| Warm sloping roofs
(i.e. sloping roofs with little or no attic or rafter ventilation) will
melt the roof snow faster and more continually than a "cold roof"
for the same installed roof insulation. This continual melt water generation
must be constantly intercepted and drained away before it is exposed to
the freezing air at the eaves. If not, the melt water will refreeze and
form ice dams and icicles. Icicles may be exquisitely beautiful but other
than that, neither ice dams nor icicles have any redeeming social value.
Ice dams cause a concentrated unbalanced loading condition at eaves. Ice
dams cause melt water to pond and back up behind the dam. This in turn
can easily lead to leaks if the roof substrate is not adequately waterproofed.
Icicles and ice dams are deadly when they fall and destructive to the
roof when they tear away. Ice dams usually take shingles, flashings, and
other materials along when they fall. Shoveling off roof snow to mitigate
ice dams is usually a case of wasted time and money with a hefty roof
repair cost thrown in for good measure. Chipping away at ice dams with
shovels, picks, etc. more often than not damages the roofing. Shoveling
from the eave partially up the roof is a waste of time and energy as an
ice dam will form just down slope of where the shoveling stopped (melt
water hits the freezing air at that point). Only if the entire roof slope
from eave to ridge is cleared of snow will the ice dam/icicle formation
cease.
Cold sloping roofs (i.e. sloping roofs where a great deal of natural
ventilation is provided at the attic or rafter space) work well to mitigate
ice dam formation. (Figure 5) With a properly designed cold roof, the
large outside air ventilation air flow below the roof surface washes
away the building heat loss migrating into the cold roof cavity. When
this "washing away" effect is substantial, little or no interior
heat warms the underside of the roof, and therefore the sloping roof
snow cover is not melted by building heat. No melt water, no ice dam/icicle
formation.
A cold sloping roof and its ice dam mitigation performance will be
discussed in section VI.
The journal article "Attic Ventilation Guidelines to Minimize
Icings at Eaves" (Tobiasson, Buska, Greatorex, 1998) presents a
case study of ice damming and formation of icicles at four identically
built buildings in upstate New York. Outside temperatures and attic
temperatures were measured. The conclusion of this paper is that icing
problems at eaves occur only when outside temperatures are below -5.6°C
(22°F), and attic temperatures are above -1.1°C (30°F) at
the same time. Tobiasson (1998) refers to this situation as "Icing
Envelope". Later mechanical ventilation in the attic space was
introduced in 56 buildings with former icing problems. This ventilation
was designed to keep the attic temperature under the critical -1.1°C
(30°F). Maintaining these low temperatures, icing problems were
avoided or significantly reduced. The article reports that "there
were no reports of problematic ice dams or icings on any of the modified
buildings". Tobiasson, Buska, Greatorex, (1998).
This test building is at Sugar Bowl, Norden, CA (near Donner Summit)
in the Northern Sierra Mountains. (Figure 6) This cabin is the now constructed
Third Case Study in the article "The Impact of Ice Dams on Buildings
in Snow Country" Mackinlay, Flood (1996).
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