Skip to content

Rainwater Collection: Design and Construction

Wrightway Onsite experts are now installing rainwater collection cisterns!

We’re bringing our extensive experience installing underground concrete precast tanks to rainwater collection.

What is a rainwater cistern? 

Rainwater (or roof-catchment) cisterns collect and store rainwater for household and other uses. A system of gutters and downspouts directs the rainwater collected by the roof to the storage cistern.

The water tank is typically located underground and can be constructed from various materials; however, we recommend precast concrete.

Typical roof-catchment cistern system.
Figure 1: Typical roof-catchment cistern system.

Why try rainwater collection? 

Rainwater collection is a sustainable and resourceful practice, especially in regions like Southern Vancouver Island and the Gulf Islands, where groundwater and surface water are scarce. 

Rainwater cisterns can supply water to households, farms, and more! Rainwater harvesting not only conserves water but also reduces your reliance on municipal water supplies. It’s a great way to contribute to environmental protection and save money.

How big is a rainwater cistern?

From 1000 liter water tanks to large 5,000 gallon water storage tanks, rainwater cisterns come in all different sizes. Which is great, since household water use varies significantly, depending on supply and usage. 

  • The average base use determined by water utilities is 7500 gallons (28,000 L) per month or an average yearly minimum need of 90,000 gallons per household. 
  • Common household planning provides for 50 to 75 gallons (200 to 300 L) a day per person, or 73,000 to 110,000 gallons a year for a family of four. 


Fun fact: one-third to one-half of the water is used for flushing toilets. 

We always recommend working with a professional to determine you, and your family’s needs before selecting a water tank. 

How to Install a rainwater cistern

Proper design and construction are essential to minimize water-quality problems. Here are some questions to ask yourself before setting up a rainwater collection system.

Which roof surface and material works best for rainwater collection:

  • Metal roofs are popular due to their effectiveness and affordability. Slate and tile roofs are excellent but expensive options.
  • Avoid asphalt shingles and wood shakes with rough surfaces, as they can reduce water flow and harbor mold and mildew.

How to install gutters and downspouts effectively:

  • Install gutters with downspouts to direct rainwater from the roof to collection points.
  • Consider the roof area and local rainfall statistics when designing your system. Typically, one downspout is recommended for every 40 feet of gutter.


How do you prevent clogs: 

  • Use leaf guard systems to prevent debris from clogging gutters and downspouts.
  • Consider debris boxes or first flush diverters to separate leaves and other debris from rainwater before it enters the storage tank.

How will you use the system? What’s the best rainwater treatment method?

  • Systems can be designed for potable (drinking) or non-potable (e.g., garden) use.
  • Proper treatment and filtration systems are essential if you plan to use rainwater for all human consumption needs.
  • Choose a rainwater storage tank rated for potable use (CSA approved) with a capacity of at least 4,546 liters (1,000 imperial gallons).

How to calculate your rainwater collection potential:

On average, each 9 square meters (about 100 square feet) of roof in the Capital Regional District (CRD) can collect 272 liters (60 gallons) of rainwater for every 25mm (one inch) of rain.

How to connect rainwater cistern to irrigation systems:

Will you be using the system for irrigation? If so a pressure pump can deliver the water on a timed system to your irrigation pipes for your watering needs.

Rainwater collection is a great environmentally friendly way to save money on your property. To ensure you’re installing the best system possible, get in touch with us here at Wrightway! Our experts are happy to help.

For more information, you can explore resources, There are numerous grants available, please check with your local municipalities for rainwater collection subsidies.

RDN Rainwater Harvesting, Team WaterSmart | Get Involved RDN

Collecting rainwater helps with CRD water supply – Island Social Trends