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Mastering the Design Issues of Installing Solar Photovoltaics on Existing Roofs

May 15, 2011

990 Commercial Street, Palo Alto, CA 94303
Phone: 650-440-6878 • E-mail:
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PV installations on existing roofs have grown rapidly, while roofing consultants’ capability
in addressing roof design, longevity, and roof structure loading has not always kept
This paper discusses types of PV systems, design impacts on the watertightness and
structural capacity of roofs, what PV installation means to roof longevity, selected appropriate
design approaches for the various systems, and pitfalls to avoid. The presentation
includes a case study and descriptions of panelized systems affixed to mounting assemblies,
peel-and-stick products for existing metal roofs, thermally applied or laminated PV systems
for single-ply roofs, and solar PV cells integrated into roof shingles.
KARIM P. ALLANA, RRC, RWC, PE, is CEO and senior principal of Allana Buick & Bers,
Inc. (ABB), which he founded in 1987. He has 30 years of experience in the construction
engineering and design field, specializing in roofing and waterproofing; energy conservation;
and alternative energy installations, including solar, exterior walls, and forensic investigation
of the impacts of water intrusion. Mr. Allana has a B.S. in civil engineering from Santa
Clara University and has received designation as a Registered Roof Consultant and
Registered Waterproofing Consultant from RCI. He is a licensed professional engineer in
California, Nevada, Hawaii, and Oregon and a regular presenter at professional meetings
and conferences.
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Installations of solar photovoltaic (PV)
systems on existing roofs have grown rapidly
in number and are expected to continue
to grow over the next decade. This explosion
has been created by building owners who
want to reduce electricity bills as utility
rates go higher and higher, by the concurrent
development of more cost-efficient PV
materials and components, by the desire of
building owners to “go green” to reduce carbon
footprints, and by time-limited governmental
and utility company incentives. And
when properly planned and financed, PV
systems will (on certain properties) increase
property values for potential buyers and
Roof consultants must keep pace with
changing design implications and become
current with their understanding of rapidly
evolving technology in PV materials and
components. All solar PV systems have
unique installation issues relating to roof
design, watertightness, and roof longevity;
and have structural complexities caused by
higher dead and live loads, wind uplift, and
seismic/thermal movement.
A review of recent history provides an
example of the need for the roof consultant
to pay attention to design implications of
solar PV. In the 1970s, rapidly increasing
prices in electricity, natural gas, and fuel oil
directly created rapid increases in the number
of solar PV systems installed on existing
roofs. Unfortunately, lack of proper design
details led to premature roof failures and
other problems that could have been avoided.
In order to learn from history, provide
the roof consultant a technical outline of
solar PV types, and to provide an intermediary
level overview of design issues, this
paper will present characteristics of PV systems
now on the market, including the
design implications for each type. It will
suggest some roof assessment techniques
to use before solar PV is installed on existing
roofs and will explore the impacts of the
installation of various solar PV types on the
watertightness of roofs.
The paper will review what PV installation
means to existing roof longevity. It will
also review selected structural, electrical,
civil, and mechanical issues, and will show
some of the pitfalls to avoid.
The solar PV systems discussed in this
paper are based on crystalline and thin-film
materials manufactured into solar panels.
Also discussed are applications whereby
thin film is affixed to metal and single-ply
roofs or utilized in building-integrated PV
(BIPV) in curtain walls, roof shingles, and
other building components.
The author will discuss structural
impacts caused by the additional roof load
caused by panels or thin film, how to design
for various types of popular systems and
assemblies now on the market, and some
simple financing strategies to convince owners
to install PV systems.
Briefly discussed is how financial issues
are interconnected with design issues. For
example, successful applications for power
purchase agreements (PPAs), lease-backs,
governmental incentives in the form of tax
rebates, and utility company incentives, all
demand detailed calculations of how the
solar PV system is being designed and how
reduced energy costs will pay for at least a
portion of the solar PV system.
