| The conclusion that
can be drawn from Figures 10, 11 & 12 is that the naturally ventilated
space in the cold roof performs the same way as the mechanically ventilated
attic in the Tobiasson study. The time periods with temperature readings
within the icing envelope when exterior temperatures are below –5.6°C
(22°F) and ridge temperatures are above –1.1°C (30°F)
are very rare and of short duration. Figure 5 underlines the result of
the graph. Some icing occurs, which is more likely due to the freeze-thaw
cycle than to ventilation problems. The previously discussed condition
of low exterior temperatures combined with snow cover on the roof is one
typical situation of the roof.
Figure 13 shows a different typical situation. December 22 and 23,
1999 are in a warmer time period. Metal roof temperatures peak in the
early afternoon.
This is an indication that the roof is not covered with snow. The sensor,
which takes these temperature readings, is located on the roof itself
on the South facing side, close to the ridge. Even on these shortest
days of the year with no snow on the roof, solar radiation heats the
metal roof significantly during the four to six hours it is exposed
to the sun, independent of the air temperature.
The condition change from a period of temperatures below freezing with
snow cover on the roof to a period of higher temperatures with the impact
of solar radiation on the temperatures readings for the south facing
metal roof can be observed in Figure 14. At temperatures below freezing,
the ribs of the metal roof hold the snow on the roof. November 12–14,
1998 is a time period of continuous air space temperatures above or
around the freezing point. This situation is caused by high outside
temperatures and increased heat transfer through the roof based on raised
interior temperatures due to occupancy change. After a time period with
enough warm air to warm up the metal roof ribs, the snow pack slides
off the slippery metal roof in the after-noon of 13 November 1998. (Figure
4) From that time on the metal roof temperature exceeds the outside
air temperatures due to solar radiation as shown in Figure 13.
The snow and cold greatly increases the complexity of roof design.
Not only are roof loads increased in ways that are not often self evident,
but such problems as ice dams and icicles can cause hazards that are
difficult to visualize during periods of warm weather. The roof designer
not only needs to understand the general principles of snow country
design, but be aware of the special conditions that apply to the site
under consideration. Special design techniques, such as cold roofs and
vapor retarders, can greatly reduce the impact of snow and cold on buildings.
Cold weather can produce many varied effects on buildings. There is
no substitute for experience when dealing with the problems of snow
and cold.
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