Here’s what you need to know before making this eco-friendly decision.
BY DAVID LENNAM
If You Can See It, It’s Worth It.
That’s not the word-for-word dictum of Bruce Greenway, but sums up the way he discusses green roofs with his clients.
The principal of Greenway Studio Architecture has designed seven green roofs on local residences, including on his own home, and maintains that the esthetic is much of the appeal for a choice that’s more expensive than traditional roofing.
As fanciful as a living roof of vegetation is, and as much a nod to eco-friendliness, they’re like a frill that often gets deleted when a new home moves from blueprint to assembly.
“For me, when I work on a project, the main thing for a new build is: Can you see it? If it can’t be seen and experienced, it tends to get cut when the budget happens and the real money comes in,” Greenway says. “With a small-scale roof, a big part of it is a psychological or emotional attachment — does it connect with your life?”
Green Benefits
The immediate benefits of green roofs often come down to scale. On a large commercial building, for instance, the rewards of that green roof are multiplied to a point where there may be a financial tradeoff. Less so on a house, or part of a house.
A green roof carries out a number of functions, though: providing shade, removing heat from the air and reducing temperatures of the roof surface and surrounding air, resulting in lower energy costs for internal heating and air conditioning.
Green roofs absorb up to 75 per cent of the rainfall and mitigate from 65 to 94 per cent of the runoff. That means less material flowing into storm sewers. There are studies that show water temperature running off a green roof is cooler than that coming from a standard roof — good news for protecting fish habitat from heat pollution. And there is the absorption of C02, plus the filtering of airborne pollutants.
And don’t overlook sound insulation and the potential for growing food on the roof.
From a purely financial perspective, however, green roofs cost more to build than conventional roofs — and they must meet certain carrying capacity standards. The underlying structure might need bigger beams, for instance, and an engineer will almost certainly be involved.
But that up-front expense can be recouped over the life of the roof. A green roof can last for 50 years — more than twice as long as standard asphalt roofing on a flat surface.
Weathering In The Green Age
Greenway explains how a standard flat-roof system is susceptible to weather.
“It gets sun, rain, hot and cold, freeze-thaw, freeze-thaw, and the roof will eventually have trouble and cause a leak,” he says.
A green roof, however, mitigates the extremes of temperature, moisture and the speed of change on the roof because of its level of coverage.
“You’re basically putting a shade and insulation over it,” says Greenway. “Because you have about four-inches of growing material, when the frost comes in, the frost stays on the top, not on the roof.”
The Flora
Choosing what greenery to grow on your roof is entirely dependent on geography.
If you’re deep in a rainforest, mosses and shade plants will find success. In a sunny location, drought-resistant plants that need little watering, like grasses, are a must.
“If it can live in the sun on a rock, it will probably do OK on your roof,” Greenway says.
Kevin Kersten of BC Green Roof grows sedum plants used as vegetation on green roofs and acts as a consultant to those keen to experience this eco-friendly alternative.
His Shawnigan Lake farm has 150,000 square feet of plants ready for the roof, including plant plugs, individual plants in pots and the most convenient options, sedum mats and green roof modules in four inches of growing medium, ready to just lay on a prepared roof.
Kersten’s expertise lets him work with architects, builders and landscape designers putting together green roofs.
“We bridge gaps between individual specialists that may exist,” he says.
Kersten acknowledges some regular stewardship of a living roof is essential.
“I always say, don’t spoil it; nature will take care of it,” he says. “That’s with five or six inches of growing medium. If you go with less, you might want to step in with some artificial irrigation.”
Layering Up: These photos illustrate how a green roof is assembled on one of architect Bruce Greenway’s projects. It’s a layer-by-layer process with membranes and filters beneath. Finally, mats of sedum are placed across the top of the roof. Note the slight slope of the roof itself that allows it to be seen and experienced from ground level. Photos by: Greenway Studio Architecture.
When Nature Takes Over
Sometimes a green roof just appears.
We’ve all seen the unintentional side effects that come from ignoring standard roof maintenance. Our moist West Coast climate can cause a buildup of destructive moss.
This kind of “green” roof is not the same. Without the appropriate layers planned to separate plants from building materials, rot and destruction is settling overhead.
While hiring a contractor and landscaper with green-roof experience is advisable, Kersten says the DIY approach is simpler than expected and relatively inexpensive.
“If you can put a tarp over your car in the winter and you can walk with a wheelbarrow and shovel some soil, you can put a green roof on,” he says.
Installing a green roof on an existing building, however (particularly one that does not feature a fairly flat roof) isn’t impossible, but it’s not recommended. They’re better on new builds, and sometimes just as a feature — over a garage, a studio space or an extension.
Greenway, who has helped friends put on a DIY green roof, says with the right support and materials, it can be as simple as buying a pond liner and going out to gather the plants that you like — though that support is coming from a professional, after all.
Make A Living
Are there disadvantages to having a green roof? Other than the initial cost, they’re just a different kind of maintenance. That might mean getting up there and doing some weeding.
Kersten cautions that the primary concern to watch for is seedlings brought onto the roof by birds. It’s good to go up once or twice a year to remove them. Otherwise, invasive root systems can cause damage, even if there is a root-penetration barrier.
“You don’t want to have any maple growing on your roof,” he cautions, “or Scotch Broom.”
While you may have a secure green-roof vision, Greenway suggests a preliminary discussion with your insurance agent will give you a list of what you need to do.
He also says a green roof, like solar panels, has to pay its way either financially or psychologically.
“It’s got to make a living,” he laughs.
Greenway believes there’s an audience of architects, designers and landscape designers who want to do green roofs, but can’t make them happen. He has heard from colleagues whose best-laid plans for green roofs in their projects were abandoned due to costs.
“Mine get built not because the clients first see these advantages and weigh the pros and cons,” he says. “My clients want these roofs realized because they fall in love with the idea and can’t imagine the building without it.”
3 GREEN ROOF STYLES
1. Intensive (Typical for large-scale roofs.)
These involve a thick layer of growing material, as much as two feet, and are ideal for planting small trees or creating a park designed to be walked on.
2. Extensive (Typical for a house.)
These have a much thinner layer of growing material, four to eight inches, and are less expensive with lower maintenance.
3. Ultra Extensive (For creating a green esthetic.)
These have almost no thickness and are basically sedum mats with a thin water-retention fleece and a drain layer. They are lighter, with less structure and limit what can be planted.
