the windows, types of windows, overheads etc, for me the amount of rain water you actually harvest doesnt seem to warrant the set up.
Alot of guys on here have a 600 ltr tank which they say lasts them a day, unless you build your own reservoir in your back garden, which I am certain would not be large enough to collect sufficient amounts to use for WFP.
Matt