Rainwater Harvesting clip artCollecting and Using Rainwater

Rainwater Harvesting has become a hot topic in recent years – even though it is a very old practice.

Here on the West Coast of British Columbia rainwater is free and abundant – especially in the winter. In the summer on the other hand, rainfall is limited, and many coastal communities can experience water shortages and depend on rainwater collected in the winter.

At Environmental Cistern Cleaning Inc. we believe that a properly sized and maintained rainwater storage can solve most water shortage issues. So why not collect a sufficient amount of rainwater not only for domestic uses such as gardening, toilet flushing, drinking or cooking, but also for commercial uses?

Rainwater Harvesting – How is it done?

Rainwater harvesting is a common practice on the Gulf Islands and on Southern Vancouver Island in the communities which are not supplied by the municipal water sources. For more information, please consult the Rainwater Harvesting Best Practices Guidebook. There is also a lot of great information in this document about collecting rainwater on the Gulf Islands from the Islands Trust.

How big does my storage tank need to be?

The most important element of the rainwater harvesting system is a large, high-quality storage tank; storage size must be sufficient to get you through the dry summer months. The drier the summers and the higher the summer use – the larger the storage tank needs to be.

The size if a storage tank which is required to meet your water needs depends on a number of factors, such as:

  1. The amount of rainfall that can be collected
  2. The size of your rain catchment area (typically the roof)
  3. Your demand for water including all indoor and outdoor uses

1. How much rainwater can I collect?

Many residents of the West Coast of British Columbia subsist strictly on harvested rainwater. Bear in mind that different areas of the West Coast can receive different amounts of rainfall and may have different rain patterns. You can find the annual rainfall in your area on the Environment Canada website. For more historical data you may also visit Historical Climate Data – Government of Canada.*

*Be aware that more precise information may be available for your location. Even across a small island or municipality, rainfall patterns may differ. Check with neighbours, town leaders, and any other information sources you can find.

2. How to estimate the area of my roof?

The total amount of water you can collect from your roof via rainwater harvesting depends on:

  • The area of your roof (the area measured as the horizontal projection of the roof including overhangs).
  • The efficiency of your rainwater harvesting system, including the system losses due to factors such as evaporation, type of roofing material, and any losses or leaks in the water conveyance system.

How to calculate your harvest potential?

Average Rainfall x Efficiency x Roof Area

How to measure your roof area?

Length along one side of a house (plus overhangs) x width of the house (plus overhangs)

It is also useful to understand the differences between metric and imperial measurement systems used in North America. Check the conversion system here.

For example:

  • 1″ of rain on 1 square foot of roof area produces 0.52 Imperial gallons of water
  • 1 mm of rain on 1 square meter of roof area produces 1 Litre of water

Can I collect from any roof?

If you want to be able to drink the water you collect, it is best to have a metal roof on the building you are collecting from. This guarantees that no particles from the roof itself get washed into your water supply.

3. How do I determine my Water Demand?

What can I use rainwater for?

The most common uses of rainwater harvesting include watering your lawn and garden, washing your car and window; non-potable uses such as toilet flushing, and potable uses from drinking and cooking to dish washing and showering.

To calculate your rainwater storage requirement, you have to understand your monthly indoor and outdoor water demands.

Outdoor Water Uses

Here are some simple guidelines that can help you determine how much outdoor water you need. The numbers apply typically during the summer as you will rarely have to water your lawn or garden during a west coast winter. For example, garden sprinklers and car washing can put the highest demand for your water storage.

  • Watering your gardens – full flow sprinkler (for 1 hour) can use about 240 imperial gallons (1100 L).
  • Car washing – running a hose (for 1/2 hour) can use about 120 imperial gallons (550 L).

Indoor Water Uses

Indoor water uses include non-potable uses such as toilet flusing and potable uses such as drinking and cooking to dish washing and showering.

To translate that information into your water storage requirements you will need to figure out how much time you will be spending at your residence. Consider these points to help you:

  • How many residents?
  • Is it a full-time residence or part-time?
  • If part-time, when are you mostly there?
  • Are there plans to live thee full-time in the future?
  • How many visitors? For how long? At what times of the year?

Type of Consumer            Water use in metric                      Water use in imperial

Average Consumer:              225 – 275 Litres/Person/Day           50 – 60 Gallons/Person/Day

Rainwater Dependant:          115 – 180 Litres/Person/Day           25 – 40 Gallons/Person/Day 

Here are calculation examples to determine your indoor water needs:

In Metric:

# of residents X 230 Litres = your household water needs per day.

In Imperial:

# of residents X 60 Imperial Gallons = your household water needs per day.


Let us help you to determine the size of your tank

Let us help you to determine how big your tank should be by taking into account all the factors that apply. Also, we would like to ask you a couple of questions to better understand your water situation.

  • Your water uses (your water demand), and
  • Amount of rainwater which you can collect (your water supply).

If you would like to know how big your water storage needs to be to accommodate your water demand, please fill out our short questionnaire.


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