DeepRoot’s Silva Cell has been installed in dozens of rooftop projects around the world. Whether it is on top of existing structures such as parking garages, or a garden terrace atop a new construction project, Silva Cells can help create structure, bio-retention for tree roots wherever installed.
There are two types of green roof installations, intensive and extensive. Extensive green roofs are only a few inches deep and are geared towards lighter roof loads and small perennials like Sedum. Intensive roofs have a deeper soil profile, that is suitable for the soil volume needs for larger trees. What are the design considerations to keep in mind when utilizing Silva Cells and trees in a rooftop, on structure planting? There are many to consider and keep in mind when specifying/designing Silva Cell into your green roof plan including vertical dimension, aggregate subbase, load, weight, slab dimensions, and anchoring.
The vertical dimension of the space is paramount: the height from the top of the underlying structure to the top of the finished pavement will ultimately determine the size (1X, 2X, 3X, or 4X) of Silva Cell used.
In almost all instances there will be layering or “build-up” of materials like vapor barrier, foam insulation, waterproofing membrane, and drainage products on the top of the underlying structure or between the underlying structure and the bottom of the Silva Cells. DeepRoot has worked with on-structure foam installations numerous times. Our clients have used hi-density 95psi foam in many of these circumstances, particularly in retrofit green roofs where the weight of the soil and trees was not planned for in the ordinal architectural design of the building. When Silva Cells are being installed over natural soil we recommend that they be installed on a minimum of 4” (100 mm) of compacted aggregate to ensure a solid foundation. When it comes to applications where the Silva Cells are being installed over concrete structure instead of natural soil, on the other hand, there can be some flexibility in the depth of the subbase.
Silva Cell systems when used in conjunction with one of our recommended pavement sections are designed to meet the AASHTO H-20 standard of 32,000 lbs per axle, or basically a large truck. Our recommended standard pavement sections range in thickness from 8” (4” of concrete over 4” of aggregate) to 15.5” (3.5” of pavers over 12” of aggregate). However, it is not always necessary to design to the H-20 standard. Frequently on structure applications have limited access, weight restrictions and other similar factors limit the type of vehicular traffic there can be or even completely eliminates it. In these instances, the design team has the opportunity to reduce the loading standard that needs to be met, which in turn can reduce the overall thickness of the pavement section over the Silva Cells and create more room for soil volume.
Weight is a major concern with any roof. Below is a chart of the weight of Silva Cells:
|Each model is approx. 10.33ft2 (.95m2)|
Lastly, it is important to anchor down the frames and plan accordingly during design and construction. In typical Silva Cell applications (not on structure) four 10” spikes per frame are used to keep the base layer of frames from moving around during construction. For on-structure applications, it may not be possible to use spikes. We recommend using small wood spacers between the frames to help keep them stationary. Scrap 2x4s or other dimensional wood is typically used as a blocking.
Here are successful examples of Intensive Green Roof, On-Structure Silva Cell projects
The Lincoln Center Bosque | NYC | U.S.
Mississauga Civic Square | Ontario | Canada
830 combination of 1X and 3X Silva Cells
288m³ ( 10,171ft3) of total soil volume
On Roof Structure
Landscape Architect: Janet Rosenberg & Studio
10 Trinity Square | London | U.K.
432 4X Silva Cells
15,256 ft3 (432 m3) of total soil volume
Subcontractor: Frost Landscapes
Project is a landscaped garden beside a hotel and constructed on a multi-story basement structure. The Silva Cell® System is installed via the same method as normal with an initial layer of coarse granular material and then the Silva Cell positioned with a layer of sand to help with the drainage.
Troy Block | Seattle | US
24 1X, 144 2x, 78 3X Silva Cells
6,529 ft3 (185 m3) of total soil volume
Landscape Architect: Swift Company
Contractor press release: https://www.pacificearthworks.com/project/troy-block/
The Broad Museum Plaza | Downtown Los Angeles | US
120 3X Silva Cells
4,111ft3 (116 m3) of total soil volume
13 trees (100-year old Baouni olive trees transplantedfrom a defunct orchard in Shasta Cascade region)
Landscape Architect: Hood Design
Silva Cells beside structural slab with up-turned beams. The trees are above the freeway and parking garage. The Silva Cells fill in the depth where thousands of cubic feet of soil will support the plazas Olive trees.
St. Regis Hotel | Chicago | USA
48 2X Silva Cells
1,180 ft3 (33.4m3) of total soil volume
On Structure, Trees, Green roof
The third tallest building in Chicago
Landscape Architect: Olin
Silva Cells were installed in 2020. Formerly known as Wanda Tower, The St. Regis Residencies will open in the third quarter of 2021.