January 2008 Vol. 50 No. 1
Cover Story
Roof
De-icing
Dis(solving) slick situations
By E. Ada Cryer
© Image courtesy Bigstockphoto.com
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very winter, a white blanket covers many Canadian towns
and cities. Despite the seeming serenity of a silent snowfall,
the resulting snow and ice can damage rooftops and also
create dangerously slick roadways and sidewalks. Results are
often devastating to the building owner. Each cubic foot of
snow contains 7 to 11 L (2 to 3 gal) of water, which freezes in
cold temperatures to threaten the roof and people below.
Ponding (i.e. the collection of water) at the base of valleys
and on flat roofs can lead to interior structural damage as areas
of the roof weaken over time and are eventually penetrated by
the meltwater. Mould and mildew will also grow.
As snow and ice shift, melt, and refreeze, they can forge a rift
between the gutter and the roof. If the meltwater seeps under
the shingle and freezes, it expands and lifts the shingle.
Meltwater seeping and freezing between the roof and gutter
will separate the two.
This damage often goes unnoticed until occupants see the
gutter jutting out from the building or experience leaks inside.
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Images courtesy Delta-Therm
➤ Figure 1
De-icing cable is installed in a triangular pattern on the roof
with drip loops to direct water into the gutter. Cable in
gutters and downspouts complete the heated drainage path.
Internal leaking resulting from roof distortion can cause water
damage to the walls and ceiling drywall. Wood beams then weaken
and wet insulation requires replacement.
Ice damming
Ice damming most often occurs when warm air inside the building
rises and melts snow from the underside. Snow melts fastest at peaks
with the highest concentrations of heat. Water drips down to the
roof ’s edge, where it meets frigid air temperatures and freezes into an
ice dam. Continuing meltwater backs up behind the ice dam and
dangerous icicles start forming along the roof edge. These dams may
also occur when there are outdoor temperature fluctuations⎯warmer
winter days may actually exasperate an existing condition. Though
they look pretty, icicles drip and cause ice slicks on walkways below,
creating a hazard with potential for injuries and lawsuits. Icicles can
also fall, injuring people and damaging property.
Design elements of the building such as dramatic slopes, glass
ceilings, and multiple peaks can compound the problem’s severity.
Dramatic slopes allow meltwater to run down the roof at an
accelerated rate, leading to the formation of longer icicles. Multiple
peaks fashion numerous points of intersection with the base of the
roof, creating areas where water is prone to pond and freeze. Glass is
an extremely poor insulator and is susceptible to melting snow
collecting atop its ceilings.
Building owners may block pedestrian access for periods of
time, especially for public buildings in urban settings where the
risk of injury is higher. Ice falling from skyscrapers gain
momentum with height and are more likely to harm pedestrians
in busy downtown areas.
The system
Roof de-icing systems can prevent serious snow- and ice-related
injuries and reduce a building owner’s liability. De-icing cables
inhibit the occurrence of ice damming. Mineral-insulated (MI) roof
de-icing cable assemblies and/or self-regulating de-icing cables create
a heated drain path for meltwater to flow safely off the roof.
A moisture-detecting sensor that resembles a hockey puck is placed
inside the gutter. The sensor is flanked by an ambient thermostat and
control panel that monitor outdoor conditions.
When snowfall begins, sensors signal the control panel to
automatically turn on the mineral-insulated or self-regulating
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➤ Figure 2
Typical installation layout for a sloped shingle roof. The tracing
width is determined by the roof slope and eave length.
melting cables. Held in place by clips in a patterned design, these
cables heat up to melt the snow or ice on the roof. The meltwater
travels down the heated drain path, preventing the formation of ice
dams, icicles, and ice slicks.
The number of sensors depends on the building owner’s budget,
design considerations, roof size, sun/shade patterns, and
manufacturer. For a suburban residential roof, typically one sensor is
sufficient; in urban settings, more are needed. For example, a multilevel roof surrounded by other skyscrapers of varying heights may
require multiple sensors installed on different sides of the building to
provide independent heated zones.
Installation
Roof materials
De-icing cables can be installed on most common roof materials
including asphalt, metal, slate, glass, rubber membrane, and wood.
Underwriters Laboratories (UL)-listed roof de-icing cables are
limited to non-flammable roof materials, whereas Canadian
Standards Association (CSA)-certified cables have no roof material
limits placed on them. However, this author cautions against
installing them on flammable roof materials such as cedar shake.
