It has been estimated that around 400,000 people in New Zealand rely on roof water and it's a fair assumption that many of them are lifestyle farmers.
Living on roof water is natural and sustainable but there are issues to think about which don't affect people on town supply.
Your water needs to be clean and safe to use. Make sure that your roof, gutters and pipes are not coated with anything toxic and are kept clean. There are a number of products you can use to divert debris away from downpipes and others that divert the first flush of water away from the tank so any dust, pollen, bird or possum faeces (or corpses) don't contaminate your water supply.
Even with diverters in place, it is sensible to use filters to clean the water if you're going to be drinking it. Filters range from straightforward filtration which removes particles to more complex chemical, biological and UV filters.
You should also disconnect your downpipes from the tank if you have work such as painting or moss spraying carried out or if you (or your neighbours) have aerially sprayed near your roofs. You can re-connect after a shower or two has washed the toxins away.
How much is enough?
You need enough water storage to ensure you don't run out if it doesn't rain for a few weeks. Calculating how much water you need is tricky - it depends on how much you'll use on a daily basis and how much rain you can expect to harvest month by month. You can be frugal and have only one water tank but if you can afford another tank then you should consider it.
See here to estimate how much water you'll use in a day.
NIWA have average monthly rainfall figures for all of New Zealand.
The calculation to work out how many litres you can harvest is roof area (in m) x rainfall (in mm) x 0.9 (to account for losses via diverters etc)
So a 300m2 roof in June in Whangarei would harvest 300x179x0.9=48,330l.
It's not just the family home roof you should consider - garages, sheds, barns, shelters and even large chook houses can be used to harvest water too.
Choosing the right water tank is important too.
Be water smart
People who are used to living on tank water use 60-100l each per day while urban dwellers who have never had to consider saving water use 180-220l per day.
Conserving water is not difficult and soon becomes second nature. A simple example is turning off taps while brushing your teeth or washing your hands. You also should watch for anywhere you may be losing water, for example from dripping taps, broken troughs or split pipes. Even a small leak if unfixed can drain a tank over time.
If you're building or renovating then install dual-flush toilets, water-saving shower heads and choose water-efficient appliances.
The good news is that roof water usually tastes wonderful compared to town supply. If you do run out of water you can buy a tank load from a water company or maybe your local volunteer fire brigade will help you out in return for a donation.