You have decided to invest in a rainwater
harvesting system, but where do you start? What
questions do you need to answer before you can build the
right system to harvest rainwater? Let’s start with
learning how much rain you can collect.
Rainwater systems need not be complicated.
They have been built and used for thousands of years.
There are 5 major components to consider:
Capture, Conveyance, Holding,
Filtering/Purification and Distribution.
The Capture system is the
place to start. For homeowners, it is the roof of your
house. The size of your roof determines how much rain
can be caught, which tells you how large the other
components need to be.
Calculating the maximum amount of rain
that can be caught on a roof is a straightforward
Maximum Annual Gallons of Rain
Capture = Annual Rainfall x Square Footage of Roof x
The Annual Rainfall in your area is
readily available on the Internet, in the local paper or
from your local weather station. The average annual
rainfall in Santa Fe, New Mexico, for example, is 14
inches (i.e. .3556 meter).
The Square Footage of Roof is
calculated by multiplying its length by its width. For
example, the roof of a single story house that is 50
feet (i.e. 15.24 meter) long by 40 feet (i.e. 12.191
meter) wide has an area of 2,000 square feet (i.e. 185.8
square meters) on which rainfall can be captured and
The last factor (i.e. .623) in the
equation, is how many gallons are in an area of one
square foot by one inch deep of rainwater.
So, putting these numbers together in one
example, the Maximum Annual Gallons of Rain Capture on a
2,000 square foot roof in Santa Fe is calculated as
follows: 14 inches x 2,000 square feet x .623 gallons =
17,444 gallons (i.e. 66,023 liters).
But the Maximum Annual Gallons of Rain
Capture is an ideal. You need to reduce it slightly due
to real world system inefficiencies.
The first reduction is due to the
inefficiency of the roofing material. Roofing materials
vary in their ability to deliver rain runoff. The least
efficient roof type for capturing rainwater is a grass
or "green" roof. If planned and installed correctly,
almost no runoff would occur in light to moderate rains
on a "green roof". These roofs are great for slowing
down and eliminating rainwater runoff, but are terrible
for rainwater catchment.
Other kinds of roofing material will allow
most of the rain to be captured, but some will be lost
due to splashing off the roof, water absorption, and
other factors. Estimated efficiency ratings are:
- For a tile/metal roof assume a 95% runoff
- For a concrete/asphalt roof assume a 95% runoff
- For a gravel roof assume a 70% runoff efficiency
- For a bare soil roof assume a 75% runoff
- For a grass roof assume a 17% runoff efficiency
Multiply the Maximum Annual Gallons of
Rain Capture by the correct efficiency rating for your
kind of roof. Using our working example and assuming a
tile/metal roof, multiply 17,444 gallons by 95% (or .95)
and you get 16,571 gallons (i.e. 62,728 liters) of
Available Annual Harvestable Rainwater.
You now know approximately how much rain
runoff you can harvest. Next start planing how to keep
debris out of the system. Install screens on each of the
downspouts or canales to prevent large contaminants from
entering the system. Screens with small holes will clog
quickly and force water over the gutters, causing water
loss or, worse, leaks into the house. Choose a screen
with holes that are larger than those of traditional
window screens. Screening material of this kind is
available from hardware stores. It will not keep
everything out, but it will keep the large debris from
entering the system and avoid unnecessary maintenance.
Next in line is your Conveyance system,
which transports the rainwater from the Capture to the
Holding system. However, before you can design the
Conveyance system, you must think about how many tanks
you may want or need and where they will be located. I
will help you with that in the next article.
for US Cities (To add others, please send with
Spreadsheet for Calculating Maximum Capture
State and City Programs and Vendors
1. Square footage of the roof of an "L" shaped house
would be the sum of the two shapes (i.e. 20x30 + 20x20 =
900 square feet of roof area. On a two or more
multi-story house, the calculation would be for only the
nonoverlapping roof space.
2. For a round roof, the area calculation would be
the Radius squared x π (i.e. 3.141592654). The Radius is
the distance from the center of the roof to the outside
3. A sloped roof will be slightly larger than the
base square footage of the house foot print due to the
slope of the roof and the overhangs. However, these
typically only add minimally to the area and are not
usually included in the calculation.
4. If potable water is desired, than the roof
material will be critical. Tile and non-painted metal
are the preferred options for drinking water as the
other materials will leach chemicals in the water that
will be very expensive, but not impossible to