Rooftop gardens or living roofs offer several environmental advantages. Their increased use in urban areas has been due their ability to decrease water runoff from the roof area. The primary intent of the living roof application is to filter the stormwater and minimize the stormwater runoff from reaching the underground infrastructure.
Environmental Advantages
Rooftop gardens or living roofs offer several environmental advantages. Their
increased use in urban areas has been due their ability to decrease water
runoff from the roof area. The primary intent of the living roof application is
to filter the stormwater and minimize the stormwater runoff from reaching the
underground infrastructure. This is of particular importance in areas with
older infrastructures because the capacity of the runoff could exceed the
capabilities of aging sewer systems. The depth of the planting medium
determines the amount of water runoff. The general rule of (green) thumb is
that the thicker the medium the higher the potential decrease of water runoff.
The vegetation further impacts groundwater by absorbing all of the pollutants
from the rainwater prior to runoff. Studies have indicated that more than 95
percent of cadmium, copper, lead and a portion of zinc can be taken out of the
rainwater, which prevents their distribution into groundwater. The vegetation
also contributes to clean air by filtering and binding dust particles as well
as naturally filtering airborne toxins.
The other significant environmental advantages of living roofs are that they
can help to reduce urban heat island effect, and they provide habitat for
diverse of wildlife.
What FMs Should Know About Living Roofs
While environmental issues and awareness have recently put these designs to the
forefront, their uses have been around for more than 100 years. Rooftop garden
use has become prominent in Europe and other
parts of the world due to explicit governmental regulations. It is estimated
that 10 million square feet of green roof systems were installed in Germany in 1989 and that some city planning
authorities in Germany
are now requiring that green roofs be put on all industrial buildings. Green
roof systems have also been widely used in Switzerland,
Italy, France, Scotland,
Monaco and Scandinavia.
Use in the United States has
been extensive in California and New York, and more U.S.
cities (Chicago, Philadelphia) are mandating their use in
downtown areas.
There is no denying the aesthetic values of rooftop garden construction.
Rooftop gardens virtually transform the single-most mundane component of a
building (the roof) by adding life, beauty and an array of colors that can be
pleasing to the building occupants, and in some instances, similarly pleasing
to the occupants in surrounding buildings. As the use of these constructions in
the United States
expands, it becomes essential that building owners and facility managers become
familiar with their weatherproofing capacity.
Rooftop garden construction poses an array of circumstances that mirrors plaza
waterproofing more than conventional roof systems. The constant presence of
moisture in the system, always avoided in conventional roof systems, increases
the reliance of a properly designed membrane system. The economical and
logistical problems associated with membrane repairs, even repairs that are
relatively minor in nature, further intensify the reliance of the waterproofing
system in this type of construction. The rooftop garden is installed for
aesthetics; the membrane is applied as a permanent waterproofing surface that
must also meet the stringent requirements of the garden design.
Waterproofing
Rooftop garden construction consists of two equally important phased
applications: the waterproofing application and the garden assembly. The
ultimate success of a rooftop garden depends largely on the proper design and
installation of these applications.
There are three waterproofing design elements that are essential to the success
of the system:
1. Support of added weight to the
building. There has to be an inherent compatibility of the rooftop garden with
the structure
that it is applied on.
2. Integrity of the waterproofing
membrane and system.
3. Location and size of the roof
drains.
The waterproofing application in rooftop garden construction typically mirrors
that of plaza deck waterproofing. In this context, the waterproofing membrane
is applied at the bottom of the system in a protected or inverted state. It is
the opinion of this author that inverted membrane application is best suited
for this type of system construction. This opinion is based on findings of
research conducted of rooftop garden assemblies constructed throughout the
world over the past seventy years. In this configuration, the inverted membrane
is protected from garden components, tools and, to some degree, plant roots.
Membrane Considerations
The time and expense associated with the repairs of the membrane prohibit the
use of an “economical” 15 to 20 year membrane system. The membrane system must
be durable enough to resist mechanical damage from gardening tools, the
penetration of plant roots and last, without repair or replacement, for the
life of the building. It is essential that the designer consider membrane
systems that have worked within this configuration in previously successful
applications. Membrane systems that are new to the roofing industry do not have
the performance record required for these applications.
It may be advantageous to consider the use of below-grade waterproofing
materials, which are typically installed in inverted plaza deck construction,
as opposed to roofing materials that are applied in warm roof (insulation over
deck) configurations, for these applications. There is a common misconception
that roof membrane products and below-grade waterproofing products are similar.
This misconception comes from a small number of materials that have achieved
success in both building component areas. In reality, the requirements of these
components end at their waterproofing capacity.
Roof membrane materials are constantly exposed to weathering elements, sun,
wind, rain, snow, ice, etc. For the most part, water buildup is removed from
these areas within 48 to 72 hours, with prolonged exposure concentrated to cold
climate winter months. Roof system materials generally have a service life of
10 to 20 years prior to the institution of remedial actions. Their remedial
construction, whether it is removal or replacement, can be accomplished
utilizing routine construction methods with minor (in most cases)
inconveniences in the building do to ease of accessibility to the
roof.
Below-grade waterproofing products are not exposed to weathering elements;
however, they are exposed to constant water buildup, which in garden
construction could exist for two to three continual months due to saturated
soil. Below-grade waterproofing systems are also continually exposed to ground
and soil chemicals. Properly applied waterproofing systems can have a service
life that matches the life of the building. This is an important characteristic,
because unlike roofing, the waterproofing materials are inaccessible and the
expenses associated with repairs and replacement are substantial, primarily due
to excavation costs.
Rooftop garden membrane construction consists of three components:
1. The waterproofing
membrane
2 Protection
Board
3 Insulation
The Waterproofing Membrane
The waterproofing membrane is the major component in this type of construction.
The membrane material must posses, as its primary characteristic, the ability
to prevent moisture from entering the facility. This key element should take
precedence in the system design over plant type and layout. The rooftop garden
is installed for aesthetics; the membrane is applied as a permanent
waterproofing surface that will also meet the stringent requirements of the
garden design. It is imperative that the membrane meets the highest material
standards.
Rooftop gardens require a membrane that meets the following characteristics:
• It resists root
penetration.
• It resists ultraviolet
rays.
• It can withstand severe
temperature changes and atmospheric conditions.
• It is flexible to meet
building movement at construction joints and intersections with vertical
elements.
• It resists surface wear during
construction.
• It can withstand attacks by
insects and microorganisms, subsoil animals, and soil
chemicals.
• It can stay in place over an
indefinite life span without deterioration.
• It prevents water
penetration.
• It can remain submerged in wet
conditions for substantial periods
of time.
There are materials and systems on the U.S. market that possess these
properties and have performed in these configurations for 30 or more years. As
with all roofing and waterproofing applications, be leery of manufacturers that
provide system warranties that are longer than they have been on the U.S.
market. The best way to access material performance is to visit similar
projects and talk with building occupants about the waterproofing performance
of the materials.
The Planting Medium & Top Dressing
There are various forms of soil composition that are suited for rooftop
gardens. The success of the plantings often depends on the types and depths of
the planting medium. This type of composition is above the scope of this paper.
A landscape architect or consultant should be contacted for these design
requirements.
Bringing Roofs to Life