Rainwater Harvesting clip artCollecting and using Rainwater

Rainwater Harvesting has become a hot topic in recent years. At Environmental Cistern Cleaning Inc. we believe that there is no water shortage, but rather a water storage problem. Rainwater is free and abundant here in British Columbia. Why not collect some water and at least use it for the garden or for toilet flushing etc.?

As a start, please read this document about collecting rainwater on the Gulf Islands from the Island Trust Fund to answer any questions you may have.

And here is some more information on rainwater harvesting, courtesy of Premier Plastics, tank manufacturer:

Rainwater Harvesting – how it is done

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.

How much water do I need?

Information required to calculate your rainwater storage needs: 

  • Monthly water demand
  • Monthly rainfall statistics
  • Roof area

There are a few straightforward calculations you can use to predict the amount of water you’ll need. These calculations will indicate averages only, please keep in mind your own habits and conservation ability. 

Indoor Water Predictions

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 

The simple calculation to determine your needs, therefore, is:

In Metric:

# of residents X (95 to 230) Litres = your water needs per day.

In Imperial:

# of residents X (25 to 60) Imperial Gallons = your water needs per day.

To translate that information into your storage requirement you’ll need to figure out how much time you’ll 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 there full time in the future?
  • How many visitors? For how long? At what times of the year?

Outdoor Water Predictions 

These guidelines will help you determine how much outdoor water you’ll need. These numbers apply typically during the summer as you’ll rarely have to water your lawn or garden during a west coast winter:

  • 1 watering can = 3.3 gal. (15 L)
  • 3 ft. shrub in hot weather (1 week) = 7 gal. (32 L)
  • 18 in. pot in hot weather (1 week) = 1.8 gal. (8 L)
  • 40 deck pots – drip water (1 week) = 50 gal (227 L)
  • 1 sprinkler full flow (for 1 hour) = 240 gal. (1100L)
  • Car washing (1/2 hour) 120 gal. = (550 L)
  • Pressure washing (1/2 hour) = 40 gal. (180 L)

How much water can I collect?

The total amount of water you can collect from your roof via rainwater harvesting depends on 3 things:  

  1. The annual rainfall in your area. Visit www.climate.weatheroffice.ec.gc.ca/.*
  2. The area of your roof (the area measured as the horizontal plane under the roof including overhangs).
  3. The efficiency of your harvesting system. This is based on factors such as evaporation loss from winds, tree cover, roofing material, and the sizing of the pipes for storm events. Well designed systems capture 75-85% of the rainfall.

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

Calculate your harvest potential: Average rainfall   X efficiency X roof area

Measure your roof area: Length down one side of your house X width of your house.

  • 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 1litre of water

Can I collect from any roof?

If you want to be able to drink the water you collect, you should 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. We had our own roof equipped with a metal roof from Stevo’s Roofing. Please go to our Links of Related Services Page for contact details.

Average rainfall and harvesting potential on BC’s Gulf islands

Precipitation averages are from Environment Canada. Harvest amounts are from a 1000 sq. ft. roof at 75% efficiency.


Annual Precipitation 

Summer Precipitation (June – Sept) 

Annual Harvest (gallons)  

Summer Harvest(gallons) 

Saturna Island (Capmon) 

33.” (840 mm)

5.1″ (130 mm)



Salt Spring Island 

38.1″ (975 mm)

4.9″ (125 mm)



Cortes Island (Tiber Bay) 

49.4″ (1255 mm)

9.1″ (230 mm)



Bowen Island 

67.8″ (1,700 mm)

9.8″ (250 mm)




Can I store enough water for the whole summer?

Yes. Many west coast residents subsist strictly on harvested rain water. The most important factor 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 required.


How big does my storage tank need to be?

Different areas of the west coast have different rainfall amounts and patterns. This changes the important factors – roof area and storage tank size – required to meet your water needs. If you plan on making it through an entire summer you’ll need to know the following:

Number of residents X number of days residing X average daily usage – summer harvest

Example: 2 people spending their weekends at the cottage for June, July, and August, using a conservative 110 Litres per day:

2 residents X 24 days X 110 Litres/day = 5,280 Liters.

This couple would need over 5,000 Liters of water to get them through the summer to satisfy their indoor needs. If their average summer rainfall harvest is only 1900 Liters, as on Salt Spring Island, they would need a minimum 3,300 Liters of storage capacity.




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