As de-icing cables come in direct contact with the roof, some
materials require a temperature check⎯the maximum sheath
temperature of the de-icing cable should be compared with the
combustion temperature of wood roofing and the melt temperature
of rubber membrane roof materials. For wood roofs, if the sheath
temperature of the de-icing cable is higher than the combustion
temperature of the wood, a fire could start. If the sheath temperature
exceeds the melt temperature of rubber membrane, the rubber roof
will melt without starting a fire. Therefore, it is imperative cables do
not go beyond the maximum combustion or melt temperatures of
roof materials.
Sheath and roof material temperatures vary by provider. Designers,
owners, and installers should verify temperatures with both
manufacturing parties prior to installation.
Layout
On sloped roofs, de-icing cables are installed 152 mm (6 in.) past
the point where the eave meets the building, inside the gutters
and the downspouts. If any downspout drains into a pedestrian
5/21/08 3:13:07 PM
Damn Ice: A Look at Ice Damming
Before: The building’s original gutter was too shallow to contain
snow, which then slid off the glass roof, creating dangerous
situations for pedestrians and property below.
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very year, this multi-purpose government building experienced
avalanching at its main entrance and interior leaking problems
due to ice damming.
While beautiful to behold, the circular building’s pitch was
extreme toward the main entrance. When heavy snowfall and lakeeffect winds met the dramatic roof slope and shallow gutter, a
dangerous situation resulted for pedestrians below. Consequently,
the building’s main entrance was closed each winter.
An engineering firm resolved the problem using roof and gutter
de-icing systems. The gutter was deepened and filled with mineralinsulated (MI) de-icing cable with a high-density polyethylene jacket
run at a low wattage (due to temperature limitations of the rubber
membrane gutter material). The MI cable was selected based on its
specified heat output, durability, non-flammability, and nondegradability. This de-icing system now keeps the main entrance
open to pedestrians year-round. ✍
area, the heating cable is extended out of the downspout into the
nearest drain.
On flat roofs, de-icing cable layout depends on drain location. If the
roof uses downspouts, de-icing cables are installed along the roof
perimeter and extended into the downspouts. If the roof has an internal
drain, de-icing cable is installed in a butterfly pattern around it.
Roofing material can also impact layout. For example, on asphalt
shingle roofs de-icing cables are traditionally zigzagged across the
roofline or installed in a triangular pattern. (Figures 1 and 2, page 9,
illustrate the layout of de-icing cables on a roof.)
De-icing cables
Mineral-insulated de-icing cables arrive on the job-site factory-built
and ready to install. They are flexible, though able to retain bends. MI
cables are circular in shape maximizing contact with the surface. As
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After: A wider, deeper gutter accommodates the roof de-icing
system that helps keep the building open all year.
they are composed of inorganic materials, they will not degrade over
time and have a lifespan of 30 years.
Copper-jacketed MI cables will patina with copper roofs and
stainless steel-jacketed MI cables blend well with silver roofs. MI
cables can also run at very low wattages to keep the sheath temperature
lower for rubber membrane and wood roof materials.
It is recommended roof dimensions be verified on site before
ordering MI cables. If extra cable is ordered, there is some flexibility;
however, insufficient cable leaves no room for flexibility.
As self-regulating de-icing cables are supplied on a reel with
termination kits, exact roof dimensions are not needed. Wattage
output varies along their heated length in response to changes in
ambient temperature and moisture (see Figure 3). These cables are
made with plastic materials and have a lifespan between eight and
10 years.
5/21/08 3:13:09 PM
Ice Melt Panels in Practice
De-icing cable is installed in the cable channel within the panels,
which are then fixed along the roof perimeter. Multiple panels
can be stacked and then covered with shingles.
The school’s de-icing system (with ice melt panels) in action.
There are no icicles along the roofline because meltwater is
draining properly through the gutter and downspout system.
A
When the de-icing system is activated, the cables heat up the
aluminum panels, which effectively melt snow and ice along the
roof ’s edge. Meltwater then travels along the heated gutters into
the downspout and reaches a drain.
Heavy snowfall in much of Canada makes roofs particularly
susceptible to damage. In its first year of operation, the school’s
system was put to the test during a February snowstorm that
dumped 396 mm (15.6 in.) of snow. Despite the heavy
accumulation, the panels successfully melted all the snow and ice
along the roof ’s edge.
