There’s no cookie-cutter approach to irrigating green roofs, according to Lynda Wightman, Hunter Industries industry relations manager. There are two main distinctions when it comes to care: extensive green roofs, which have 2 to 6 inches of soil and are generally comprised of native plants and sedums, and intensive roofs, which are 6 inches and deeper and are essentially conventional gardens that happen to be located on the roof of a building.
In extensive green roofs you have more coarse aggregates that have poor capillary qualities and do not conduct water effectively. Intensive roof soil is much more like traditional landscapes with a higher level of organics and it conducts water much better.
Because green roof soils are so porous, drip and micro irrigation won’t give you much spread. However, you can irrigate to specific plants. This option is good for smaller areas and planters, but you have to put the tubes so close together that the cost is prohibitive, Wightman said. And if you try using sub-irrigation, you end up over-irrigating.
To get around that problem, wrap applications can be used to spread the water better. In this case, a geotextile fabric is wrapped around the tubing.
Mat tubing is another option. Here, inline tubing with a wrap is attached to a mat that’s then buried at root level. “Now you have a total wicking which is what you need for that coarse aggregate type of soil,” Wightman said. This type is very common in Europe.
Wightman does not recommend using point source PVC or micro sprays for extensive roofs. “If you have any kind of wind at all, it’s gone immediately,” she said.
Rotating stream nozzles have a larger droplet size to combat wind loss, lower precipitation rates, various pop-up heights, check valves for slopes and very uniform applications. It also cleans plant surfaces as it waters.
“Don’t use fixed sprays. They dump out a ton of water in a little tiny area,” she said, which means any fertilization will be leached through immediately.
The three-legged stool.
“I don’t think we put enough forethought into this whether you’re designing, installing or maintaining green roofs,” Wightman said, noting that all three are crucial to a sustainable green roof.
“This is the roadmap for the installation,” she said, noting that the key is making sure it’s easy to read.
Dual systems are becoming more popular with underground and aboveground systems. “They’re more costly, yes. Healthier plant material? Yes,” she said.
Be sure to include installation details as well as a management plan and a schedule of controllers for all seasons. “Site inspections are cheap insurance policies. They ensure that what’s getting installed is as per your plan,” she said.
Make sure you’re using a certified licensed contractor or company, she said, and make sure the installer gets training from the manufacturer. It’s also key to understand the local codes and install the system to the specifications of the design.
“So many think that when you design and install a green roof, you don’t have to maintain it,” Wightman said. Her tips include making sure you have good maintenance access to the roof, using a certified contractor and manufacturer training.
And make sure you know how to winterize, she said. “How are you going to get the compressor up there? How are you going to start up in the spring?”