3 August 1998
in place on slippery roofs. Internally drained membrane roofing systems with a slope of
incj/foot avoid these problems. Switching from internal drains to scuppers can lead to
problematic, dangerous icings (figure 16).
10. ICICLES AND ICE DAMS. Icicles and ice dams can form along the eaves of inadequately
insulated and ventilated roofs of heated buildings that drain to cold eaves (figure 17). Where
eaves are not present, such ice may form on the walls below (figure 18). Icings at eaves
prevent snow load reductions by sliding until that ice warms up and either melts or breaks free.
Falling ice is a hazard (figure 6). Icings at eaves can be avoided when attic ventilation systems
are able to keep the temperature of the roof from rising above about 30F when the temperature
outside is about 22 F. When it is warmer outside, icings usually do not grow and when it is
colder outside, less attic ventilation is needed. Equations for sizing attic ventilation systems are
presented in CRREL Miscellaneous Paper "Ventilating Attics to Minimize Icings at Eaves." The
extra cost of adequately insulating and ventilating a roof to prevent icings is easy to justify since
the water that ponds behind ice dams usually leaks into the building causing significant
problems. Efforts to remove icings with hammers, axes (figure 19), chain saws, and such usually
damage the roof. On existing buildings, electrical heaters may be needed to keep tunnels melted
through small ice dams (figure 20). The tunnels prevent water from onding on the roof and
leaking into the building. Electric heaters are relatively easy to install along the eaves of a roof
with asphalt shingles (figure 21). Installing electric heaters on standing seam metal roofs is more
difficult. Guidelines are available in CRREL Miscellaneous Paper "Electric Heating Systems for
Combating Icing Problems on Metal Roofs." Essentially alnew roofs should be designed so that
they do not require electrical heaters.
11. SNOW GUARDS. Snow guards are objects used to hold snow on slippery roofs (figure 22).
Many slate and metal roofs require snow guards to protect people and property. Snow guards
may also be needed on barrel vaults and other such roofs with smooth membranes. Some snow
guards are attached mechanically while others are adhered to the roof surface (figure 23).
Design loads on snow guards should be based on the assumption that friction between the snow
and the roof is zero. Multiple rows of snow guards spaced well apart up the roof (figure 24) are
better at holding snow in place (i.e., avoiding the large dynamic loads created by sliding snow)
than one row of last-resort snow guards placed near the eaves. A short snow guard on a long
roof without other snow guards must be able to resist all the snow located within outward 45
angles up slope of its location. The loads at the ends of such a snow guard are about twice the
average load on it. The design load on a snow guard should be less than half of any failure load
reported by its manufacturer. In high risk situations, (e.g., entrances and emergency exits of
schools) allowable loads on snow guards should be even lower. Design guidance, test, data,
and performance standards on snow guards are limited so they should be used with caution.