“There was a little bit of rain last night and we have a big roof, but how much water fell on our roof that we could have collected?”

Well, first you would need to know how much rain actually fell. You can find out by listening to the news or checking with the weather bureau, but this will often be the amount of rain that fell somewhere near you rather than what fell in your exact area. Alternatively, you can use a rain gauge in your own yard to measure rainfall.

Then you need to determine the actual area of your roof - and luckily you only need to do that once! Your house plans are probably the easiest way to do this, but you may be able to estimate the roof area if you know the square metres of your home (be careful if you have a two storey home, though!)

Then, for every square metre of roof area you can expect to collect about 1 litre of water for each millimetre of rain fall.

So if it rained 8ml last night, 8 litres of rain fell on each square metre of roof. If your roof is 10 square metres, you could have harvested 80 litres!

With that much water, it becomes clear why harvesting water can be so important - even water off a garage or shed can make a difference to how much watering you can do over summer.