Soka-Bau Green Roof Case Study - SCHL

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nnovative buildings
Soka-Bau Green Roof Case Study
Wiesbaden, Germany
Background
Soka-Bau (Zusatzversorgungskasse des
Bauhauptgewerbes VVaG), which
administers an extended pension fund
for the German construction industry,
recently built a new administrative
complex in Wiesbaden, the capital of the
regional state of Hessen in west-central
Germany. It rents some of the building
to tenants, such as the City of
Wiesbaden.
Soka-Bau is a useful case study because
it demonstrates several aspects of green
roof design and maintenance, including
• a range of greening types, such as
extensive green roofs, intensive green
roofs and vertical plantings;
• the use of plant structure to optimize
interior and exterior environments;
• novel design features, which should
inspire
designers of green roofs;
• lessons learned.
November 2004
Photo courtesy of Anneliese Latz
Figure 1: Conceptual landscape plan

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Design Description
General
The project’s primary goal was to create optimal
environmental conditions (for example, climate,
lighting, energy) for the building occupants. The
architect and landscape architects thoroughly
analysed the site and the potential effects of sun,
shade, wind and other climatic variables on the
building. The analysis led to special design solutions
that used planting structure to attain optimal
conditions for natural light and ventilation, solar
shading and passive air conditioning. The site
includes both extensive- and intensive-green
roof elements.
Figure 2 shows the main building components
of Soka-Bau (labelled A–D). The backbone of the
new building is a ground-level concourse (A),
on which are perched four overhanging office
blocks (B). East of the concourse is an underground
parking structure and to the west, several
underground cisterns. The large building to the
south (C), with enclosed courtyards is the
renovated former administrative building. The
small building (D) at the southern-most end of
the site is a kindergarten.
Extensive green roofs
Figure 2: Schematic site plan
Extensive-green roofs cover the upper levels of the office blocks (B), as well as the
renovated building (C) and the kindergarten (D). They are not accessible to the public
and cannot be seen by the building occupants. These green roofs serve ecological functions,
such as retaining stormwater and fostering urban biodiversity. Like all green roofs, they
also help to extend the life of the waterproof membrane by providing protection from
UV radiation, temperature extremes and mechanical damage.

The design consists of gentle furrows placed diagonally to the building. The
substrate is 10 cm (4 in.) deep in the valleys and 15 cm (6 in.) deep at the ridges.
The valleys are planted with sedum and other low-growing plants. The ridges
support slightly larger plants, such as iris, wildflowers, grasses and aromatic herbs.
A total of 15,000 bulbs were added between the plants and several moss species
established themselves voluntarily.
A weather station on one upper roof measures climate variables, including rainfall
and wind speed. A central computer stores and analyses data, and the rainfall
data automatically controls the irrigation system. In high winds, the light louvres
on the building facades are automatically stabilized.
Intensive green roofs
The boxwood bands, courtyards and terraces are all intensive green roofs and
were detailed as shown below. Intensive green roofs employ deeper growing
plants, which support larger root systems.
Boxwood bands
Figure 3: Schematic section—elevation
Curvilinear bands of boxwood are located atop the connecting concourse between
the office blocks. This area is prone to air turbulence and the swirling boxwood
hedges reduce the effects of the turbulence. The spiralling pattern punctuated with
roses is the designer’s representation of turbulence and appears like arabesque
filigree when viewed from the offices above. The idea was conceived when the
landscape architect collaborated with a specialist from Great Britain, who tested the
building and the plant structures in the wind channel. The preliminary architectural
design proposed curved corners for the office blocks, which would have effectively
reduced the wind channel, but cost constraints forced shelving of the design.
As Figure 6 shows, from the air Soka-Bau is distinguished from surrounding
buildings by the imaginative plant patterns on its roofs.
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Courtyards
Nestled one level below the boxwood bands are the courtyards, which can be viewed
from above or from adjacent meeting rooms. The design consists of colourful gravel
paths, a few taller plants, such as Kousa dogwood among interspersed ferns and other
woodland plants, and stacked limestone sculptures.
