They are growing from the top of the Natural History Museum of Utah, sprouting from the terraced sides of the LDS Conference Center, putting roots onto the new downtown Public Safety Building and across the street at the Main Library. They are green roofs.
Originating in Scandinavia, where sod torn up for a new home was sometimes replanted on the roof of the dwelling, the idea of what a green roof is has changed and expanded as it’s moved to new continents and different climates.
Here in the arid West, green roofs are not the lush green elevated lawns of the Old World, but they still provide the same benefits: rainwater management, cooling and energy conservation, increased biodiversity and wildlife habitat, noise insulation, water waste treatment and, sometimes, food production.
A DIY green roof doesn’t have to start with your house, and it probably shouldn’t. Learning on small projects—say, a green roof dog house or bird feeder—is a great place to start before graduating to sheds and, down the road, a home. For projects big and small, Here are a few tips from the experts.
Why am I doing this?
Ed Snodgrass, green roof researcher, nursery owner and co-author of Small Green Roofs (2011: Timber Press), asks his clients this question first: Why do I want a green roof? By refining your intent, says Snodgrass, you can decide what design you want all the way down to the plants.
Do you want to grow food on your roof? That will take a certain kind and amount of soil. Do you want an aesthetic flower garden? You might need to plan out an irrigation grid. Or, do you want a waterwise garden that cools your building and feeds pollinators? Get yourself ready for a green roof that won’t be uniform with every inch covered in plants.
Is my roof ready?
A DIY green roof project is a big endeavor requiring serious thought and planning. Designing the roof – what kind of soil will you use, how will you ensure good drainage, what will you plant—can’t even start until you cover the basics.
According to Snodgrass, there is a distinct “hierarchy of failures,” the worst possible of which is roof collapse. Research the weight load for your structure. Utah homes should have roofs with a weight-bearing capacity of at least 30 pounds per square foot (double check the local building codes). A single inch of soil can weigh seven pounds per square foot, so most structures will require additional support to hold the weight of a green roof.
The second worse failure, says Snodgrass, is a leaky roof. Eventually, bad leaks can compromise the structure, but in the short term, you don’t want to put in all the work laying soil and plants only to find the waterproofing doesn’t work and needs to be redone.
Compared to these factors, other complications like soil blowing away or plants dying are minor. They might require more labor, but they won’t hurt the building.
Choosing soil and plants
Scandinavians grew their green roofs in soil taken straight from the ground. Some modern green roofs still use garden soil, but there are plenty of other options. Crushed brick, pumice, expanded clay and shale are good options especially for a drought-tolerant green roof that will require good drainage.
When considering weight, your choice of soil substrate is essential. Specialized mixes like Utelite, an expanded shale aggregate substrate used on green roofs at the LDS Conference Center and the Natural History Museum of Utah, can cut the soil weight by half.
In 2010, the Denver Botanic Garden planted a 1,180-square-foot green roof using it as a test case to research whether green roofs could be water-efficient in water-thirsty areas. Four years later, horticulturist Amy Schneider considers the garden a success.
Since the roof’s installment Schneider has been collecting data on garden’s 112 plants, enabling the botanical garden to compile from these records a list of well-adapted, hardy plants thriving in a garden that receives a one-inch watering every seven weeks.
Schneider hasn’t always been able to water so little, and not every plant has survived. Here is what she’s learned:
• Establishing roots can take some plants two, even three years. The first year, the garden may require daily watering, with each subsequent year needing less as plants put down roots.
• Plant biodiversity equals a healthier dry-conditions green roof. Mixing sizes and types of plants can shade their roots, creating conditions that allow for less water.
• Plant from seedlings, not by broadcasting seeds. Seeds are difficult to establish on a dry-climate green roof since they need constant moisture to germinate.
Here are a few of the best plants for your rooftop garden:
- Penstemon pinifolius, a creeping herbaceous perennial with red flowers that attract hummingbirds
- Ipomopsis, a genus of tall flowering plants with red trumpets (one of the few plants that can start by seed)
Books and magazines
High and Dry: Gardening with Cold-Hardy Dryland Plants, Robert Nold, 2012
The Professional Design Guide to Green Roofs, K. Dakin, L. Benjamin, M. Pantiel, 2013
Small Green Roofs: Low-Tech Options for Greener Living, E. Snodgrass, N. Dunnett, D. Gedge, J. Little , 2011
Journal of Living Architecture http://livingarchitecturemonitor.com/index.php/journal
Red Butte Gardens. Visit the gardens (located at 300 Wakara Way), find gardening tips on their website, attend the plant sales, talk to knowledgeable staff. Fall Bulb and Native Plant Sale on Sept 26 and 27. RedButteGarden.org
Grow Wild Nursery, a local native and water-wise nursery with wildflowers, cacti, grasses, conifers and more, 372 E 2100 S, growwildnursery.com
Denver Botanic Gardens. In the gardens navigator, click on “plant search” and under the “choose location” option enter “green roof” to see information on all current plants on the garden’s green roof. http://navigate.botanicgardens.org/ecmweb/ECM_Home.html
Cities Alive Green Roof and Walls Conference, Nashville, Nov 12-15, 2014. www.citiesalive.org