The paper includes lessons learned from
actual case studies and real-life examples of
design issues faced by the roof consulting
professional, in the following format:
1. Definition and description of solar
PV systems
2. Brief overview of solar thermal systems
3. Types of solar PV materials
A. Thin film
B. Crystalline silicon
4. Mounting systems for roof installation
of solar PV
A. Low-slope solar PV thin-film
adhered systems
B. Low slope solar PV panel systems
C. Solar PV panels mounted to
standing-seam metal roofs
D. Solar PV systems mounted via
penetrating mounts
E. Nonpenetrating, nonballasted
solar PV panel roof-mounting
F. Nonpenetrating ballasted solar
PV panel roof-mounting racks
G. Snow loadings and snow drift
mounting and load issues
H. Carport-mounted, shade-structure
mounted, ground-mounted
and tower-mounted tracking
solar panels.
I. Other types of solar PV systems
– concentrators
J. Building integrated PV (BIPV)
5. A brief list of solar PV manufacturers
6. Design issues faced by the roof consultant
A. Roof assessment
B. Physical constraints
C. Solar PV and new roof warranty
D. Sustainability of solar PV system
over time
E. Structural loads created by the
solar PV system
F. Fire code design issues
G. Electrical, mechanical, and
other disciplines
H. Peer review of design
I. Maintenance of the PV system
and the roof
J. Nonengineering design issues
7. Financing solar
A. Price
B. Alternative financing methods
C. Key things to remember for the
economic case
8. Contractors and suppliers
A. Criteria for selecting a solar contractor
B. What questions should be
C. Safety criteria
D. Roof and site integrity – can the
firm provide it?
E. Roofing and construction experience
A solar PV system generates electrical
power through the conversion of solar ener-
gy – first into direct current (DC) electricity,
and then into alternating current (AC) electricity.
In conversion to DC electricity, sunlight
falls on a material such as crystalline silicon
(C-Si), either in the form of mono-crystalline
silicon or poly-crystalline silicon.
Other PV materials include amorphous silicon
(a-Si), cadmium telluride (Cd-Te) and
copper indium selenide/sulfide (CIGS). See
Figure 1 depicting the process that converts
sunlight into electricity.
Crystalline silicon and cadmium telluride
cells are typically assembled into a
PV panel. These panels are then mounted
on ballasted racking systems or penetrating
standoffs on the roof. PV thin-film sheets
containing amorphous silicon are integrated
into PV panels and roof membrane products;
cylindrical roof-mounted products
containing copper indium selenide/sulfide
are installed on racks; and building-integrated
PV (BIPV) components containing
various materials are integrated into curtain
walls and even roof shingles.
PV panel systems can be mounted on
carports and other ground mounts and can
be installed on tracking systems rotating in
one or more axes to take maximum advantage
of the sun as the earth rotates.
The power generated by PV panels, roof
membrane systems, cylindrical products,
and BIPV products varies by system type
and manufacturer. Panels—sometimes
called “modules” in the industry—are connected
in “strings” or “arrays.” Multiple
arrays and strings are connected together
at a combiner box.
The power output from PV systems is
highest on a bright day with relatively mild
ambient temperatures and drops as the
modules heat up (such as on a very hot
day). There is no power output in the dark
and there is no stored energy in the panels/
modules themselves.
Panels are oriented in a manner to provide
the best access to sunlight. This means
they are typically mounted on the south or
southwest roof plane of a steep-sloped roof.
On a low-slope roof, panels can be laid flat,
but the power conversion efficiency is
reduced below the efficiency of steep roof
systems. To take best advantage of sunlight
on a low-slope roof, PV panels are mounted
on racks tilted to the sun at the “azimuth”
angle and compass direction appropriate for
the geographical area and site.
The desired azimuth angle (the orientation
of the panel or module to the sun)
varies by latitude of the site, but actual
installed azimuth angle will vary, due to factors
such as amount of space available for
the array, wind uplift issues, and aesthetic
At the earth’s equator, flat panels would
be at the most efficient angle to the sun. In
the continental United States, the most efficient
azimuth angle of orientation to the
sun varies between approximately 26º and
47º from horizontal, depending on latitude
(See Figure 2 for a solar resource map of the
United States).
The typical roof area required for singlefamily
home use is about 400± sq ft,
Figure 1 — How electricity is generated in a solar PV system.
Figure 2 — Solar resource map of the United States produced by the National
Renewal Energy Laboratory for the U.S. Department of Energy.
