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April 2008
guest feature article/recommended readings
A green roofing resource
By James R.
Kirby, AIA
Graphics Courtesy Professional Roofing Magazine
Publisher's Note: This article was originally printed in the April
2008 issue of Professional Roofing Magazine, published by the NRCA.
Reprinted with Permission, © Copyright 2008 National Roofing Contractors
Association With
the proliferation of the sustainability movement within the construction
industry, green roofing is touted as one of the ways to make roof
systems more environmentally friendly. Recognizing this trend,
NRCA began developing a green roofing manual in 2004. And in 2007,
NRCA published
The NRCA Green Roof Systems Manual—2007 Edition.
The
manual provides in-depth information about green roof system projects.
The information contained in the manual was developed by
NRCA member contractors and applies to roofing contractors,
designers, manufacturers, and third-party individuals involved with
green roof system design and installation.
Although some
information is provided about green roof system components, such as
drainage layers, moisture-retention layers and filter fabric, if you use
the manual, you primarily will find specific information about
selection, design and installation of waterproofing membranes
appropriate for green roof systems.
The manual
The
manual is divided into four main sections: General Green Roof Project
Considerations, Green Roof System Guidelines, Green Roof System
Construction Details and Glossary.
Considerations
The
General Green Roof Project Considerations section provides an overview
of general information concerning the design and installation of quality
green roof systems. The information is intended to complement
The NRCA Roofing Manual series and should be used as a guideline to
prepare proper specifications.
The manual defines a green roof
system as "a roof area of plantings/landscaping installed above a
waterproofed substrate at any building level that is separated from the
ground beneath it by a man-made structure. A green roof system consists
of a waterproofing system and its associated components—such as
protection course, root barrier, drainage layer, thermal insulation and
aeration layer—and an overburden of growth medium and plantings."
As with all roof system types, proper design, quality materials and
quality workmanship are needed to achieve a green roof system that will
perform satisfactorily for its anticipated service life. Appropriate
maintenance during a green roof system's service life also is necessary
to ensure its success.
Also provided in the General Green Roof
Project Considerations section is information about pre-bid and pre-job
conferences; material storage and handling; temporary green roof
waterproofing systems; weather considerations (such as high- and
low-temperature installation, wind and precipitation); deck and
structural design considerations; surface inspection; slope and
drainage; expansion joints and control joints; curbs and penetrations;
flashings (such as membrane base flashings, sheet-metal counterflashings
and terminations); quality assurance and water testing; overburden;
warranties; and recommendations for building owners.
Guidelines
The Green Roof System Guidelines section provides in-depth technical
information about green roof systems' design and installation; however,
NRCA recognizes green roof system practices vary considerably
throughout the U.S. because of the variety of conditions that exist and
various roofing materials available.
Selected plant varieties
commonly dictate a green roof system's necessary depth of growth medium.
This also helps guide the specific component design.
Of course,
when reroofing with a green roof system, a roof deck's structural
capacity should be considered. Because of the varying depths of growth
medium, NRCA's manual
divides green roof systems into the following three primary categories: -
Extensive (shallow): a green roof system with an engineered
soil-based growth medium approximately 2 to 6 inches deep
-
Semi-intensive (moderate depth): a green roof system with an
engineered soil-based growth medium approximately 6 to 10 inches
deep
- Intensive (deep):
a green roof system with an engineered soil-based growth medium
greater than 10 inches deep
A fundamental idea behind the manual's development is that a green roof
system is a combination of waterproofing and roofing concepts. Because a
green roof system is likely to have moisture present within the system
most or all the time,
NRCA considers a waterproofing membrane appropriate. And because
green roof systems include detailing similar to all other roof system
types (parapet walls and roof drains, for example), construction details
for green roof system waterproofing membranes are based on termination
concepts NRCA
considers appropriate for all roof system types.
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A semi-intensive (moderate depth) green roof system |
Performance attributes, environmental features and common system
components for green roof systems are included in the manual, as well.
Possible performance attributes include increased thermal efficiency,
noise reduction and extended membrane service life. Likely performance
attributes are stormwater management, increased efficiency of
roof-mounted photovoltaic panels and aesthetic benefits. Economic
benefits, as well as LEED® (Leadership in Energy and Environmental
Design) benefits, also may be realized.
The manual's green roof
system details provide design and installation information for: -
Hot-fluid-applied polymer-modified asphalt membranes
-
APP and SBS polymer-modified bitumen sheet membranes
-
EPDM membranes
- Polyvinyl
chloride membranes
- One-
and two-component fluid-applied elastomeric membranes
Specific installation parameters, including minimum membrane thickness
and ambient temperature recommendations, are provided for each
recommended green roof system waterproofing membrane type. ASTM
International standards for materials and application are provided where
appropriate.
NRCA
recommends membranes be adhered to decks to provide the most stable
substrate for green roof system waterproofing membranes. Therefore,
thermal insulation used in a green roof system is installed above the
membrane or below the roof deck. Positive drainage should be provided by
sloping the structural deck.
Construction details
The
manual's Green Roof System Construction Details section provides
technical information and construction details concerning the design and
installation of quality green roof systems. The section's introduction
is intended to supplement the special notes on the individual
construction details by providing additional information about green
roof system components and deck types.
For each membrane type,
construction details are provided for eight common conditions. The
details are arranged according to membrane type to assist roofing
contractors with the submittal process. Additionally, details are
provided for substrate crack preparation, nonmoving cold joints,
cementitious board joints, sidewall penetrations, area dividers and
paver edges.
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Low parapet flashing for thermoplastic waterproofing
membrane—extensive (shallow) green roof system |
Construction detail concepts for green roof waterproofing membranes are
similar to those pertaining to common roof system types. Some
similarities follow: -
Membrane base flashings are recommended to be a minimum of 8 inches
above the surface of the membrane and 4 inches above the
overburden's top surface. For green roof systems, mechanical
protection is recommended.
-
Sheet-metal rain collars are recommended for penetration details,
and soft-metal pipe flashing is recommended for vent stacks.
-
Sheet-metal counterflashing is recommended for membrane terminations
at skylights, scuttles and smoke vents.
-
Sheet-metal expansion joint covers that provide movement capability
are recommended; however, for green roof systems, expansion joints
should be formed of concrete.
-
Drain details include membrane compressed between the drain bowl and
clamping ring.
-
Sheet-metal copings are recommended to include a continuous front
cleat and appropriate fastening in the back face. The coping's
uplift resistance should meet building code requirements.
NRCA believes an important part of a green roof system project is
the verification of the integrity of the system's waterproofing
membrane. Before subsequent application of necessary components—from
root barrier to growth medium—NRCA
strongly recommends a water test be performed.
A water test may
consist of a flood test or flowing water test depending on the
structural capacity of the deck and slope; each typically lasts a
minimum of 24 hours. A water test documents the performance of a green
roof system's waterproofing membrane before placement of system
components. A water-test verification form is provided in the manual.
A valuable resource
I encourage specifiers, manufacturers and installers of green roof
systems to reference
The NRCA Green Roof Systems Manual—2007 Edition when designing
and constructing green roof systems.
The manual also is
available on CD-ROM. The CD-ROM version contains CAD-formatted
construction details intended to be altered to meet specific project
parameters. To purchase the manual, log on to
shop.nrca.net
or call NRCA's Customer Service Department at (866) ASK-NRCA (275-6722).
James R. Kirby, AIA, is NRCA's associate executive director of
technical communications.
Publisher's Note: You can reach Jim at:
jkirby@nrca.net. See this article online at Professional
Roofing Magazine
here.
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