spat
.700 Nitro Express
You only need a couple gallons a day of potable water, but you probably want a lot more that doesn't need to be potable.A lot already covered; rain run-off from the roof will be loaded with nasty chemicals if the roof is asphalt typy shingles. Yes you can drink it but it's not good for you, metal roof=good to go.
Get yourself a copy of Ball's Blue Book of home canning and start water bath canning this fall.
We can and freeze dry and dehydrate stuff, it's now just my wife and I so we don't eat much. We always have leftovers, they get frozen in boil bags for short term or for long term get freeze dried and packed in mylar bags or vacuum packed in canning jars.
You can go as far as you want from a couple cans of Tuna to cases of everything!
The vast majority of water doesn't need to be drinking quality.