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depending on location, generating approximately
2,500± watts. For commercial and
large-scale applications, the required roof
area will vary, based on electrical load and
the size of the available roof area (shading,
mechanical equipment, etc.).
PV panels and other components come
in a large number of sizes, shapes, and
uses, with products available across the
country from more than 25 manufacturers.
Panels are installed by hundreds of contractors
and “integrators.”
Not only are the method of mounting
and type of system important to the roof
consultant; so, too, are the method of
installation and type of the complete PV
system – including the wires and conduits
carrying the DC power away from the panels,
the inverter system that converts DC
power into AC power, the utility company
connection, and other support components.
PV-generated electricity is most often used
only for supplying the electrical needs of a
user, although surplus power can also be
sold to electrical utility companies. See
Figure 3 showing the typical components of
a roof-mounted solar PV system.
In addition to PV technology, solar thermal
energy (STE) systems also take advantage
of sunlight in order to generate heat
and are available in the low-, medium-, and
high-heat ranges. Low-heat systems (less
than 100º) are typically used in pool heating,
space heating, and process heating.
Medium-temperature systems (100 – 204º)
are typically used for domestic or commercial
hot water. High-temperature systems
(under 1500º) are usually large-scale utility-
company-owned systems, designed to
concentrate sunlight to make steam to drive
STE systems are not covered in depth in
this paper, although some of the same
design issues (structural load, wind load,
impact on the roof assembly, roof longevity,
and roof maintenance) come into play.
These thin-film
and crystalline materials
transform solar
energy to electricity.
A. Thin Film
Thin film began to
see widespread commercial
deployment in
the early 2000s,
whereas crystalline
silicon panels saw
widespread commercial
use in the 1970s.
Thin films are available
from a number of
different manufacturers and can cost less
per watt than traditional silicon panels;
however, they can require significant additional
space due to their lower power efficiency,
leading to higher overall system
Thin film, when integrated into solar
panels, is used in roofing retrofit situations;
however, when integrated into single-ply
roofing materials for retrofits, a complete
reroof may be required, depending on the
individual circumstances and condition of
the roof.
Thin-film materials, whether mounted
into panels, directly applied to roofs, integrated
onto single-ply roofing, or integrated
into BIPV, are available in these raw forms:
• Amorphous silicon (a-Si)
• Copper indium gallium selenide
• Cadmium telluride (CdTe)
See Figure 4.
Amorphous silicon (a-Si)
Amorphous silicon (a-Si) applications
have a lower efficiency rating, often by half,
compared to crystalline silicon (c-Si), but
are more flexible in their applications. Thinfilm
applications thus require larger roof
space but provide cost efficiencies and some
weight savings, as a-Si layers can be made
thinner than crystalline applications.
During the manufacturing process, a-Si is
deposited at very low temperatures (as low
as 75º). This allows for deposition not only
on glass, but plastic as well. This makes it
a candidate for a roll-to-roll process.
The weights of a-Si applications in single-
ply roofing vary from manufacturer to
manufacturer. And as noted, relatively larger
roof space is required.
Figure 3 – Typical components of a roof-mounted solar PV system.
Figure 4 — Flexible thin film on a large roof.
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Copper Indium Gallium Selenide
CIGS cells tend to be less expensive,
due to lower material costs and potentially
lower fabrication costs (although crystalline
technology is also dropping in production
cost). A research paper prepared by the U.S.
Department of Energy showed an efficiency
of nearly 20% research testing (higher efficiency
levels than a-Si and crystalline silicon);
electricity production efficiencies may
be lower, however, depending on application
and installation.
One CIGS manufacturer makes cells to
take advantage of direct sunlight, diffuse
sunlight, and reflected sunlight from the
roof surface. With a white roof, these panels
can capture up to 20% more light than a
black roof, according to the manufacturer.
Thus, the reflectivity of an existing roof is a
critical factor for this type of system. Roofs
lacking the required reflectance due to age
or soiling or both may require a significant
increase in cleaning frequency, increasing
costs and affecting long-range economic
Also, there is a relatively short commercial
deployment history of cylindrical CIGS
modules compared to traditional crystal silicon
Cadmium Telluride
CdTe cells are not as cost efficient as
crystalline, but are suitable for large-scale
production. CdTe is the only thin-film PV
technology to surpass crystalline in cost
effectiveness when used in utility scale
As with other thin film, the weight per
sq ft of these systems varies by manufacturer.
