Is Your Roof Solar Ready?
By Mark Robins, Senior Editor,
Posted
09/07/2012
Here are a few things to consider and prepare for before
installing solar
More and more buildings in America are realizing the benefits of
solar photovoltaic (PV) systems. According to the "U.S. Solar
Market Insight" a quarterly publication of Washington, D.C.-based
Solar Energy Industries Association
(SEIA) and GTM Research, the United States began 2012 with the
second highest quarter for PV installations ever. More than 18,000
photovoltaic systems totaling 506 megawatts came online in the
first three months of the year. Reduced electricity bills, solar
rebates, tax credits and net metering are the contributors to this
solar boom.
What can be done to a metal roof to prep it for
successful PV installation? Not a lot is really necessary. One
reason is metal roofs are the most compatible roof format for PV
installation. A modern standing seam roof system is easier, faster
and less expensive to mount solar on than any other roof type. Not
only can it be "direct-attached" and penetration-free using seam
clamps and PV kits, but metal is the only commercial roof type that
offers a service life in excess of the solar PV.
"All other commercial roof types will require replacement before
PV life expires-a very expensive proposition when the roof is
covered with a solar array," says Jerry Heininger, environmental
products coordinator for Englert Inc., Perth Amboy,
N.J. "In the residential market, slate and clay tile may offer
similar service lives to standing seam, but the array cannot be
installed without some roof disassembly and/or penetration of the
roof. So the preferred 'solar ready' roof is clearly standing seam
metal. This advantage is so compelling that often asphalt shingles
are replaced with standing seam prior to solar installation."
Before installation
Before installation, every PV project begins with a site and
roof evaluation to determine factors such as solar orientation,
wind loads, roof warranties, useful life remaining in the roof
system, usable roof area and location of the building's electrical
service. A review of a building's energy consumption will show how
much energy can be saved. Prior to installation, the correct
permits and a plan check can finalize an installation plan that
includes the number of panels, positioning and the overall PV
design.
"Prior to any solar installation, we will determine the roof
type, age, warranties remaining, manufacturer and condition," says
Kate Riedo, solar development manager at Tecta America Co., Rosemont,
Ill. "We recommend that solar be installed on roofs with at least
15 years of life remaining. If necessary, Tecta will do
repairs/replacements to bring the roof up to 15 years. [We] always
follow the original roof manufacturer's guidelines for installing
solar PV and file the necessary warranty continuation paperwork on
the customers' behalf."
The roof should be walked and a report prepared
showing any deficiencies such as cracks, open seams, ponding water
or punctures. "This is especially true if the roof is older than 10
years," says Monique Hanis, spokesperson at SEIA. "Prior to
installation, open seams should be sealed and any damage repaired
in areas within the solar footprint to maintain the roof's
integrity. Inspect the roof for sags and other abnormalities." A
sag or deep depression may indicate a structural weakness in the
support system that may require correction.
"Tecta hires an outside structural engineer to evaluate the roof
and determine the excess load capacity," Riedo says. "Prior to any
work on the site, Tecta will develop a site specific safety plan
and set up appropriate safety measures including safety flags,
OSHA-approved cable systems and/or guard rails. Tecta also
designates a staging area(s) on the roof, keeping safety and weight
considerations in mind."
Load limitations
A structural analysis must be performed on a metal roof to
ensure its load limitations will not be exceeded. Crystalline solar
modules weigh about 3 pounds per square foot (psf). A metal roof
must be capable of taking this added collateral load. Most
pre-manufactured trusses are designed with 7 to 9 psf of collateral
load designed into the truss (in addition to wind and snow
loads).
"After drywall and light fixtures, there is still a range of 4
to 5 psf collateral load that can be added," Heininger says. "With
most conventionally framed (bar joist) commercial structures there
is also some 'wiggle room' in structural design. With
pre-engineered metal buildings this may or not be the case. In any
event, on existing construction it is a good idea to check out the
structural capacity with a licensed engineer or the original
manufacturer of the structural system. On new construction, be sure
the structural design can accommodate the required 3 psf."
Roof size matters
Roof size must be considered during PV solar evaluation and
preparation. Roofs need a certain amount of south-facing roof space
to accommodate a PV system of any appreciable size. In the northern
hemisphere, southern exposure to the sun is ideal to harness its
full force. However, panels can be oriented to the southeast or
southwest without substantially decreasing performance.
Exposure to the panels should be free of obstructions and
shading. A shade analysis examining roof equipment, overhanging
trees or surrounding buildings can determine if a roof is suitable
for a productive PV array.
PV system sizes are usually referenced in kilowatts like 1,000
watts of rated DC power. A typical crystalline module is about 240
watts-although they vary-and takes about 18 square feet of gross
roof space, so approximate about 80 square feet of gross "usable"
roof area for each kilowatt of system size. One kilowatt of panels
can be fit on about 100 square feet.
Systems are usually sized in accordance with the power consumed
by the occupant. "The objective is normally driven by economics,"
says Heininger. How large a system will satisfy a significant
portion of the occupant's power demand without waste and payback
his investment in the shortest time possible? We have seen
high-energy-use commercial facilities where every square foot of
usable roof space was employed, and other cases where only 20 or 30
percent of available space was used."
PV installation and preparation is easier for large, flat,
unadorned roof areas. Roof vents, antennas, satellite dishes or any
other roof fitting may need to be relocated. "If the roof area is
cut up, there is typically more installation time needed," says
Alan E. Burnett, PE, associate at Gales Associates Inc. in Mountain
View, Calif. "Open roofs with low slopes and simple shapes like a
rectangular outline provide more efficient installation. Access and
staging are typically easier for low‐slope roofs."
High-sloped roofs will typically require a penetrating PV
racking system that meets wind and snow requirements of the loads
for its region. To determine wind loads, Hanis recommends checking
local building and safety departments for specific requirements.
International building code wind speed charts can determine basic
wind speeds for U.S. locations.
[sidebar]
Solar prep: new versus existing roofs
Solar panel installation preparation is different for existing
building roofs than it is for new building roofs. For existing
roofs, the contractor should inspect the roof to confirm that
existing features will not be affected by the new
photovoltaics.
Also, the contractor should verify that the remaining roof
service life is similar to the PV service life. "If the PV system
will be mounted to the roof structure, the penetrations should be
flashed in a watertight manner and the roof warranty should not be
compromised, if the roof is under warranty," says Alan E. Burnett,
PE, associate, Gales Associates
Inc., Mountain View, Calif. "If under warranty, the contractor
should ensure the roof manufacturer is notified and approves of the
roof repairs for warranty compliance."
For new building roofs, the PV layout needs to be coordinated
with other roof‐mounted items such as mechanical units and
electrical conduits. If the PV will be connected to the roof
structure, the curbs or stanchions should be installed prior to the
roof installation to avoid having to repair the recently‐installed
roof. "These PV support items will need to be 8-inch minimum above
the finished roof surface and should not obstruct drainage,"
Burnett says. "For some projects, the PV system can be a separate
design‐build item and the contractor will need to coordinate his
work with the PV designer and contractor."