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Under the green roof: Ground control in northern California

Case Studies | October 1, 2008 | By:

Stabilizing a “living roof” with biodegradable geotextile: Interlocking plant trays help the new California Academy of Sciences building blend into hilly surroundings.


Lead architect Renzo Piano envisioned the new California Academy of Sciences (CAS) building as an organic part of its setting, San Francisco’s Golden Gate Park. So his design crowned the building with a 2.5-acre living roof, a swath of native vegetation blanketing 7 mounds that echo throughout the surrounding hills.

Said Piano: The roof design “is like lifting up a piece of the park and putting a building under it.”

The mounds posed a major challenge for the green roof design firm tasked with realizing Piano’s ambitious design. Ultimately, the modular, biodegradable geotextile system that was invented specifically for the CAS building made the mounds—and thus the vision—a reality. The CAS green roof won a 2008 Award of Excellence from Green Roofs for Healthy Cities, the industry’s trade association.

Building description

The new California Academy of Sciences building will help the 155-year-old institution fulfill its mission “to explore, explain, and protect the natural world.”

When done and all museum moves are completed, the building will shelter more than 20 million natural history specimens, the Kimball Natural History Museum, the Steinhart Aquarium, the Morrison Planetarium, 8 scientific research departments, the world’s deepest living coral reef exhibit, a 4-story rain forest, and 20 African penguins. (View the “PenguinCam” at

A pilot project of San Francisco’s ambitious green building legislation, the CAS building is certified LEED-platinum. LEED (Leadership in Energy and Environmental Design®) is a nationally recognized program administered by the U.S. Green Building Council (www.usgbc. org/leed).

In total, the CAS building is 410,000ft2, with about 100,000ft2 as public space. The roof is 197,000ft2, which includes about 2.5 acres of green roof and 2 acres of solar panel canopy. The green roof is officially named The Osher Living Roof, in honor of the San Francisco-based Bernard Osher Foundation, which donated $20 million toward the new building.

The green roof is rectangular. (“It’s about 1 football field long and about 1-and-a-half wide,” said Paul Kephart, executive director of Rana Creek, the architectural firm on the project.)

A viewing platform in one corner will allow visitors to see the groundbreaking roof up close. In the center, a glass ceiling allows natural light to fill the rain forest exhibit below.

Seven mounds designed to mimic the hills of San Francisco surround the glass ceiling. Three large mounds with slopes in excess of 60° flank the glass dome in a triangular formation. Four smaller mounds sit between the large mounds. The mounds range in height from 9-25ft. Small skylights speckle 2 of the 3 large mounds. The skylights operate automatically, opening and closing vents as necessary to maintain the building’s interior at an optimum temperature.

A band of 60,000 photovoltaic cells form a perimeter around the green roof; the cells are expected to generate approximately 213,000 kilowatt-hours of energy annually, fulfilling up to 10% of the building’s total electricity requirements.

Gravity and the BioTrays

The biggest challenge that the architects faced when designing the nuts and bolts of the green roof was, essentially, gravity: “Soil retention on the curvilinear, steep domes,” said Kephart, the Rana Creek executive director and holder of the patent on BioTrays.™ “How we would retain the soil on those steep slopes was a major design question.”

Designers did a feasibility study of existing green roof soil-retention products, but none proved suitable for the CAS building’s severe slopes. Then architect Renzo Piano had an idea, explained Kephart. “He asked me to develop a biodegradable container that could be used to put the plants on the roof and then [the containers] put on the roof.” That is, Piano asked him to create containers that could be used to transport the seedlings to the roof site and then installed in the roof once they were there.

Kephart, who had been working with plants and biodegradable containers of various sorts for 20 years, took the idea a bit further. He created a sort of plantcontainer “Swiss Army Knife.” BioTrays are 3in. deep and 17in. square, porous, interlocking, and biodegradable. They simultaneously serve as plant propagation, transportation, and installation. This tri-purpose design saved time, labor, and money in the CAS green roof project.

BioTrays featured 2 other innovations. Traditional modular green-roof trays are made of plastic. BioTrays are made of coconut coir—a fibrous coconut-industry waste product from the Philippines—and a type of tree sap, natural latex. Once installed in a green roof system, the coir biodegrades during the course of several years, giving the plant roots time to grow through the trays and interlace with the roof’s soil-layer medium, forming a living mat that will secure soil even on steep slopes.

A third innovation, in Kephart’s words, was “the inoculation of the coconut coir with mycorrhizal fungi, a beneficial biological inoculant for the growing medium and the tray. The fungi lock phosphorus in the soil, helping plant development and creating healthy roots that retain more water.”

Together, these 3 innovations made the BioTrays an ideal solution to the problem posed by the steep slopes of the CAS living roof. Roof installation of about 48,000 BioTrays—filled with 1.7 million plants—began in May 2007. Green Roof Solutions, a Chicago-based green roof product maker, assisted with the prototyping, manufacturing, and distribution of the trays.

Geosynthetics and the underlayers

As with all green roofs, the CAS building’s living roof comprises multiple layers of materials that together protect the building and provide the conditions that top-layer plants need to thrive. In total, there are 7 layers for the CAS green roof, including the drainage, soil, and plants.

The bottom layer of the CAS green roof is concrete. On top of that is the waterproofing layer: A hot-applied monolithic membrane (including fabricreinforced assembly), a fluid-applied rubberized asphalt. The reinforced membrane weighs approximately 1.4 lbs/ft2. This is the waterproofing layer.