Concern with the toxicity of the cadmium
has been expressed by many potential
users. See Figure 5.
B. Crystalline Silicon (c-Si)
Crystalline silicon (c-SI) materials come
in two types: monocrystalline and polycrystalline.
It is used by the semiconductor
industry and is the material used in over
80% of all PV today. Generally, it provides
12% to 21%+ cell efficiency, generates 13 to
17 watts per sq ft, and has extremely low
degradation/reduction in efficiency over
time. As one of the original PV technologies,
it has a history of over 40 years of fielddeployed,
successful installation. For retrofits,
this is the most practical system,
assuming the roof is in good condition,
because the system can be relatively easily
set on an existing roof.
Monocrystalline (Single Crystal)
Monocrystalline is the original PV technology
invented in the 1950s and has over
40 years of history and reliability behind it.
Monocrystalline modules are composed of
cells cut from a piece of continuous crystal
cylinder sliced into thin circular wafers.
Because each cell is cut from a single crystal,
it has a uniform, dark blue color (see
Figure 6).
Polycrystalline (Multi Crystal)
Polycrystalline entered the market in
1981, but is similar in history, performance,
and reliability. Polycrystalline cells
are made from silicon material, but instead
of being grown into a single crystal, are
melted and poured into a mold. This forms
a square block that is then cut into square
wafers with less waste of space and material
than round single-crystal wafers. As the
material cools, it crystallizes in an imperfect
manner, forming random crystal boundaries.
The efficiency of energy conversion is
slightly lower. The size of the finished module
is slightly greater per watt than most
monocrystalline modules. The cells are also
different in appearance from single crystal
cells. The surface has a jumbled look with
many variations of blue color.
In the United States, mono and polycrystalline
panels are available from at least
25 manufacturers. The so-called “typical”
panel has a size of approximately 14-17 sq
ft, generates <200 to >240 watts, and
weighs approximately 40 pounds—or, from
a very Rough Order of Magnitude point of
view—2-3 pounds of dead load per sq ft of
panel, not including wires, connectors, conduits,
other equipment, and loads created
by wind, seismic, or thermal movement. See
Figure 7.
Low-slope Solar PV Thin-Film Adhered
Single-ply thin-film solar PV roofing systems
are applied directly to the roof, either
in the factory or in the field. For example,
thin-film solar PV has sometimes been
adhered with adhesive to existing metal
roofs or has been factory-applied to singleply
roof membranes. Single-ply and other
low-slope adhered systems are limited in
application due to the condition of the existing
roof membrane and insulation, how
Figure 5 — Typical CdTe panel.
Figure 6 – Typical monocrystalline
panel. Because the sizes and outputs
vary, the designer needs to check
manufacturer specifications.
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many roof membranes are already in place
(as limited by local code), how many roof
penetrations must be dealt with, wind
uplift, and similar factors. However, thinfilm
systems, either in single ply or manufactured
into the panelized systems
described above, offer significant weight
and structural advantages.
Low-Slope Solar PV Panel Systems
These panel systems are laid flat on the
roof or are secured or ballasted in place,
often by only their own weight, with or without
separate protection pads. If laid flat on
the roof without being secured, wind uplift
and ventilation of these panels can be problematical
if not properly designed. In a
retrofit situation, this can have a serious
impact on remaining roof life and the safety
of the panels.
Solar PV Panels Mounted to Standing
Seam Metal Roofs
Panel systems can be mounted to standing
seam metal roofs with clips (see Figure
8), although clips are not approved by all
governmental agencies (for example,
California Division of the State Architect,
involving K-12 schools and some other local
government-owned buildings in California).
See Figure 9.
Solar PV Systems Mounted via
Penetrating Mounts
Penetrating roofing systems for roof
retrofits include specialized mounts affixed
to the roof structure, although this does
cause the membrane to be cut, possibly
affecting the existing warranty.
