Description of Water Cisterns

A cistern is a covered watertight tank for storing collected rainwater for a catchment.

A catchment means the surface from which rainfall is collected and may include roof or ground surfaces. A steel or tile roof is an acceptable surface for a catchment, whereas a wood shingle roof is not an acceptable surface. The risk of contamination is greater with a ground surface catchment than an appropriate roof surface catchment.

Water Quality

There is no guarantee that water collected by roof collection will be clean water. Therefore, the user should chlorinate, disinfect or treat the water prior to consumption.

Cistern Location

A cistern should be located:

* adjacent to the dwelling;
* a minimum of 30 feet from any part of a sewage disposal system;
* a minimum of 100 feet from any areas of barns used by livestock;
* on a higher ground elevation than the sewage disposal system, with the surrounding area graded to provide drainage away from the cistern;
* within the setback requirements of the applicable land use zone;
* within an area not subject to flooding.

Cistern Design

Cistern should be watertight with a smooth inside surface. In case of failure or leaks, cisterns should not be closed in. Inlet, outlet and waste pipes should be screened. Cistern drains are not permitted to be connected to wastewater or sewer lines.

Manhole openings should have a watertight curb with edges projecting a minimum of 4 inches above the level of the surrounding surface. The edges of the manhole cover should overlap the curb and project downward a minimum of 2 inches. The cover should be provided with a lock to minimize the danger of contamination and accidents. Where there is potential for accidents, the building inspector may require the installation of a safety fence. Large cisterns should have two manholes or vents for cross ventilation & cleaning.

Cistern Size

The size of the cistern will depend on the number of users, the number of plumbing fixtures and the recharge rate of the cistern. With conservation with consumption can be greatly reduced. The data provided below assumes consumption of 100 gallons per day per bedroom. Deficits in the annual yield of catchment can be made up by increasing the size of the cistern accordingly and by storing well water
or trucked water in the cistern.

Data for Gabriola Island:

Annual Rainfall 40 inches
Run Off 75% of total rainfall
Daily Water Consumption/Bedroom 100 gallons
Period between rainy periods 60 days
Cistern Size/Bedroom 6,000 gallons
Annual Water Consumption/Bedroom 36,500 gallons

Area Catchment (sqft) Annual Yield of Catchment
500 7,790
1,000 15,570
1,500 23,360
2,000 31,140
2,500 38,930
3,000 46,720
3,500 54,500
4,000 62,290

Calculations:

Formula: Minimum volume of cistern= 100 gallons/day x # of bedrooms x 60 days

Example: 12,000 gallon cistern = 100 gallons/day x 2 bedroom x 60 days

Formula: Minimum catchment area = 100 gallons/day x # of bedrooms x 365 days

Example: 3,500 to 4,000 sqft = 100 gallons/day x 2 bedroom x 365 days

Note: This document is presented for information purposes only & adherence to the details contained within this document do not preclude the Building Inspection department from requiring building permits & engineering of a cistern.

Regional District of Nanaimo

Water Delivery
Water delivery is available on Gabriola Island through Dan Foley of Summer Rain at #247.9136.