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The Complete Guide to Rainwater Harvesting

Collecting: Rain Water Catchment

An effective rainwater harvesting system has three components.

  • The storage tank, which not only holds the accumulated the water but also serves as the source for the recycled water.
  • The transport network made up of pipes, filters, and valves through which the water flows into the tank.
  • The initial catchment, which is arguably the most important aspect of a rainwater harvesting system. After all, if you can’t capture the runoff water in the first place, its transport and storage are irrelevant.

In most cases, the catchment is simply comprised of a home’s roof and its guttering. The rain that falls onto a slanted roof runs downhill into the gutters, where it flows through to the downspout openings that lead to other pipes. So it’s important to make sure that gutters are placed on all sides of a roof where the water runs off. Otherwise, the rainwater falls to the ground and is wasted.

How much rainwater can you collect in a single rain event?

There is a rule of thumb which can estimate how much water you can capture, based on the size of your roof and the amount of rainfall in a given shower or storm. The basic formula is 45% of the number of inches of rain multiplied by the square footage of your roof. The result is the number of gallons of harvested rainwater that you will be able to collect and store. In other words, if you know the size of your roof (if its rectangular, simply multiply the length by its width) and the estimated size of a given rain event in your area (many weather sites can provide you with this data), then you multiply those two numbers together and then take 45% of the product. Keep in mind that you only use the areas of your roof which are connected to your rainwater harvesting system.

The resulting amount of rainwater you can capture may surprise you. In fact, if you are using a 55-gallon storage barrel, your roof only needs to be 11 feet by 11 feet for an inch of rain to fill up your barrel (which further illustrates the need for an overflow pipe).

Secondly, it is vital that your gutters remain clog-free so that the runoff water flows smoothly into your harvesting system. Debris such as leaves and pine needles can block the water from its intended destination, causing it to spill over the sides of your gutter and onto the ground (which can lead to additional problems).

That’s where a Gutter Helmet® gutter cover system can help. The reverse-curve design allows rainwater to trickle over the guard and into your gutters, while helping solid debris to slide off onto the ground and stay out of the gutters entirely. You’ll never have to worry about cleaning out clogged gutters again!

But Gutter Helmet® gutter covers are valuable to a rainwater harvesting system for another reason, too: they act as a filter for the runoff water flowing into the pipes.

Typical rainwater harvesting systems require the installation of a “roof washer”—a vertical pipe placed along the transport network of pipes which is designed to remove the initial, dirtier water that flows from a roof before it travels into the storage tank. With Gutter Helmet®, the particulates that normally contaminate runoff water never make in into the gutters, but instead slide onto the ground along with leaves and other debris. Therefore, when a Gutter Helmet® system is in place, the runoff water is cleaner than it otherwise would be—which may negate the need for a roof washer further along in the process.

So when deciding on the catchment portion of your rainwater harvesting system, be sure to compute how much water you will capture and how you will filter it. Only then can you concern yourself with what to do with the water that winds up in your storage tank.