Terrace
Source: Landeshauptstadt Wiesbaden
Figure 6: Aerial photograph showing the distinctive structure of the roof gardens
On the same level as the courtyards there are terraces to the east and the west edges
of the concourse. The western terraces are wider and a little sunnier than the eastern
terraces. Curved metal bands separate fine gravel surfaces of different colours. Boxwood
hedges planted along the curves gradually slope upwards as if they were bending
towards the light. As with the swirling boxwood bands, these hedges help to buffer the
wind that travels from the southwest towards the northeast. The form is different because
the wind is gentler in this location.
The gravel surfaces are sparsely planted with bulbs and other low-growing species that
come and go with the seasons. The shaded areas contain shade-tolerant species such as
geranium. The western terrace, next to the cafeteria, is an outdoor dining patio and is
partially covered in stone slabs matching the typical curves.

Other types of greening
Groves and hedges
The tree groves to the east are planted above the underground parking and the groves
to the west partially cover the water cisterns. The growing medium is 1 to 1.2 m
(3.2 to 3.9 ft.) deep. Each area between the office blocks is planted with a grove
of fine-leaved trees (Gleditsia, Robinia or Crataegus), which provide summer shade
while still letting in natural light. The meadow mix contains grasses, clover, squill
and iris. The privet hedges adjacent to the west side of the main concourse are
planted in curving rows that are an extension of the boxwood hedges on the
terraces above. Both the grove and hedge elements are intended to buffer the
prevailing southwest winds and cool the western facades.
Light courts
The light courts are at the lowest level and provide natural light to the
underground offices and natural ventilation to the underground parking. Vines
that wind around wire cables are planted along the parking structure as a green
facade. They are intended to partially shield the view of parked cars from the
office windows opposite. Vine growth will be restricted so that it does not
block natural ventilation for the open parking structure.
The light courts are a challenging environment for plant growth as they are in full
shade, vulnerable to cold drafts and, because of the overhanging structures, do not
get as much precipitation. The designers used vertical elements, such as columnar
oaks and vines and limestone sculptures on yellow-white gravel, to lighten the space.
Fog irrigation brings the necessary humidity. The light courts have a primordial
quality with the limestone sculptures reminiscent of dinosaur fossils set amid
ferns, vines and fog.
Technical aspects
Growing media
The extensive green roofs are built using the single-layer construction method,
in which the drainage layer and growing medium are combined (see Figure 4).
The resulting growing medium has a relatively high pore volume that supports
rapid drainage.
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Courtesy of Wigbert Riehl. Translated by Goya Ngan.
Figure 4: Extensive green roof (single-layer construction) cross-section
In this case, the substrate is composed of lava rock, pumice and zeolite with a particle size
distribution of 1–10 mm (.03–.39 in.) It does not contain any organic material, a composition
German landscapers have used successfully for more than 10 years. It has several advantages
over organic-rich growing media. It does not decompose over time and the particle size
distribution remains stable. It is slump-free after installation and compression. Other
characteristics include a silica-buffering system, pH regulation, high cation-exchange capacity,
low dissolvable-salt content and a high nutrient-retention capacity. After fertilizer has
been applied very little of it washes away. In fact, water off these extensive green roofs was
tested at a laboratory and found to conform to regulation drinking-water quality.
Because the roofs are high and subject to strong wind load, the growing medium also
had to conform to requirements for resistance to wind uplift. Finally, the growing
medium has a high frost-tolerance, which limits physical weathering and the hydraulic
changes that would otherwise reduce the permeability of the substrate.
The growing medium for the intensive green roofs is lava rock, pumice and zeolite with a
particle size distribution of 0-8 mm (0–.3 in.) (see Figure 5). Added to it is a 6.5 per
cent weight organic component of steer manure and green compost. Many of the
characteristics are similar to the single-layer, extensive-growing medium. However, it has a
higher water retention capacity, higher nutrient retention capacity and supports stronger
plant germination and growth.