Ice melt panels are especially attractive to architects who wish to
avoid the triangular or zigzag cable patterning along the gutter
line. These panels melt snow and ice without compromising the
building’s esthetic value. Moreover, they lower installation costs
because they use about a third as much cable⎯ panels are installed
around the building’s perimeter, typically requiring less cable than
required for the standard triangular pattern. Although panels are
hidden, they are easily accessible for repairs and maintenance. ✍
school opted for ice melt panels to retain its roof ’s beauty
while preserving its life. Considerations included avoiding
seam leakage, ice damming, and stress on gutters. Of
paramount concern was the prevention of icicles and ice slicks
where children would be walking or playing.
One row of 0.2 x 1.2-m (8 x 48-in.) angled section panels were
installed along the 3.5:12 pitch roof edge where snow and ice
typically accumulated, and the cable angled towards the gutter.
A parallel row of flat panels was installed to transfer heat past the
point where the eave met the building. Flat panels were also placed
on both sides of the valleys.
Within the 1.2 x 0.2-m (48 x 8-in.) panels, roof de-icing cables
were neatly fitted into channels located on the panels’ edge, after
which the cover was snapped on. The panels were then installed
atop the roof ’s waterproof membrane and concealed under asphalt
roof shingles using screws, nails, or adhesives. Only the aluminum
de-icing cable channel cover remained exposed.
Aluminum is used because of its strong heat transfer properties.
Both MI and self-regulating de-icing cables contain ultraviolet
(UV) inhibitors to prevent sun damage.
Cable costs depend entirely on the manufacturer. Typically, MI
cable is less expensive than self-regulating cable. However, selfregulating cable’s ability to adjust wattage output in response to
temperature can lower operating costs. In terms of performance,
they both create the same type of heated melt path.
Many installers select self-regulating cables because they can be
purchased at a local distributor and do not require field verification
of roof measurements. Electrical engineers typically prefer mineralinsulated cables because their inorganic composition does not
contribute to fire, and they last longer.
Controls
When choosing a control model, the following questions should be
asked of the manufacturer:
1. How many sensors can be installed?
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2. Are the sensors aerially installed at the roofline or inside
the gutter?
3. Do the sensors have moisture sensors on both sides? How high
does the water have to be in the gutter to touch the moisture
sensors? (This is important to ensure all meltwater drains.)
4. How long is the timer?
5. How does the de-icing system shut off?
Regardless of model type, controls typically operate on the same
principles. They activate the de-icing cable system when the following
two conditions are met:
• the ambient temperature falls below 4 C (40 F); and
• the sensor detects snow, ice, or moisture.
These conditions ensure the de-icing cable operates when moisture is
present and it is cold enough for water to freeze.
Once the de-icing system is activated, the control continues to
check moisture and temperature at specified intervals. When the
sensors detect a change in conditions, the control deactivates the de-
5/21/08 3:13:13 PM
➤ Figure 3
For example:
• aluminum and copper clips are attached to standing-seam metal
roofs using double-sided airplane tape, attaching metal to metal
without pulling off;
• clips are attached with roof adhesives to asphalt roofs;
• clips are hooked over slate shingles in slate roofs; and
• specialty clips are used for retrofitting on existing slate roofs and
Spanish tile roofs.
The lowest ambient temperature at which adhesives can be installed vary
by manufacturer⎯some cannot be installed at temperatures below 0 C
(32 F). Adhesives typically take 24 to 72 hours to cure. Consequently,
clips should be installed one to three days before cable installation.
Otherwise, there is a good chance the clips will be pulled off the roof by
the cables’ weight.
Drain path
A drip loop is an important feature on sloped roofs designed to direct
meltwater into the gutter. Hangers are used to attach de-icing cables
to the inside of the downspout.
This graph depicts how a self-regulating cable will adjust
wattage output in response to ambient temperature change.
icing cables until both conditions are met again.
A remote indicator and activation device may be used. This system
allows the user to observe the de-icing system’s operational status as
well as start the de-icing system in the event moisture is not present
on the sensor, but residual snow on the roof is of concern.
Depending on the time of day and location of the sensor, variations
in detection can occur due to roof temperatures, sunlight, and wind.
Consequently, installers need to place the moisture sensor where
snow detection is most likely.
The roof can also be separated into control zones based on sun/
shade patterns. (The sunny side of a roof melts faster than the shady
side.) Also, urban settings with buildings of varying sizes located on
the same block radius may cause differing sun and shade patterns. At
least one control and sensor is installed per zone.
If only one control is installed for the roof, the system may either
be oversized or undersized depending on the sensor’s location. For a
small roof with an oversized system (meaning the sensor is located
on the shady side), it may not make a significant financial difference
in terms of operating costs for the sunny side to operate longer than
necessary. However, a large roof with an oversized system will
encounter much higher operating costs.