The system providing the best answer to
wind load, thermal movement and azimuth
angle issues, is the system attached to the
roof structure or building frame. This
method does require additional roof penetrations,
although with proper design can
be accommodated – see Figure 10 for one
example of a penetrating mount on an existing
gravel ballasted built up roof. The roof
consultant will need to review and implement
NRCA recommendations regarding
clearances, on a case-by-case basis.
Figure 7 — Typical polycrystalline
panel. The dimensions of these panels
vary widely, depending on the
manufacturer. One of the more common
sizes is a panel approximately 39
inches wide, 65 inches in length
(approximately 17 sq ft), and 1.8 inches
thick. The shipping weight is
approximately 40 pounds. Because sizes
vary so widely, the designer must check
specifications with each manufacturer.
Figure 9 — Penetrating roof mount on gravel ballasted BUR. The same detail
would be applicable to an embedded aggregate roof.
Figure 8 — Panel mounting on standing seam metal roof.
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Nonpenetrating Nonballasted Solar PV
Panel Roof-Mounting Racks
Nonpenetrating nonballasted roof solar
racks consist of PV panels being set in
metal frames placed on the roof over rubber
or EPDM pads. These systems are somewhat
similar to ballasted systems, but
instead of ballast being added, take advantage
of the weight of the solar PV system
and the racks themselves. Some of manufacturers
claim their systems can sustain
winds of 90 to 120 miles per hour; however,
some manufacturers claim only the capacity
to accommodate a 30-degree angle from
the roof deck at such wind speeds, an angle
that may be less efficient in generating
power in some parts of the country where
an azimuth angle of up to 47º from horizontal
may be necessary for optimal solar efficiency.
Nonpenetrating Ballasted Solar PV
Panel Roof-Mounting Racks
These ballasted systems consist of
metal frames with the addition of weight in
the form of bricks or other engineered material.
The manufacturers of some ballasted
systems claim ballasted systems can withstand
a greater tilt angle. The roof consultant
should exercise due diligence by asking
for backup engineering calculations from
the manufacturer. Both nonballasted and
ballasted roof racks create additional
weight, possibly causing the entire system
to exceed the limits for the roof structure.
Regardless of the PV type installed, the live
and dead loads need to
be calculated to determine
if the roof structure
can accommodate them.
Snow Loadings and
Snowdrift Mounting
In some parts of the
country, solar PV installations
will result in
greater roof loads from
the weight of fallen or
drifting snow, the weight of the racks, and
the weight of the panels. Taken all together,
the roof structure may not accommodate
the extra weight.
Carport-Mounted, Shade-Structure-
Mounted, Ground-Mounted, and Tower-
Mounted Tracking Solar Panels
In addition to the roof mountings of the
solar PV systems discussed above, PV systems
can be mounted on existing or new
carports, shade structures, ground mounts,
and tower mounts to provide for tracking of
the sun for greater efficiency.
Other Types of Solar PV Systems –
Concentrator systems are not covered in
great detail in this paper; however, these are
some of the concentrator systems now
available, typically for large-scale, utilitycompany-
sized systems.
Heliostat concentrators are special towers
used to concentrate the sun’s energy,
typically in the form of heat, on a central
point to generate steam to drive turbines,
creating electricity.
Concentrated solar panels use a series
of lenses to concentrate the sun. These panels
are thicker and heavier than the typical
solar panel. Concentrated solar panels may
also be mounted on sun-tracking, groundmounted
towers or panels.
Building integrated photovoltaic systems
(BIPV). Solar panels, thin-film systems
and other solar PV are now found in:
• Curtain walls (see Figure 11).
• Roof shingles, where the solar component
is also the shingle.
• Shade structures.
There are many, many manufacturers of
solar PV panels. A not all-inclusive, but
rapidly changing list:
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Figure 10 – Roofmounted,
ballasted solar
panel system with
polycrystalline panels.
Note the limited
clearance between the
panels and the roof
deck, possibly creating
the need to remove the
panels for the purposes
of roof replacement or
possibly even for normal
roof maintenance, leak
repair, or cleaning.
Figure 11 —
Building integrated
PV (BIPV) in a
curtain wall.
Advent Solar
Amonix Inc.
Atlantis Energy System, Inc.
BP Solar International, LLC
Canrom Photovoltaics, Inc.
DayStar Technologies, Inc.
Duro-Last Roofing Inc.