Irrigation
All the roofs have drains and downspouts that connect to two underground cisterns on
the west side of the property. These are connected and each has a capacity of 1.35
million L (296,958 gal.). The entire irrigation system is supplied by water from the
cisterns. A separate cistern permanently filled with city water is used for the fire
sprinkler system within the building. Roof water is not used for the sprinklers
because the levels fluctuate and the possibility of sediment clogging the sprinklers.
There are three different irrigation systems at Soka-Bau. There are soaker hoses along
the hedges and in a ring around each tree. In the light courts, fog nozzles are used both
for irrigation and esthetics. Spray sprinklers irrigate other planted surfaces, such as
between hedges and the extensive roofs. They are only required on the extensive
roofs during the establishment period and in the future will probably only be used
during extreme drought.
The spray sprinklers at the boxwood bands and terraces are not necessary for plant
growth, as the soaker hoses provide enough irrigation, but they create a fresh, humidified
microclimate. The sprinklers turn on in the early morning to simulate the valley inversion
effect. When employees come to work, the environment is cool and refreshing.
Maintenance
Courtesy of Wigbert Riehl. Translated by Goya Ngan
Figure 5: Intensive green roof cross-section
Maintenance is by a full-time, highly qualified gardener, hired during construction.
Because the gardener worked with the landscape architect and contractors, the
aims of the design can be implemented.
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The landscape maintenance requirements at Soka-Bau are high: the gardener has kilometres
of hedges to prune, along with about 200 species and 90 varieties of plant to care for.
During the establishment period, he follows a strict, weekly weeding schedule in the
hopes of reducing this to four or five weeding sessions a year.
Some plants, such as the trees and shrubs in the courtyards, have to be heavily pruned
back as the design supports only a limited weight and wind loading. The grass and meadow
mix under the groves is mowed in early June and late October. Integrated pest management,
such as releasing natural predators, avoids chemical pesticides. The boxwood hedges are
fertilized every three months with a chemical fertilizer and yearly with chicken manure.
Lessons learned
Soka-Bau, with its enormous scope and progressive design, was an excellent green-roof learning
ground. The following lessons learned from studying Soka-Bau can be applied to many
green-roof projects and should help future green-roof designers and builders avoid mistakes.
The importance of guidelines
Designers in Germany are fortunate to have Green Roof Guidelines which thoroughly
cover the design, installation and maintenance of green roofs (FLL–Forschungsgesellschaft
Landschaftsentwicklung Landschaftsbau e.V.).
These guidelines correspond to the general Flat Roof Guidelines (ZVDH–Zentralverband des
Deutschen Dachdeckerhandwerks). One of the parallel requirements for roofs with less than
five-degree slope is that waterproofing at the parapets should extend at least 10 cm (4 in.)
above the finished grade or planting level. The parapet acts as an emergency overflow if drains
become plugged and the roof floods. Damage to the adjacent facades and roof penetrations is
avoided by extending the waterproofing at least 15 cm above finished grade at these locations.
The trouble on this project was at the extensive green roofs, where the architect designed
the parapet flush with a concrete slab edge, which itself was flush with the planting level. These
roofs are virtually flat (6-cm {2.3-in.} grade change over 20 m {65.6 ft.}) and contain only one
drain inlet. Although the drainage was tested before planting and found to be adequately
rapid, the roof did eventually flood over the parapet, onto the building facades and
damaged the wooden components. The parapet had to be rebuilt so that it was 10 cm
(4 in.) above planting level. As an extra precaution, an eavestrough was added. Now the
rainwater can be retained on the roof and released slowly without causing damage.
Guidelines, like those published by the FLL, do not yet exist in Canada and there are no
specifications relating green-roof construction to existing roofing guidelines or standards.
Check for leaks before installing green-roof components
At the terrace level, there were leaks in the waterproofing membrane. They were
discovered only after the planting was complete. The plant material and green-roof

components had to be completely removed for the membrane to be repaired.
