The following is an excerpt from Small Green Roofs by Nigel Dunnett, Dusty Gedge, John Little and Edmund C. Snodgrass (Timber Press, 2011). The excerpt is from the section Projects: Green Roofs on Sheds, Garden Offices, and Studios.
Location: Raleigh, North Carolina
Owner and designer: Emilio and Kathryn Ancaya of Living Roofs, Inc.
Context: Private residential, garden building
Size: 8 × 10 ft
Main purpose of green roof: Reduce stormwater runoff, provide habitat for birds and pollinating insects, aesthetic
This new shed was part of the redesigned backyard landscape which included a fruit and vegetable garden, workshop, greenhouse, and green-roofed garden shed. Emilio and Kathryn Ancaya were in the process of redesigning their backyard and needed a garden shed, and they decided this would be a perfect place for a trial green roof. They knew the garden shed roof would be very visible from the house, so they wanted to integrate the building as best they could into the landscape. Additionally, they wanted to attract birds and pollinators to the roof.
Designing and planning
Although they now own a green roof company, Emilio and Kathryn built the shed as their first green roof project, long before the company was formed. The experience was key to their discovery of the potential of green roofs. Emilio and Kathryn designed the structure themselves, based on information from Ed Snodgrass at Emory Knoll Farms, the internet, and various books. ‘The most challenging part was designing the drains. We came up with our own solution which works great,’ says Emilio.
A standard waterproofing membrane was fixed to the roof. On top of this was placed a root barrier, a permeable landscape weed block fabric. Because the roof has a 7:12 (30°) slope, a wood grid system was used to keep the substrate on the roof. The grid was elevated 20 mm (0.75 in) above the roof surface to enable water to drain beneath. Substrate was laid to a depth of 10 cm (4 in) and was composed of a 4:1 mix of expanded shale and compost. Vegetation was introduced as plug or pot-grown plants grown by Emilio or obtained from Emory Knoll Farms and included species of Delosperma, Sedum, Talinum, Euphorbia, and Sempervivum. The roof is watered very occasionally using a garden hose, and an annual application of organic slow-release fertilizer is applied.
Success of the green roof
The roof has delivered many benefits and has been a great success. According to Emilio, the only challenge is the roof ridge, which dries faster than the rest of the roof. ‘The view of the shed from anywhere in the backyard is definitely a great sight to see,’ says Emilio. The green roof significantly reduces stormwater runoff as compared to a traditional roof (such as asphalt shingles, metal) and the roof definitely attracts birds and pollinating insects. Every spring small sunflowers sprout on the roof. These seeds are dropped by birds who feed on the roof after visiting the bird feeders. Cooling was not expected but is a great additional benefit: the building is a cool retreat during hot summer temperatures. And the roof has been a great learning experience, as it was the first of many green roofs for Emilio and Kathryn.
If starting again with the roof, Emilio says that he would do some things differently. They would add water retention fabric under the entire system to help retain or at least slow water drainage, as well as an additional layer over the ridge to help retain moisture along the dry peak. Finally, they would have cut the rafter tails back further from the roof deck edge, because water comes in contact with the rafter tails for prolonged periods when the roof drains.