Energy Photovoltaics Inc.,
Evergreen Solar Inc.
First Solar LLC
Global Solar Energy Inc.
Innergy Power Corporation
Iowa Thin Film Technologies
Kyocera Solar Inc.
Matrix Solar Technologies
Mitsubishi Electric & Electronics USA
Mitsui Comtek Corp.
Pacific SolarTech
RWE Schott Solar Inc.
SANYO Energy (USA) Corporation
Sanyo Semiconductor Corporation
Sharp Manufacturing Company of America
Shell Solar Industries LP
Solar Power Industries, Inc.
Solar World-USA
SolyNdra, Inc.
Spire Corporation
Sunpower Corporation
Sunwatt Corporation
Sunwize Technologies LLC
Terra Solar Global, Inc.
Trina Solar
Tideland Signal Corporation
United Solar Ovonic LLC.
And many more!
Each of these manufacturers offers
varying sizes, dimensions, power output,
configuration, ease or lack of ease of roof
installation, design issues to be addressed,
familiarity to the installer, and warranties.
These differences will have an impact on the
design and how the roof consultant goes
about preparing plans for installation.
The information shown below is not
intended to be an all-inclusive list of every
issue faced by the roof consultant when
dealing with solar PV installations on an
existing roof. As with all building components,
issues dealing with solar PV are complex
and extensive. But based on our firm’s
experience reviewing existing roofs for their
capacity and capability for solar installations,
we have prepared a brief overview of
design issues to consider. The buildings we
have reviewed include airports, multifamily
residential properties, schools, and commercial
A. Roof Assessment.
Roof age. What is the age and condition
of the existing roof? Does it need to be
replaced now?
Remaining Service Life. Will the
remaining roof life be concurrent with the
service life of the solar PV system? Will it
need to be replaced before the service life of
the solar PV system ends, creating more
costs? Solar PV manufacturers provide estimates
of service life. These need to be compared
to the remaining roof life during the
design process.
Existing Warranty. What is the impact
to the existing roof warranty? Who was the
manufacturer, and what is the actual warranty?
What are the rules and requirements
about penetration? Some warranties are
also voided if a new type of roofing material
is joined to the existing material.
Watertightness. Are there issues to be
addressed before solar is installed, and will
these issues be worsened by the installation
of solar? Have all existing leak conditions
been repaired?
Drainage. Is the existing drainage system
adequate, and will it be made worse or
even made better (not likely) by the solar
Chemical Compatibility. Will the existing
operations, especially in manufacturing
or food operations, be compatible with the
roof life, the solar PV material, and the solar
PV system installation? Will contaminants
or particulants discharged from the building
cause problems?
Impact on Structural Load. The roof
consultant needs to collect weight data on
the entire solar PV system to determine if
the existing roof can accommodate the new
live and dead loads. If the roof consultant is
not a licensed structural engineer or architect,
a licensed professional will need to be
retained in order to provide those answers.
B. Physical Constraints.
These issues to be addressed are fairly
straightforward and as follows.
Adequate Roof Space. Is there sufficient
roof space available on the roof to handle
the entire electrical need?
Existing Mechanical Equipment. How
much mechanical equipment is on roof?
Will it conflict with the panels, and if so, can
it be moved or removed?
Distances Between Solar PV
Components. How is the design of clearances
and separations between solar PV
panels and other equipment impacted by
local fire codes?
Power Runs. Are conduit runs possible
from solar to electrical tie-in?
Additional Space on Ground for Solar
PV. Is space required on the ground for a
partial or complete ground-mount system,
and is the space adequate in size to accommodate
what cannot be accommodated on
the roof?
Ground Enclosures for Solar PV
Equipment. Will new ground structures be
required and can these ground structures
take the form of parking shade structures?
Locations of Ground Enclosures.
Where will space on the ground or inside
the building be provided for an inverter and
equipment mounting system, and is it sufficient
in size, location, and proximity to the
solar PV system? Is a separate building
required for this equipment?
Trenching. Will trenching be required
from the solar PV system to the electrical
tie-in point?
