Rainwater Harvesting: The Guide

Rainwater Tank

The practice of collecting rainwater for later use isn’t a recent thing. It is, in fact, an ancient practice. Why it’s still practiced in numerous places to this day has to do with the fact that, compared to mains water, rain is free and clean.

But in order to gain the most benefits from the use of rain, it’s important that it’s properly collected. That said, it’s not enough to just place a barrel under part of the roof where water flows down hardest (although it works to collect water to be used for one or two chores). The first thing you need to do is to be aware of these basic components:

  • Catchment area: From the name itself, this component refers to part of the rainwater collection system that catches each drop of rain. This is usually the roof of your home.
  • Conveyance system: This moves water from catchment area to storage container. A good example is a downspout network.
  • Storage system: Keeps rain clean and safe so it can be used in the future. For a large property, the best option is a number of rain water tanks connected together.
  • Distribution system: This component distributes water stored in the tank to parts of the home where they need to be used. The pump is the most common option.

You should then answer the following questions:

1. What do you need rainwater for?

Do you want to practice rainwater harvesting because you love the idea of not using a lot of water in the garden? Or do you want to build your own home system because your toilet uses so much water each flush and you do not know the exact unit to replace it with? The answer to this question determines how your setup should be.

2. Which uses of rainwater are allowed in your place?

There are places that do not allow rainwater harvesting at all. There also are places that allow the use of collected rainwater for toilet / urinal flushing and outdoor irrigation only. That said, it makes a lot of sense to check the laws of your locality even before you think of the appropriate materials for your rainwater collection system.

3. How much rainwater does your household need in a certain period?

Recall how you will be using the collected rainwater. Next, make a daily and weekly estimate of the number of times you and members of your household do the tasks the collected rainwater will be used for. Those information alone should allow you to determine the right tank size.

Once all questions have been answered, you should then proceed with building your system. Here are the things to keep in mind:

Catchment Area

Not all roofing materials are suitable for rainwater harvesting. For this type of system, the best roofing material would be steel. A few porous materials like concrete tiles, clay tiles, and wood shingles, are also suitable for this type of system, although they will soak up some rainwater that falls on them.

Many roofing materials that are not mentioned above may contain contaminants like heavy metals, which are harmful to your health.

Downspouts / Eavestroughs

Fully benefiting from the catchment area also means equipping your home with correctly sloped and sized gutters and the right number of downspouts. Otherwise, much of the water that falls might end up overflowing, especially during heavy rains.


When it comes to the tank to purchase, you have two options. The first is the poly tank. Poly tanks are lightweight, but durable. The second is a metal tank. Metal tanks are very durable. These also are heavier than poly tanks, hence making them great for windy places.

Rainwater Conveyance Piping

A conveyance network includes all the plumbing that will take rain from rooftop to storage tank and those which will distribute stored rain to all end-use points.

Protective Measures

A properly-designed system should include an overflow pipe, which protects against damage associated with the tank overflowing during heavy rains. When setting up the system, it’s important to aim the overflow pipe away from foundations and other structures.

Homeowners should also protect their systems from freezing. One of the ways to do this is by setting up below-ground parts  well below frost line. Another is to completely drain seasonal systems when the end of the season they’re intended for comes.

Backflow prevention devices are another type of protective measure. Useful in plumbing systems where rain conveyance piping can be interconnected with the potable water piping of the household, they ensure that any water that isn’t potable does not end up flowing backwards into your potable water system.

Filtration and Treatment Components

Roof debris like leaf litter as well as local air pollution can contaminate the rain that falls from the sky. To prevent contaminated rainwater from getting into the storage tank, add pre-filtration devices, which remove any contaminants that might be or have settled on the roof,  to the conveyance system.

Some examples of pre-filtration devices you can set up include downspout filters, first-flush devices, and gutter guards.

Post-storage Treatment

Some type of disinfection or filtration treatment can help to further reduce potential health risks when it’s time to use the collected rainwater.

A rainwater harvesting system can cost quite much. Fortunately, you can get back all the money you invested for the system in the form of monthly savings associated with the use of rainwater in place of mains water for certain chores.