By Christina Herbers, P.Eng, LEED AP

Sustainable building design is part of our future. Reducing a buildings energy use and use of natural resources is the first step. One low cost item that can be done is to incorporate a rainwater harvesting system into your next building design.

The main rule for sizing a rainwater harvesting system is that the supply, the volume of water to be captured and stored, must equal or exceed the volume of water used, also called demand. Assume that in Alberta, we can only meet part of the building's water demand for six months from approximately mid-April to mid-October.

To determine the buildings total annual water demand, for an existing building, you would review the historic water use records. For new construction, you would project the water demand based on building usage, number of occupants, and days that the building will be used per week.

For your building, you will also need to determine:

What the captured water will be used for, such as grey water reuse for toilet and/or urinal flushing.

The maximum amount of water that can possibly be captured.
The collection surface area (or area of the roof).
And the daily volume of rainfall.

The maximum amount of water to be captured (in mm) multiplied by the area of the roof (in m2) equals the total possible rainwater that can be captured in Litres.

You will then need to size the pump and storage tank for the optimal system performance, refer to drawing above.

Check out these resources below for tips on how to incorporate this green aspect into your next building.

https://www.cmhc-schl.gc.ca/en/co/grho/grho_016.cfm

https://www.thebalance.com/design-a-rainwater-harvesting-system-in-6-steps-3157815