Because this took place during the growing season, some of the terrace plants
died and had to be replaced.
This emphasizes the importance of properly checking for leaks before installing of
green-roof components. This can be done at the division between structural-trade
work and landscape-trade work, as shown on the details for extensive and intensive
green roofs in Figures 4 and 5.
Provide maintenance access and storage
All green roofs require access for maintenance personnel and equipment. The
architects chose form over function and did not provide access to the boxwood
bands because it would visually spoil the detailing of the facade.
The hedges need frequent tending and the only access is by ladder from the terraces
or through the washrooms, where there is a very narrow door blocked by a 70 cm
(2.3-ft.) high horizontal bar. Once through this obstacle, maintenance personnel
must jump from one ledge to another, carrying their tools and buckets of hedge
clippings. There is an ongoing discussion with the architect about building a door
in the facade. Gardening tools and materials are stored in an easily accessible side
room of Building C.
Ensure that growing medium is weed-free
Intensive roof greening provides seeds with ideal germinating and growing conditions.
It is important to specify growing medium and plant material that it is weed-free
to reduce labour-intensive weeding. If large weeds and seedlings are not removed
the roots could eventually damage the waterproofing membrane.
Understanding plants
Architects and building owners are generally not accustomed to having plants
interact so intimately with a building. It is important to have a good understanding
of how plants behave and what should be expected of them, depending on whether
it is an extensive-green or an intensive-green roof.
The aim of an extensive-green roof is to create a largely self-sustaining community of
plants. A common approach in Germany is to specify a wide selection of suitable plant
species. During the establishment phase, a process comparable to “survival of the fittest”
occurs, and certain species thrive and a few die out. Meanwhile, volunteer species
brought in by the wind or by birds, establish themselves. Recognizing what should
stay and what should be removed, such as tree seedlings, is the art of green-roof
maintenance. Although the maintenance is only required once or twice a year, it
should be done or at least supervised by an experienced green-roof professional.
Intensive-green roofs are highly artificial environments and the plants are
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completely dependent on proper maintenance to satisfy their needs. The plants require
the correct balance of water and drainage and a continuous supply of nutrients. They
need to be kept at the correct size and are not permitted to outgrow their sub-strate
nor exceed their weight and wind-load allowance.
The fact that plants are living things can create some logistical difficulty with construction
timing. Temporary storage must be available, though it should be recognized that there
are limits to how long plants can tolerate these situations. Plant mortality for certain
parts of this project was high, mostly due to lengthy storage during hot weather.
Architects and building owners should seek the guidance of a landscape professional experienced
with green roofs for design, construction and maintenance. The green roofs at Soka-Bau have
benefited from having a permanent gardener on staff whose on-site training was started
during construction with the landscape architect transmitting the aims of the project.
Highlights
Building type Administrative building complex
Location Wettinerstrasse 7, Wiesbaden, Germany
Status Completed in 2003
Landscaped area 34,000 m 2 (365,972.9 sq. ft.)
Total site area 34,000 m 2 (365,972.9 sq. ft.)
Landscape budget 5 million euros (about $7.9 million Cdn, September, 2004)
Total construction budget 200 million euros (about $318.6 million Cdn,September,2004)
Owner–developer Soka-Bau
Architect Herzog + Partner, Munich
Landscape architects
Green roof contractors
Acknowledgements
Latz + Partner, Ampertshausen (design) Latz + Riehl,
Kassel (construction documents and contract administration)
Fa. A. Fichter, Dreieich (new building A and B)
Fa. Dachland, Mainz (building C)
Wigbert Riehl, Latz + Riehl, www.riehl-LArchitekten.de; Anneliese Latz, Latz + Partner,
www.latzundpartner.de; Landeshauptstadt Wiesbaden, www.wiesbaden.de; Herr Vetter,
Soka-Bau gardener
Written by Goya Ngan, Landscape Architect, ngan@gmx.net. Photographs by the author
unless otherwise indicated.