Avoiding Pitfalls with Large Skylight Design and Maintenance

By Jordan Swail, BESc, PEng (ON), BSS
This paper was presented at the 2024 IIBEC
International Convention and Trade Show.

INTRODUCTION
Modern skylights are complex building
envelope elements that allow natural light to
enter buildings, while providing environmental
separation for water, air, and heat. Beyond these
basic functions, skylights offer the potential for
architects and designers to blend interior and
exterior spaces, transform atriums, and bring
natural light into the core of large buildings.
The impact large skylights have on the way a
building feels and functions can be significant.
However, the design of these complex systems
can have costly and disruptive effects to building
operations teams if they are not designed with
maintenance in mind.
Skylights consist of three basic components,
including the structure or frame, the glazing
material, and the sealants. Skylight frames are
designed to support and retain the glass, and
in drained systems they are also designed to
collect rainwater and drain it to the exterior. In
cold climates the glazing materials almost always
consist of sealed insulated glazing units, which
eventually fail and require replacement. Skylight
sealants also fail over time, and depending on
the skylight design, they can sometimes require
diligent maintenance to remain watertight. In
order for skylights to function properly, they
require maintenance, and good skylight design
considers not only their aesthetics, but also their
durability and maintenance requirements.
LARGE SKYLIGHT DESIGN
Large skylight systems can typically follow one
of a few common industry principles to remain
functional and exclude water. Common skylight
types include:
• Face-sealed skylights, which have one layer of
sealant to prevent water entry. These systems
are often designed for easy access to maintain
exterior seals.
• Simple framed drained rainscreen skylights,
which follow a dual barrier approach, but
often rely on sealants at hidden joints
without raised/shingled/continuous drainage
channels. These systems utilize framing

often found in commercial curtain wall
systems, with little if any design changes to
accommodate the increased rain loads that
skylights experience.
• Modern drained skylights with raised/
shingled/continuous drainage channels, which
follow an improved dual barrier approach
and incorporate frame detailing to reduce
reliance on sealants. These systems typically
feature dual-barrier frame connections that
are shingled and raised where one drainage
channel connects to another.
Face-sealed skylight systems reduce
complexity by relying on one seal at the outer
face of the glass-to-glass surface. These joints
do not offer redundancy, but they can be
easier to diagnose and repair when water leaks
occur. Because there is only one seal, the leak
location can be traced at the interior, and the
area requiring repair can be quickly diagnosed.
These systems are often designed to support
maintenance personnel loads, to permit sealant
repairs to be readily undertaken.
Simple framed drained skylight systems
utilize extrusions that have not been significantly
modified or adapted from vertical glazing
applications for use as skylights. The reason for
their use is often related to manufacturing cost,
as one simple extrusion profile can be used
for rafters (verticals) and purlins (horizontals),
but they do not include raised and overlapped
drainage channels. The joints of these systems
rely on difficult-to-access sealant, which requires
maintenance to remain watertight and reduce
leakage over time. Despite these limitations,

simple drained skylights are not uncommon due
to their low cost.
Modern drained skylight systems can be
designed to include raised and overlapped
drainage channels, where each layer shingles
over the next and is raised out of the drainage
pathway below, providing a pathway for water
to drain in the system, with minimal reliance
on sealants. These framing systems are more
expensive than adapted curtain wall frames,
as different extrusions are used for rafters,
purlins, and others, but they can significantly
improve long-term performance and reduce
the frequency of leaks. In certain design
situations, variations of these skylights have
been used in bent profiles to eliminate joints
in rafters altogether, such as the bent-rafter
skylight design.
Detailing of skylight glass retention can also
impact the risk of water entry. Low-profile caps
are common at skylight purlins to reduce the
volume of water that can be trapped above
each mullion as they impede drainage over the
exterior skylight surface. Modern two-sided
structural silicone skylight systems take this
approach one step further, by eliminating
the caps altogether. By eliminating the caps,
structural silicone systems allow water to easily
drain from one glazing unit to the next, without
presenting a risk or water retention or entry at
leading-edge gasket seals.
SKYLIGHT DESIGN FOR
MAINTENANCE
Regardless of the skylight design
implemented, all skylights will require some
form of intermittent maintenance such as
cleaning, sealant repairs, and sometimes
localized glass replacement. Undertaking
these procedures is often complicated by
the fact that skylights are often in elevated,
hard-to-reach places, and may present fall
hazards to maintenance personnel. For that
reason, the first step in designing skylights
for maintenance should be to maximize the
durability of the skylight system, to reduce its
maintenance needs.
Minor cleaning and sealant repairs are often
undertaken using poles, with no access to the
skylight required. High-slope skylights can often
be cleaned via bosun chairs or boom lifts. Use
of bosun chairs or platforms that transfer load
to skylights should always be reviewed by a
qualified engineer to confirm that the skylight
surface is designed to support anticipated
loading. Regardless of the cleaning method, all
personnel accessing the areas around skylights
should be made aware of whether the skylight
presents a fall hazard or can be used as a
working platform.
Large skylight sealant replacement and
leak repairs often require access to portions of
skylight systems glazing, which can be difficult
to reach. During this work, tradespersons need
to reach portions of the framing that cannot
easily be accomplished from adjacent roof areas,
and they often require the use of an engineered
scaffold, crane, or boom lift. The difficulty in
working and accessing sealants for replacement
is a good reason for designers to ensure that
reliance on hidden sealants is minimized.
Replacement of hidden sealants frequently
requires removal of glass, as the joint sealants
requiring repair are at the back of the skylight
framing sections.
Glass replacement typically requires both
a crane to lift the glass and a safe working

