Your Annual Rainfall
If you don’t have 30 inches or more of annual rainfall, it’s going to be difficult for you to harvest enough rainwater to make a significant difference, for two reasons: (a) you’ll pay for a lot of surfacing, piping, and storage for a small benefit, and (b) you live in a dry climate where most of the water that falls on your roof will evaporate before you can store it. By the way, if you have less than 30 inches of annual rainfall, you should read it as handwriting on the wall and clear out, but that’s another story.
If you’re in the U.S., go to The Weather Channel, enter your Zip Code, and look for the Averages page. Then check the graph of rainfall by month. Calculate the annual water consumption for your household, If you’re one of those fortunate enough to have 30+ inches of annual rainfall, read on. This could work.
Harvesting rainwater from your roof can be effective and safe, but your roof contains at all times deposits of bird and animal droppings, as well as dead insects, dust, and bacteria from plant matter. We outline below the steps you need to take to improve the safety of your rainwater.
Structure of Your Roof
You want a surface that will catch as much rain as possible, of course, and you want to introduce as little crud as possible. Our current favorite is Galvalume, a factory-enameled galvanized steel, but we will change this if something more attractive surfaces. We live in a hot, humid climate where summer cooling is the big challenge, so the Galvalume helps us by shedding heat. If you live in a cold climate, this will be a difficult choice for you. You’ll need to compromise between having a dark roof that absorbs heat and an impervious one that collects rainwater better but bounces back more heat.
Beware of using Galvalume close to salt water. If you live within a mile of ocean or bay, use aluminum instead.
You will arrange a network of gutters and downspouts that flow by gravity to a cistern, the top of which is below the lowest point on your roofline. You will want to fit all your gutters with some kind of screening to reduce the flow of solid matter into the water supply, and you will equip the junction between the gutters and the cistern with a rainhead that screens out any solid matter that gets through the gutter screening.
First Flush Diverter
When rain falls, the first few gallons off the roof will contain the bulk of the crud you want to keep out of your cistern. So the most important step you can take by far to improve the quality of your drinking water is simple and inexpensive, a first flush diverter.
The first flush diverter design we prefer consists of a T-shape formed by two pipes, a horizontal pipe through which all rainwater flows on its way to the cistern, and a vertical pipe that acts as the diverter. The vertical pipe is fitted with a floating ball. When rain falls, the diverter catches those first few gallons in the vertical pipe, until the water in the vertical pipe force the ball up into the joint of the pipes and seal it off. For the remainder of the rain-shower, the ball stays up in the joint and the rainwater flows to the cistern. When the shower stops, the vertical pipe slowly empties itself through a trickle valve at the bottom so it’s ready to trap the first few gallons of water from the next shower. Every few months, you’ll need to open the diverter and clean it out.
Check to see the driest two months in your area during the year, Cut the rainfall in those two months by 50%, and subtract and then size the cistern to 25% of that total. We’re talking a pretty big cistern.