C. Solar PV and New Roof Warranty
Before the purchase, obtain a copy of
the warranty from both the solar PV system
and the roof manufacturers, and address
these issues:
Ongoing Maintenance. What continued
maintenance of the roof and solar PV is
required as a condition of the warranty? Is
the purchaser required to sign an agreement
requiring annual or other periodic
maintenance, and is such maintenance
Financial Strength of Company
Holding Warranty. Is there financial
strength behind the solar PV warranty, or is
it an insurance policy from an insurance
Extreme Environment Exceptions. Is
maritime or extreme environment deployment
approved in the warranty?
Fine Print. What does the fine print
D. Sustainability of Solar PV System
Over Time
Compatibility Between Service Lives.
Will the roofing last the term of the solar PV
financing? How long will the PV system
Obsolescence of Solar PV System. Will
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the solar PV and all associated systems
maintain their manufactured
integrity and last the term of the
solar PV financing or warranty?
E. Structural Loads Created by
the Solar PV System
The structural load can take the
form of dead load caused by the
weight of the panels, wires, collectors,
connectors, conduits, mounting
racks, and other materials.
Live-load issues can be created by
wind loadings, thermal movement,
and other factors. Design requirements
to address these issues vary
by state and local conditions. Some
examples of dead loads have been
provided in this paper; however, the
manufacturers should be contacted
for specific engineering details.
Building officials from
California to New York may, upon
submittal of a building permit
application, request calculations of
the ability of the affected lateral
system components to resist additional
seismic loads and the
impacts of thermal movement created
by the solar PV system equipment.
F. Fire Code Design Issues
Markings. Solar PV systems, including
the individual components, should normally
be marked with weatherproof materials
indicating they are solar systems in order to
provide safety from electrical shock to those
working around the systems, such as firefighters
in the case of a roof fire, for example.
Many jurisdictions require nameplates
displaying voltage ratings of the components.
Many jurisdictions also require locations
of power disconnects to be clearly
Access and Pathways. Fire codes in
most jurisdictions require adequate access
to panels, adequate paths of travel between
roof-mounted solar PV equipment, access to
the roof from the ground level (varies by
jurisdiction), and emergency egress from
the roof. Some jurisdictions have been
known to require a 6-ft wide safety path
around the perimeter of the roof. Some
jurisdictions require the path of travel to be
over structural elements.
Size of Arrays. Some jurisdictions limit
the size of individual arrays.
Nonhabitable Buildings. In some jurisdictions,
the fire requirements do not apply
to nonhabitable structures such as carports,
shade structures, and other groundmounted
arrays, although a vegetation-free
area of 10 ft is required in some jurisdictions.
Figure 12 — A solar “garden.”
Figure 14 — Possible
maintenance and
safety issues caused
by conduit lying on
Figure 13 — Possible roof
maintenance issues created
by thermal movement.
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Impact on Fire Rating of Roof
Assembly. The designer needs to determine
if and how the PV system will impact the fire
rating of the existing roof assembly; this
information may be required when permits
are requested.
G. Electrical, Mechanical, and Other
Design Disciplines
Solar PV systems are unique, requiring
design by electrical engineers of adequate
wiring sizes and conduits; design by
mechanical engineers to provide for existing
roof-mounted mechanical equipment;
design by structural engineers for structural
issues; and civil engineering design of
trenching, etc. Solar system power modulation,
ground-fault, and short-circuit potential
need to be studied, including how well
wires are protected in metal and other raceways
and how they are protected from the
weather. Thermal movement also impacts
conduit runs.
H. Peer Review of Design
Solar PV installations, being so complicated,
are an excellent candidate for peer
review by other engineers.
I. Maintenance of the PV System and
the Roof
One of the issues often forgotten by the
designer is maintenance, including that of
the roof, the PV system itself, the electrical
conduits, and the racks, to name a few.
Figures 12-14, showing some of the possible
long-term problems. The answer to these
problems can be found in proper design.
J. Nonengineering Design Issues
• Use appropriate materials; reflected
light is an issue on campus settings.
For example, solar installations on a
single-story building can reflect glaring
light into existing or planned
taller buildings.
• Sizing matters.
• Aesthetics are important.
• As they say: location, location. Solar
can go anywhere, right? No.
• Make sure the PV system remains in
continuous operation.
• Beware of security issues during
installation and operation.
