1 June 1998
10. COLD REGIONS CONSIDERATIONS.
a. In cold regions, roofs that drain over their eaves may develop icicles and ice dams in cold
weather. Technically and economically, low slope membrane roofs with internal drains are often
a better choice for large buildings in cold regions. When systems that drain over their eaves are
chosen, ice that forms at eaves causes water to pond on roofs. If the roof is not completely
waterproof (most are not) leaks are inevitable. Since building heat, not the sun, is the primary
cause of ice dams, metal roofs placed over heated buildings should be well insulated and,
whenever possible, ventilated.
b. Insulation versus Ventilation. Insulation alone will reduce, but not eliminate, icing
problems. Serious ice damming has occurred on unventilated roofs with a thermal resistance (R-
value) in excess of 5.3 K*m2/W (30 degrees F*h*ft2/Btu). Ventilation is not always easy to
incorporate into metal roofing systems, but it is an appropriate feature for most metal roofs that
slope to cold eaves in cold regions.
(1) Attic Ventilation. For reasonably well insulated roofs with no heat sources in the attic
space, designers should use the 1/150 attic area rule used to size openings for natural ventilation
as discussed in the ASHRAE Fundamentals Handbook. Continuous inlets are needed along
eaves and continuous exhaust openings are needed along ridges. Large openings in far apart
gable ends are not very effective at ventilating the central portion of large attics. Where motors,
hot air ducts, or other heat sources exist in a cold attic, calculate the increase in ventilation
needed to remove that heat in addition to rising heat from the heated building below. Dining
facilities almost always require such calculations. Air handling ducts located in cold attics should
be well insulated.
(2) Ventilation of Cathedral Ceilings. When ventilation is provided in narrow air ways
between metal roofing and ceiling insulation, the 1/150 attic area rule may not provide enough
ventilation to minimize icing when the length of the airway from the eaves to the ridge exceeds 6
m (20 ft.). Such narrow airways should be at least 38 mm (1- in.) deep for airway lengths up to
6 m (20 ft.). For longer airways, airway depth should be increased to a minimum depth of 76 mm
(3 in.) for airways over 24 m (80 ft.) long. Whenever possible such airways should be
interconnected, not isolated. Interconnections facilitate ventilation of valley areas and areas
above and below large roof penetrations that block isolated airways.
(3) Design Temperatures for Ventilation. Ventilation needed to avoid serious ice
damming should be based on maintaining the attic (or airway) temperature below -1 degree C
(30 degrees F) when the outside temperature is -5.50 degrees C (22 degrees F). The
calculation method presented in the ASHRAE Fundamentals Handbook is appropriate for these
design temperatures. Such calculations may indicate that natural stack ventilation will not suffice
for some buildings. If mechanical ventilation is used, it should supplement whatever natural
ventilation can be provided. Mechanical ventilation should be thermostatically controlled so that
it is energized only when the attic temperature is above -1 degrees C (30 degrees F) and the
outside air temperature is below -5.50 degrees C (22 degrees F). By placing undampened
exhaust fans near the ridge of roofs needing supplemental mechanical ventilation, the fan
openings can also serve as exhaust openings for natural ventilation.
c. Sliding Snow. Icing at eaves can prevent snow sliding off slippery metal roofs. Because
of this, ASCE 7 does not allow the metal roof of a heated building to qualify as a slippery surface