Rainwater Harvesting Questions

What can I use rainwater for?

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

Can I store enough water for the whole summer?

Yes. Many west coast residents subsist strictly on harvested rainwater. 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 much water can I collect?

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

  1. The annual or monthly 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 along one side of your house X width of your house or roof section.

  • 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

Note: Treated wood or Asphalt roofs are not recommended for potable water collection.

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)



How much water do I need?

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.

Information required to calculate your rainwater storage needs:

  • Monthly water demand
  • Monthly rainfall statistics
  • Roof Area

Monthly Water Demand

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 Dependent:

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 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 requirement.


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 Litres.

This couple would need over 5,000 Litres of water to get them through the summer to satisfy their indoor needs. If their average summer rainfall harvest is only 1900 Litres, as on Salt Spring Island, they would need a minimum 3,100 Litres of storage capacity (5,000 – 1,900 = 3,100 Litres).

How much filtering do I need to do?

The amount of filtering required in your system depends whether your usage is for outside use, toilet flushing or drinking.

There are two basic considerations:


Primary screening can be screens that cover the entire gutter system to take out twigs, leaves and needles from close-by trees before entering the downspouts. There are a number of products on the market that do this job. Keep in mind accessibility for periodic cleaning.

If debris from trees is less of a problem then filter screens (rain heads) can be installed at the top or bottom of the downspouts. Some devices are designed to shed debris (self-cleaning) and others have a collection pocket or canister that can be emptied periodically.

A third method is to install a filter basket that fits inside the 16″ lid opening at the top of most storage tanks. The filter basket can be used in addition to gutter and downspout screens and has the added advantage of accommodating an optional fine cloth filter that will filter out pollen as well. Keep in mind all screens and filters must be easily accessed.

The more screening and filtering you can do before water enters the tank means there will be less sediment buildup in the tank, and less future cleaning.


Further filtering on the outlet from the tank depends on the type of use.

a) If used for outside use only (garden, washing windows or cars) then no additional filtering would be necessary.

b) Water for toilet flushing and laundry needs to be filtered to a higher level to ensure pipes and valves do not clog up or become damaged from particles in the water.

c) Water for drinking or human contact must be further filtered to one half a micron to ensure the proper function of an ultra-violet (UV) disinfection unit to ‘purify’ the water for human consumption or contact. As the water quantity needed for drinking or showering is relatively small, a separate tank or separate connection to the main storage tank would save the cost of unnecessarily disinfecting water that would be going to outside use or toilet flushes.

A professional person familiar with UV disinfection must be consulted for this application.

There are a number of filtering devices on the market, and a rainwater professional can advise what is best suited for your application.