How and why to finance solar need not
be difficult if the following issues are taken
into consideration:
A. Price
What’s the best way to pay for solar PV,
and is the price reflective of all needed
options? The overall question is: Does solar
make financial sense for the given unique
situation? The big picture is: Utility costs
are increasing rapidly, utility costs are and
will continue to be volatile, rebates and tax
credits are limited, and money spent on
energy is money not spent on growth. So
solar enables a consultant to control and
predict utility costs and save money.
B. Alternative Financing Methods
Cash Purchases. These are great. . . if
you have the money. Some financial analysts
have shown a 10-18% return on
investment and an 80% to 98% reduction in
energy costs.
Federal and State Incentives.
Traditional Financing Using a Capital
Loan or Lease.
Municipal Leases for Municipal
American Recovery and
Reinvestment Act (ARRA) Stimulus
Power Purchase Agreements (PPA).
This financing method allows control of utility
costs without capital investment, providing
increased savings over time.
Municipalities that cannot otherwise benefit
from tax incentives can, in effect, allow others
to reap the benefits at much lower costs
to the municipality.
• The finance company installs, owns,
and operates the solar plant on the
owner’s site. There are a number of
different PPA providers, prices and
terms can vary greatly, and the
proven ability to commit and close
as promised is critical.
• The building owner does not pay for
the equipment or its maintenance.
• The building owner is buying energy,
not equipment.
• Owners buy energy from the system,
and only as much energy as the system
• Because this is an agreement to buy
energy, not a lease or a loan, the
financing agreement is not reflected
on the client’s balance sheet.
• Credit quality is key: can the parties
fulfill their obligations for 20 years?
C. Key Things to Remember for the
Economic case
• Utility costs are increasing unpredictably.
• Incentive levels decline over time,
often much faster than expected, so
there is urgency to act quickly.
A. Criteria for Selecting a Contractor
The issues to be addressed when selecting
a contractor follow:
• Is the “contractor” an integrator or a
contractor? Is the work self-performed
or is the work performed by
a subcontractor?
• What is his quality of construction
and experience with similar projects?
• Does the contractor/integrator have
a strong balance sheet?
• Are good communication practices,
procedures, and methodologies in
• Are safety procedures and practices
in place?
• How many subcontractors will he
• What are the quality standards to
reduce future costly repairs?
• What methods does the contractor
use to avoid delays and problems,
increasing enthusiasm for the project
within the community/
constituency, reducing stress on
business operations, reducing the
business/organization’s risk for onsite
accidents, improving the integrity
of the roof or site, and eliminating
repair disruptions in years to come?
B. What Questions Should be Asked?
• Is this integrator/contractor experienced
in construction?
• Does this integrator/contractor
understand the whole building?
• Will the integrity of the roof system
be protected?
• Does this integrator/contractor
deliver quality construction on time
and on budget?
• Is this integrator/contractor an
expert in every step of analysis,
design, sourcing, and cost-control?
• Can this integrator/ contractor be
trusted to work safely and prevent
costly accidents?
• Does the firm have the ability to execute
on time and on budget?
C. Safety Criteria
• Are there full-time safety managers?
• Is there a written safety plan?
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• Are there on-site audits?
• Are personal fall-arrest systems in
• What is the Experience Modification
Rate (EMR)?
• How do rates compare?
D. Roof and Site Integrity – Can the
Firm Provide it?
E. Roofing and Construction Experience
• Is infrastructure in place to support
complex projects?
• Is there construction planning and
operations excellence?
• Is there a local, cross-trained workforce?
• Are managers experienced with
solar projects?
• Are the appropriate licenses held?
(In California, for example, B, C-10,
and C-46 contractor’s licenses.)
• Are there NABCEP certified technicians?
• Are project superintendents highly
Installation of solar PV on existing roofs
is complicated, complex, and ambitious,
but worth the time and effort through cost
savings and the ability to have a positive
impact on the environment. Most importantly,
the issues addressed and answered
in this paper will assist the roof consultant
in keeping pace with the rapidly changing
practices in the installation of solar roofing
systems. The financial means and methods
do exist to allow installation of solar PV systems
on many existing roofs across the
United States. Following these simple
guidelines will lead to a successful, rewarding