Summer is officially here! 

While we are enjoying the sunshine, the plants in our gardens are also feeling the heat and getting thirsty. Same goes for the vegetation on our rooftops.  Hence, we come to the big question, do we need to irrigate our green roofs?

Is irrigation necessary?

Irrigation is recommended but not necessary for most green roofs in Canada. A properly designed green roof needs little supplementary water and over 90% of our projects in ON and BC do not have an irrigation system. However, a hose bib is required on the roof for emergency watering during extended periods of heat and drought.

Water requirements are project specific.

Factors include:

• Climate: The climate varies widely across Canada – temperature, humidity, wind, annual precipitation and the precipitation pattern all affect irrigation needs.

Microclimate: While most green roofs are exposed directly to sun and wind, others may benefit from some shade and shelter by nearby buildings.

Plant selection: Succulents such as sedums can store water in their leaves and stems. Native plants and their communities are also more adapted to the local climate.

System buildup: Retention layers such as fleece and mineral wool increase water retention and reduce irrigation needs. Check out this online Green Roof Retention Modeler that lets designers vary the green roof buildup and see how it affects the water stress on the plants.

Owner expectations: Many native plants or succulents can survive heat and drought, but they may look “tired” or go into dormancy from water stress. Irrigation can keep the roof lush and green.

What type of irrigation system works best?

Spray, rotors and drip irrigation work well on green roofs. The choice is project specific taking into consideration of plant types, distribution uniformity, irrigation efficiency and site constraints. A lawn sprinkler or a garden hose work fine on smaller green roofs.

• Spray and rotors produce more uniform water distribution but also higher wastage as water is intercepted by the foliage or evaporates into the air instead of going to the root zone. Larger, taller plants around the spray heads can sometimes block the water stream, too. Irrigation efficiency typically ranges 0.4 – 0.7 for spray and 0.6 – 0.8 for rotors.
• Drip lines deliver water right to the root zone. Typical green roof growing medium is granular in nature so tight line spacing is required to achieve uniform distribution, which can be costly. Retention fleece promotes lateral water movement and allows wider spacing between drip lines. Irrigation efficiency of drip systems is typically higher at 0.7 – 0.9.

Preferred Flow Rates

The following are preferred flow rates used in manual irrigation.

1″ water source:

• Static pressure of 70 psi at point of connection
• 20 GPM (dynamic measurement)

3/4″ hose bib:

• Static pressure of 70 psi at point of connection
• 12 GPM

How much and how often?

How much irrigation is necessary? Water green roofs deeply and infrequently to develop resilience.

A good guide is the water retention capacity – expressed in l/m2, equivalent to mm. This is the amount of water needed to bring a green roof from bone dry to saturation. This will give the green roof a good soak without wasting water.

The green roof may feel dry on the surface but remain moist deeper in the substrate. Lift the layers (for soilless systems) or dig your finger deep into the growing medium. If the substrate feels moist, there is no urgency to water. Otherwise, water to saturation as above.

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