platform for the contractor. Often the systems
designed for personnel support are inadequate
to support skylight glass on their own. In order
to reduce the frequency of glass replacement,
designers should consider the durability of
the components in use by considering the
following:
• Minimize water potential for reaching
and sitting against edge seals or laminate
interlayers. When detailing framing, ensure
that there is adequate clearance for water to
drain around the glass, drainage tracks are not
filled with sealant, and glass setting—blocks
do not restrict drainage.
• Utilize durable edge-seal construction for
insulated glass units. Skylight glazing unit
edge seals undergo greater heat and loading
stress than window glazing units due to their
orientation and should be sufficient for their
intended use case.
• When specifying tempered glass ensure units
are heat-soak tested. Specify materials and
use manufacturers with a proven history of
quality control and consider undertaking plant
visits to confirm manufacturers are following
fabrication standards.
SKYLIGHT ACCESS METHODS
Skylight access methods will depend on the work
being completed, the skylight design, and local
health and safety requirements of authorities
having jurisdiction. When access to a skylight
system is required, the method of access will
depend on the skylight configuration, capacity,
and type of work required. Common access
methods will include one of the following: a
permanent engineered moveable platform
(such as a gantry system), temporary working
platforms (such as scaffolding), or glass design
for maintenance loads. In some situations, cranes
can also be used for skylight access or to support
skylight materials.
Gantry systems are engineered moving
scaffolding systems, which can be used to
allow regular access for skylight cleaning,
maintenance, or inspections. These systems
are typically designed by specialist structural
engineers and should be incorporated from the
outset of the skylight design. The high initial
cost of gantry systems is often justified based on
the high cost of repeated temporary scaffolding,
or in situations where structural support
conditions are too complex for temporary
scaffolding to be installed. These systems are
typically not sufficient or designed to support
loads of glass replacement, and separate
scaffolding or craning is often required in those
situations.
Temporary scaffolding work platforms are
expensive if used on a repetitive basis, and
although they can often be designed to suit
the specific repair, they are time-consuming
and have limitations in their configuration.
Scaffolding systems can be designed to support
glass and personnel loads and may be designed
so the use of a separate crane is not required
during glass replacement. As a result, temporary
scaffolding is a very common method of access
used for glass replacement work.
The cost of installing temporary scaffolding
work platforms or a permanent moveable gantry
system can be a considerable expense when
considered on a per-use basis, which we have
observed exceeding $40,000 CAD for a single
glazing unit. In order to reduce access costs, it
is becoming more common for modern skylight
glass to be designed to support maintenance

work platform requires complex structural glass
analysis, the cost premium can be more than
offset by maintenance savings. By designing
glass for maintenance loads, the expense
of separate access systems or temporary
scaffolding access can sometimes be eliminated,
saving money and reducing the financial burden
of skylight maintenance.
New glass designed as a working platform
is typically designed to support a single worker
per glass unit and not the load of replacement
glass, which is typically supported by a crane.
One additional benefit of this approach is that
it also reduces the risk to unknowledgeable
trades that may be working near skylights—
whether it be sitting on, falling, or tripping
onto the skylight, there is a reduced risk to
other workers.
Glass designed for maintenance loads can
vary in its design applications, but it is commonly
used in face sealed structural glass systems
where regular access is anticipated. It has
been our experience that many older skylight
designs have the potential to be retrofitted to
accommodate personnel loads, by analyzing and
upgrading glazing components with modern
high-performance laminates that are compatible
with fully tempered glass. In northern climates
where snow loads are significant the base
skylight structures are often adequate to support
personnel loads, and it is often the glass that is
the limiting factor in supporting maintenance
loads. Structural glass analysis and design can
confirm whether this is a practical solution,
and the limitations of this approach should be
confirmed with a qualified engineer.
CONCLUSION
In summary, there are many solutions available
for the durable design, maintenance, and
access of large skylight systems. Commercially
available skylight systems vary significantly in
their performance, cost, and longevity, and it
is important for the correct skylight system to
be chosen for each application. Although the
industry has developed retrofit and temporary
solutions to undertake skylight maintenance,
the most cost-effective approach is to consider
skylight durability and maintenance from
the start.

ABOUT THE AUTHOR
Jordan Swail, BESc,
PEng (ON), BSS, is an
Associate with RJC
Engineers. He has
significant experience
in complex building
enclosure retrofit
projects throughout
southern Ontario and
has led several of
RJC’s most significant
glazing retrofit
projects, focusing on building renewal, energy
efficiency, and greenhouse gas reductions. His
expertise spans heritage restoration, glazing/
cladding, and the roofing sectors. In addition
to his professional work, he sits on the IIBEC
Southern Ontario Board of Directors in the role of
Marketing Director.