1 June 1998
from which snow will slide unless it has an R-value of at least 5.3K*m /W (30 degrees F*
h*ft2/Btu) if unventilated or at least 3.5K*m2/W (20 degrees F*h*ft2/Btu) if properly ventilated. The
ability of metal roofs to shed snow by sliding can be an advantage and a disadvantage. When
snow sliding off the roof does not damage the roof itself or create hazards to people, property,
and lower roofs, sliding is beneficial since it reduces snow loads on the roof. However, sliding
snow can create several hazards. It can drag plumbing stacks and other roof penetrations with
it, ripping holes in the metal roofing, which voids the waterproofing sealants. Snow cornicing at
eaves can create large overhanging loads. Cornices may grow large enough to curl around and
damage wall and windows. They may break off and damage the building near the ground also.
d. Snow Guards. Snow guards may be needed to hold snow on slippery roofs and should be
at the bottom of the roof, as the roofs are generally attached at the eaves and float at the ridge.
Usually snow guards should not be mechanically attached through the metal roofing into purlin or
other supporting members since that voids the floating features of most metal roofing and
results in leaks. Adhesive systems can work well if properly installed. Peel and stick systems
have not proven successful.
e. Electrical Heating Cables. Electrical heating cables are able to melt small drainage
tunnels through ice dams. Properly isolated and ventilated metal roofing systems should not
require electric heaters to prevent ponding behind ice dams. However, such heaters may be
needed in some isolated areas such as valleys that are difficult to ventilate properly. Too much
heat can increase icing instead of solving icing problems. Whenever possible, heating cables
should be installed under the metal roofing to avoid the inevitable maintenance problems
associated with exposed heaters. Attaching heating cables on top of metal roofing is difficult.
Perforated stainless steel clips attached with special, neutral curing silicone adhesive works well.
Such clips should support the heating cables directly. Indirect securing using plastic cable ties is
not recommended since such ties survive only a few years at most. When the roof is heated, the
metal fascia below must also be heated or large icicles will form. On fascia, icicles tend to form
at the edges of heated tunnels melted by isolated heating cables. Vertical baffles, installed on
either side of heating cables can prevent this, provided that additional cables are used to warm
the baffles and the fascia between them. Fascias below valleys containing heating cables should
be baffled and have a section at least 0.3 m (1 ft.) long fully heated so no ice can form on it.
Much more heat is needed on the fascia than in the valley. If the drip edge of the fascia is not
heated, icicles will form there also. Electrical heaters should be thermostatically controlled to
activate only when the outside temperature in the shade is below 1 degree C (34 degrees F).
When inexpensive thermostats are used, they are best set at 4 degrees C (40 degrees F) to
achieve this control objective. Often snow guards are needed where heating cables are installed
to prevent heater damage from sliding or creeping snow.