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Case Studies, Features | September 1, 2008 | By:

Stabilizing a “living roof” with biodegradable geotextile and interlocking plant trays helps 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 1 hectare (ha) living roof, a swath of native vegetation blanketing seven mounds that would echo the surrounding hills. In a press release, he is quoted as saying that the roof design “is like lifting up a piece of the park and putting a building underneath.”

The mounds posed a major challenge for Rana Creek Living Architecture, Carmel Valley, Calif., the green roof design firm tasked with realizing Piano’s ambitious design. Ultimately, the modular, biodegradable geotextile system Rana Creek invented specifically for the Academy 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 green roof infrastructure trade association, in the “Extensive Institutional” category.

The building

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 completed and fully moved into, the building will shelter more than 20 million natural history specimens, the Kimball Natural History Museum, the Steinhart Aquarium, the Morrison Planetarium, eight scientific research departments, 20 African penguins, the world’s deepest living coral reef exhibit and a four-story rainforest.

A pilot project of San Francisco’s ambitious green building legislation, the CAS building is certified LEED® Platinum. LEED, short for Leadership in Energy and Environmental Design, is a nationally recognized program administered by the U.S. Green Building Council (USGBC). In all, the CAS building is 36,900m2, of which roughly 9,000m2 will be public space when the building opens in fall 2008. The roof is 17,730m2, which includes about 1ha of green roof and 0.8ha 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 a football field long,” said Paul Kephart, executive director of Rana Creek, “and about one and a half wide.”) A viewing platform in one corner will allow visitors to see the groundbreaking roof up close. In the center, a glass ceiling seemingly floats over the piazza below.

Seven mounds meant to mimic the hills of San Francisco surround the glass ceiling. Three large mounds with slopes in excess of 60° flank the glass ceiling in a triangular formation. Four smaller mounds sit between the large mounds. The mounds range in height from 2.7m –7.6m. Small skylights speckle two of the three 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 forms a perimeter around the green roof; the cells are expected to generate approximately 766,800MJ of energy annually, fulfilling up to 10% of the building’s total electricity requirements.

The BioTrays

The biggest challenge that Rana Creek Living Architecture faced when designing the nuts and bolts of the green roof was, essentially, gravity. “Soil retention on the curvilinear, steep domes,” said Paul Kephart, 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.”

Rana Creek 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 roof and then be put on the roof.” In other words, Piano asked him to create containers that could be used to transport the seedlings to the roof site, and installed in the roof once there.

Kephart, who had been working with plants and biodegradable containers of various sorts for 20 years, took the idea further. He created a sort of plant container “Swiss Army Knife.” BioTrays are 76.2mm deep and 431.8mm square, porous, interlocking and biodegradable. They work for plant propagation, transportation and installation. This tri-purpose design saved time and labor in the CAS green roof project, cutting down on costs.

BioTrays featured two more innovations. Existing modular green roof trays were 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 over the course of several years, giving the plant roots time to grow through the trays and interlace, forming a living mat that will secure soil on even steep slopes.

The 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 locks phosphorus in the soil, helping plant development and creating healthy roots that retain more water.”

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

The underlayers

Like all green roofs, the CAS building’s green roof comprises multiple layers of materials that together protect the building and provide the conditions that plants need to thrive. Rana Creek designed all seven layers of the CAS green roof, including the drainage, soil and plants, and wrote the specifications for installation.

The bottom layer of the CAS green roof is concrete. On top of that, Jensen Corporation Landscape Contractors hot-applied membrane (Fabric Reinforced Assembly), a rubberized asphalt. The reinforced membrane weighs approximately 68.8kg/m2, according to the company. This is the waterproofing layer. On top of the membrane sits the insulation: two layers of 50.8mm-thick high-density extruded polystyrene.

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 American Hydrotech’s Hydrodrain®, a geocomposite drainage system comprising a three-dimensional drainage core and a nonwoven, needle-punched filter fabric. The filter fabric is bonded to the ridges of the face of the core.

On top of the drainage layer, Jensen Corporation Landscape Contractors laid down 76.2mm of a custom soil medium developed by Kephart. The medium is an organic mineral mix that contains, among other things, mycorrhizal fungi and scoria rock. Then came the BioTrays, which contained an additional 76mm of the soil medium, as well as the seedlings. Kephart estimated that 3,090m2 of soil medium were used in the roof.

Running through the entire roof, on a 7.3m by 7.3m grid, are 203mm deep, 304.8mm wide rock-filled gabion baskets, which help with drainage and soil retention.

In all, the CAS green roof weighed 1.18 million kg and cost about $17.00/sq ft, typical for a green roof.

Active ASTM standards developed by the ASTM Green Roof Task Force under E06.71 include:

  • E2396-05 Standard Test Method for Saturated Water Permeability of Granular Drainage Media [Falling-Head Method] for Green Roof Systems
  • E2397-05 Standard Practice for Determination of Dead Loads and Live Loads associated with Green Roof Systems
  • E2398-05 Standard Test Method for Water Capture and Media Retention of Geocomposite Drain Layers for Green Roof Systems
  • E2399-05 Standard Test Method for Maximum Media Density for Dead Load Analysis of Green Roof Systems
  • E2400-06 Standard Guide for Selection, Installation, and Maintenance of Plants for Green Roof Systems

The plants

One of the trickiest parts of designing a green roof system is choosing the green — that is, the plants. If you choose wrong, your green roof will be a brown roof. For the CAS green roof, distinguished Academy botanist Frank Almeda led a team that put more than 35 plant species through Survivor: Steep Dry Roof.

“What we did,” Almeda said, “is create mock-ups that mimicked the slope of the hills. At the end of [the two-year testing period], we decided on nine species. It was a matter of how well they performed. We had them on these mock-ups with very little water and no fertilizer.” The mock-ups 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 are complete, the roof will not be artificially irrigated or fertilized, so 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.

“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 four.” They are sea pink, beach strawberries, self heal, and stonecrop. Sea pink attracts moths and butterflies; beach strawberries’ fruits are attractive to native birds; self heal attracts hummingbirds and bumblebees; stonecrop is attractive to butterflies.

The winning plant species were propagated in BioTrays at the Rana Creek Nursery 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 15cm, 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 are still being irrigated (as of June 2008) to help them get established. So far, they are thriving.

“We’ve had flowers on the roof 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 continues 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 being 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 6°C cooler than a standard roof, which helps to decrease the urban heat island effect; absorb about 98% of stormwater, which could prevent 7.6 thousand m3 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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