On top of the membrane sits the insulation: 2 layers of 2-in.-thick, highdensity extruded polystyrene, commonly known as “blueboard.”

Next is the drainage layer, critical for preventing soil erosion and root rotting.

“I used a reservoir board underneath the soil,” explained Kephart. “Essentially, I tried to subsurface or subflow as much water as possible to get it off the surface so we wouldn’t have erosion.”

The product used for drainage is a geocomposite drainage system that includes a 3-dimensional drainage core and a nonwoven, needlepunched geotextile filter fabric. The filter fabric is bonded to the ridges of the face of the core.

On top of the drainage layer is 3in. of a custom soil medium developed by Kephart. The medium is an organic mineral mix that contains, among other ingredients, mycorrhizal fungi and scoria rock.

Then the BioTrays, which contained an additional 3in. of the soil medium, as well as the seedlings. Kephart estimated that 3,600yd2 of soil medium was used in the CAS green roof.

Running through the entire roof, on a 24ft x 24ft grid, are 8in.-deep, 12in.-wide rock-filled gabion baskets, which further help with drainage and soil retention.

In all, the CAS green roof weighs 2.6 million lbs and cost about $17/ft2, typical for a green roof.

The plants

One of the trickiest parts of designing a green roof system is choosing the green—that is, the plants. If the choices are wrong, the green roof could turn into a brown roof.

For this green roof, distinguished CAS botanist, Frank Almeda, led a team that put more than 35 plant species through their version of Survivor: Steep Dry Roof.

“What we did,” Almeda said, “is we created mock-ups that mimicked the slope of the hills. At the end [of the 2-year testing period], we decided on 9 species. It was a matter of how well they performed. We had them on these mock-ups with very little water and no fertilizer.” They resided on the roof of the old CAS building prior to its demolition.

When the installation and initial grow-in stages of the CAS green roof were completed, artificial irrigation and fertilization ceased, and the testing team left the plants to fend for themselves.

When choosing the plant contenders for the first testing round, the team looked for several characteristics: native to the area, attractive to some form of wildlife, tolerant of low-moisture environments, tolerant of shallow soil, and able to play nice.

“They are all native species,” Almeda explained, “but they don’t all grow next to one another in nature. Everything finds its niche. On a living roof, things are a little different. We have to make sure that anything that is too aggressive doesn’t stay there.”

At the end of the testing period, the team chose 9 hardy winners: 5 annuals and 4 perennials.

“We wanted to create an environment that would attract butterflies and insects and moths and birds, so we chose species that we knew would attract animals to the roof. The reason we didn’t want the entire roof to be grasses is because they are actually wind-pollinated—they don’t attract any insects or birds.”

Almeda calls the perennials, which will be on the roof year-round, the “fabulous 4.” They are: sea pink, beach strawberries, self-heal, and stonecrop. Sea pink attracts moths and butterflies; the fruit of beach strawberries is attractive to native birds; self-heal attracts hummingbirds and bumblebees; stonecrop is attractive to butterflies.

The annuals—no nicknames here— are: tidy tips, miniature lupine, California plantain, California poppy, and California goldfield. These species are attractive to bees, butterflies, and other organisms, Almeda explained, because they produce both pollen and nectar.

The winning plant species were propagated in BioTrays beginning in spring 2007. While the seedlings grew, the trays were open to the elements. An assortment of uninvited plant guests appeared, borne by birds and wind.

“We found an interesting collection of willows growing in the trays,” Almeda said. “I call willows ‘the water hogs.’ What would happen [if the willows were allowed to grow on the roof] is they would steal all the water from the other plants. Also, the soil profile on the roof is only 6in., and willows grow roots much deeper than that.”

In the end, the willows had to go.


Installation of the basic components of the green roof went as planned. The plants were irrigated through summer 2008 to help them get established. So far, they are thriving.

“We’ve had flowers on the roof ever since we started planting it,” said Almeda. “The self-heal has had flowers continuously, the beach strawberry is flowering, the stonecrop is flowering, and so is the sea pink. During the spring season is when we will get a flush of flowering from all the annuals. It’s going to be interesting because what happens when you have annuals in any setting is that no one year is the same as the next. One year you may get a wonderful show of a particular species, and the next year you may get them only in spotty numbers.”

Installation of the green roof continued through the summer of 2008 with the development of an exhibit garden, which surrounds the roof ’s visitor platform. This garden will spotlight about 45 native plant species, including buckwheat and seashore daisy.

“We’re also seeing native species that have arrived on their own,” said Almeda. “Several species of the annual monkeyflower have shown up on the roof.” While not all the native species that arrive will be allowed to stay; some, like the monkeyflower, will be.

The roof is monitored for biodiversity. Bees, butterflies, and hummingbirds have all been spotted.

The CAS green roof promises to benefit people as well as wildlife. The roof will absorb carbon dioxide, a greenhouse gas; provide excellent insulation; stay an average of 10° cooler than a standard roof, which helps to decrease the urban heat island effect; absorb about 98% of stormwater, which could prevent 2 million gallons or more of runoff every year; and extend the lifespan of the underlying roofing materials by up to 50%.

Shelby Gonzalez is a freelance writer specializing in environmental and sustainability issues. She lives in Bolinas, Calif.,

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