Collecting Rainwater on the Roof

Off Grid Rainwater Collection - Complete How To Guide

May 27, 2019
(last update May 27, 2019 )

Daniel Mark Schwartz Profile Picture

For those of us still paying for water to drink, wash in, and irregate our plants, the idea of collecting free rain water from our roofs seems naturally appealing. There is a lot of confusion out there about the best way to harvest and store the water that falls naturally on your roof. But it doesn’t have to be that hard.

How do you collect rain water? You can do it in in a few easy steps:

  1. Ensure you are legally allowed to collect rain water in your area
  2. Install gutters, if you don’t have them already
  3. Choose a suitable “first flush” system (optional)
  4. Route water to a large above or below ground storage tank (more info below)
  5. Install a pump (optional)

If you are confused about the best way to collect water in your situation, read below to where I have collected complete answers to the most cost common questions people have about rain water collection.

Installing a rain water collection system need not be a trial, but there are a few things you need to know. The first is find out whether you are legally allowed to collect and use the water off your roof. This varies state by state and you will need to check with the government office with authority over water use in your state to determine what you are allowed to do. See map below for an overview of the level of rain water regulations by state.

Rainwater Harvesting Regulations Maps, source: energy.gov

What You Need to Buy for a Rainwater Harvesting System

The next question that people often ask is what hardware they need for their rain water system. Here is the short checklist:

  • Gutters and drains
  • Downspout filter / diverter
  • Rain barrel or cistern
  • Extraction spigot and/or pump
  • First flush system (if desired)
  • Pipe to run between diverter and barrel (if needed)

Gutters and drains are they same type you typically find installed in homes. You don’t need anything special, but you do need to consider the number and location of any downspouts you have in the system.

In order to collect all the water off your roof you will need either a rain barrel at each downspout, or to pipe the water from each downspout to your cistern. Piping water can be quite a bit of extra work, and may not be suitable for above ground systems unless you are OK with long runs of pipe visible along the walls of your home. But, on the other hand, having a rain barrel on some downspouts may not be desirable because they are either in a difficult to reach location or they are in a location where the barrel would be in the way. So, when installing gutters, reduce the overall number and install them in locations where they work well with your rain barrel or cistern placement.

What is a Downspout Filter / Diverter

The simplest way to integrate your rainwater collection system, for home scale designs, is to use a commercially available diverter such as this Fiskars Rain Barrel DiverterPro Kit. What this product does is:

  1. Make sure any leaves or debris pass through the downspout and don’t enter the rain barrel
  2. Divert water to the rain barrel until it gets full, in which case excess water can pass through the downspout

How Do I Choose a Rain Barrel or Cisterns for My Rainwater Harvesting System

Before you choose your rain barrel size and material, you need to be clear exactly what you require from your container, including:

  • Will it be above ground or below ground?
  • How much water do I intend to collect?
  • Where do I plan on using the water?
  • Do I want to drink the water or use it on plants?

The most important question is whether you want an above ground system or below ground system. Above ground is much less work / cost to install, but may be visible and susceptible to freezing if you live in a cold climate. Below ground is more protected, but requires running pipes, digging a hole in the ground for your cistern, and probably a pump to get the water out. In either case, be sure the barrel or cistern you buy suitable for above ground or below ground use. Some plastics degrade in the sun, and so require burial for long term use.

The next big question is how big of a cistern to get. If you intend to collect all the water you can, see below to calculate how much water you can expect to collect in a year.

If you have a specific use in mind, like watering a garden, consider investing in a water flow meter and measure how much you use over a typical week, then multiply that by the number of weeks each year that you need to water.

Remember, you can always add more water storage later if you decide that you need more.

Water flows down hill, so you need to consider where in relation to your tank you plan to use the water. If your tank can be up on some blocks or uphill from where the water will be used, then it is simplest to get a tank with a spigot on the bottom for a low cost, albeit potentially low pressure, gravity fed water system. Otherwise, you will need to pump the water in to a pressure vessel or elevated water reservoir before use.

Metal Water Reservoir

Lastly, remember that if you intend to drink the water you have collected, make sure that your water container is safe for human consumption. Water tanks sold on the market will say if they are rated to be used for drinking water.

What is a First Flush System

First flush systems deal with the fact that water coming of the roof may be dirty for the few gallons before the roof has had time to be cleaned off. This potentially contaminated water is called the “first flush.” And a first flush system is designed to separate the initial downpour from the rest of the collected water

Even if you don’t plan to drink your rainwater, extra dust and organic matter in your cistern may lead to sediment algae growth in the tank that requires cleaning over time.

A first flush system need not be complicated, and generally consists of a tee in the line between your downspout and rain barrel. Downward on the tee is a length of pipe with a small valve that lets water very slowly drip out the bottom.

When the rain first starts, the downward pipe will be empty, and fill up with the first flush. Once full, the rainwater will continue on to the storage container. Between rain storms, water will slowly drip out the bottom of the first flush diverter, leaving it empty and ready for the next downpour.

How Much Water Can I Harvest in a Year

How much water you can collect in a year depends on how much rain you get and the size of your roof. Here are the steps to calculate how much rain you can harvest in a year

  1. Look up how many inches of rain you get on average. A quick search for “average annual precipitation in (your city)” will usually result in a quick answer.
  2. Determine the square feet that your roof covers. If you know the square footage of your house (or first story if you have several), then you can use this. Otherwise you can find out by:
    • Going to Google Maps
    • Find your house in satellite mode
    • Right click on a corner of your house and choose “Measure distance”
    • Click around the house on all corners, then click on the first one to close the shape
    • Read the “Total area” from the bottom of the screen
  3. Calculate (number of inches of rain per year) x (roof area in sqft) x (0.0114) = (gallons of rainwater per year)
Calculating Roof Area on Google Maps

Remember this number is an average, so you may get more or less depending on the year. First flush systems or other inefficiencies in the collection system (leaky or over full gutters for instance) will result in less water being collected.

Is it Safe to Drink Rainwater

Rainwater is generally safe to drink when it falls from the sky, but has the potential to be contaminated with biological or chemical contaminants during the collection and storing process. Because of this, the CDC recommends filtering rainwater before drinking.


Free Off Grid Guide

Thinking about going off grid? Subscribe for a free 30+ page PDF that helps you get there! My gift to you.

You will receive your free guide, exclusive discounts, and occasional announcements