If the system is undersized (meaning the sensor is installed on the
sunny side), the shady side melt path will not be activated long
enough to drain all the water off the roof.
Therefore, the engineer should visit the building to see how sun/
shade patterns fall to ensure a sufficient number of sensors is
adequately placed.
Clips and adhesives
Attachment methods for de-icing cables vary by roof material. Clips
and adhesives are designed to hang cable in place on most common
roof materials without roof penetration.
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Design considerations
Gathering information about a roof is important when considering a
de-icing system. The following characteristics should be considered:
• plan drawings;
• elevation drawings;
• available voltages;
• roofing material;
• roof dimensions;
• section of roof, gutter, and downspout system intended
for heating;
• heating of valleys;
• gutter length and width;
• length and number of downspouts;
• shape of roof (i.e. flat or sloped);
• relation of gutters, downspouts, and eaves to each roof portion;
• gutter, downspout, and eave material;
• conduit distance to the nearest junction box system; and
• desire to retain roofline’s esthetic value.
For sloped roofs, further considerations include roof pitch and the
distance eaves overhang from the building wall.
A choice then has to be made between mineral-insulated (either
single-conductor or dual-conductor) and self-regulating de-icing cable.
If esthetic value is a concern, ice melt panels can be added to conceal the
de-icing cables altogether. (See “Ice Melt Panels in Practice,” page 12.)
General roof de-icing system guidelines
Correct design, installation, and maintenance of a roof de-icing
system are required to ensure safe and reliable operation:
1. For metal eaves and gutters, use a maximum of 15 W
per 0.3 m (1 ft).
2. For composition roofs, use a maximum of 8 W per 0.3 m (1 ft).
3. For optimal results, de-ice eaves, gutters, valleys, and downspouts.
4. To calculate de-icing cable lengths, use the manufacturer’s
recommended application design for the roofing material and pitch.
5. When calculating maximum circuit lengths, refer to the
manufacturer’s datasheet.
5/21/08 3:13:17 PM
Energy Consumption
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s building owners aim to save on operating costs, it is imperative the roof de-icing system be neither undersized nor oversized.
To estimate hourly operating costs, the length of the roof should be multiplied by a factor of three to yield a rough cable estimate
for the roof, gutter, and downspout portions of a roof de-icing system.
Calculation to determine cost per hour
Length of roof = 30 m (100 ft)
kW rate per hour = $0.08
De-icing cable wattage = 8 W per 0.3 m (1 ft)
Estimated total length of cable = 30 m (100 ft) x 3 = 90 m (300 ft)
Total wattage = 90 m x 8 W per 0.3 m = 2400 W = 2.4 kW
Cost per hour to operate the system = 2.4 kW x $0.08 = $0.19
Self-regulating de-icing cables may provide further cost savings because they lower wattage output as ambient temperature rises;
conversely wattage output is raised as ambient temperature drops. However, the lifespan of self-regulating cable is lower than that of
mineral-insulated de-icing cable. ✍
Ground-fault protection of equipment (GFPE)
In Canada, all exposed roof de-icing systems are governed by
Canadian Electric (CE) Code Rule 62-300, “Electric Surface Heating,”
which requires the use of 30-mA ground-fault protection of
equipment. Most manufacturers of roof de-icing systems provide
GFPE inside the load-switching contactor panel.
Instruction manual
Many skilled professionals, particularly those with a firm
understanding of roof de-icing systems, may be tempted to ignore
the instruction manual and rely on acquired knowledge. However,
properly training installers, electricians, and others involved in the
process can only come from reading the instruction manual prior to
installation practices.
Properly installed roof de-icing systems require no maintenance.
The author recommends cleaning leaves from the gutter and taking
caution not to damage any gutter cables in the process. ✍
E. Ada Cryer is an owner of Delta-Therm, co-ordinating the
company’s American Institute of Architects (AIA) continuing
education program. An authorized instructor for the Radiant Panel
Association (RPA), she was a member of the RPA board of directors
for three years. Cryer has been working in this field since 1988. She
can be contacted via e-mail at [email protected]
Contents of Construction Canada are copyrighted and are reproduced by FosteReprints with consent of Kenilworth Publishing Inc.
The publisher and Construction Specifications Canada shall not be liable for any of the views expressed by the authors,
nor shall these opinions necessarily reflect those of the publisher and Construction Specifications Canada.
www.delta-therm.com
toll free: (800) 526-7887 / fax: (847) 526-4456
[email protected]
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