Is Your Roof a Good Candidate?
When evaluating an existing roof for PV potential, look at factors such as the load-carrying capacity of the deck and framing,
the type of roofing system and slope, the roof’s age and condition, building height, and wind and seismic loads.
Roofs should be classified as good, better and best based on
their ability to comply with the overall design strategy. Good
candidates include roofs that can accommodate increased loads
without modification. Other good options are protected membrane roofs or conventional roof membrane assemblies that have
been constructed with high compressive strength, as well as
roofs that have been designed with a positive slope for drainage.
Always be sure to use thicker membranes when installing solar
on single-ply membranes. Avoid installing solar on roofs with
light wood decks or autoclaved aerated concrete as blocks and
One of the fundamental
choices is whether to install a
penetrating, mechanically fastened, ballasted or laminated
system. For low-slope roofs,
NRCA recommends the use of
a penetrating system where the
panels are attached through
to the building structure. This
approach can be accommodated by installing curbs, sleepers
or posts as an integral part of
the roof system.
Ballasted systems are also
popular as they are less expensive to install, minimize the risk
to the building during installation and increase the provider’s
return on investment. If curbs,
sleepers or posts are used, they
Racks can dislodge if they aren’t
properly secured against wind.
Fires can obliterate both the solar panels and roof.
must be designed and attached to the building frame to with-
stand the live loads, including seismic and wind. An inability
to recognize environmental loads such as snow and wind can
lead to system failure or collapse. Penetrating and mechani-
cally fastened systems can be designed with a high degree of
predictability vs. the ballasted system. These options can be
designed to accommodate almost any building and roofing
system, but due to cost and ease of installation, most PV
systems have a ballasted design.
One example of a ballasted system places solar panels on
a racking system, usually constructed from aluminum and
weighted down to keep them in place. If a ballasted PV system
is installed on new construction, most membrane manufacturers require the use of a thicker membrane (such as a minimum
of 60 mm in the case of single-ply) and high-density cover-board installed directly under the membrane. This approach
provides added durability and reduces the risk of accidental
damage. A ballasted solar system weighs about 5 pounds per
square foot, increasing on the outside row and at the end of
panels as an added measure against wind. While this weight
can be easily accommodated in new construction, existing
roofs often do not have the spare capacity to accommodate the
Smart Installation Practices
Changing a protected membrane roof or aggregate ballasted
roofing system to a conventional roof or using a modified bitumen or single-ply membrane that weighs less than the existing
roof membrane will often provide the necessary spare capacity required for the PV installation. Changing the roof system
or membrane just to accommodate solar installation may not
always be the best choice. Removing the aggregate surfacing
from a built-up roof system and capping it with a single-ply
membrane can have risks, including the difficulty of finding
leaks if there are punctures and water has spread beyond the
point of origin and become trapped.
ONE OF THE
ALL PHOTOS DOUGLAS FISHBURN