| daestrom 2005-07-19, 11:25 pm |
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"legg" <legg@nospam.magma.ca> wrote in message
news:aj7od11g953c27rq891pf0tbmr7ml8tc21@4ax.com...
quote:
> On Mon, 18 Jul 2005 12:59:48 -0300, Derek Broughton
> <news@pointerstop.ca> wrote:
>
>
> The snow is self-insulating, so the lower layers tend to melt due to
> heat escaping from the house. The problem is worse in geographical
> areas (and in normal thawing seasons) where wide ambient temperature
> swings occur during the 24hr day. In continental areas, this swing can
> easily be 15degC (37degF), daily, any time of year.
>
> Melted snow doesn't run off so much as bead and compact or creep in a
> slurry, to be caught by the next temperature dip. Ice is a lousy
> insulator, as the air content is removed - so a sort of equilibrium
> occurs - thick snow forming ice to the level where the original
> insulating effect is no longer sufficient to melt the snow.
>
> Roof slopes tend to show little regard to geography, nowadays. Its
> fashion, convenience and low cost that sells.
>
Actually, ice build up is *not* caused by daily temperature swings
above/below freezing. Take it from someone that gets 160 inches a year
average snowfall. Ice forms when heat in the attic melts the bottom layer
of snow, and the water runs down out over the eaves where the outdoor
temperature underneath the eave is below freezing. When the outdoor
temperature is *above* freezing, the ice at the eaves melts and any water
melted under the main roof doesn't re-freeze before dripping off the eaves.
The key to preventing ice dam build up is to keep the attic air space as
*cold* as possible. Lots of soffit vents, and either ridge or gable vents
(or both) to keep the attic air space as close to outdoor temperature as
possible. If the home has 'cathederal' style ceilings, we put a lot of
insulation in the ceiling, but leave a 2 to 4 inch gap between the top of
the insulation and the roofing. This allows a cold channel under the
roofing to prevent heat leaking through the insulation from melting the snow
on the roof.
If part of the roofing gets exposed to the sun, and it is a cold day (below
freezing), this can also build up some ice. The sun on dark shingles will
melt some snow around the edges of the exposed shingles and the resulting
water will re-freeze as it runs down the roof towards the eaves. But in
heavy snow country this isn't much of a problem since very little of the
roof is exposed to the sun :-(
In deep snow country, the pitch of the roof and wind exposure are issues
with regard to how much snow accumulates. Surprisingly, sometimes a low
pitch roof (such as 5/12) accumulates fewer drifts than a steep (12/12)
roof. While the windward side of both roofs are swept clean of snow by fair
winds, the lee side of a steep roof often accumulates much deeper drifts
because of the swirls/eddys as the wind trys to rise up over the windward
side then 'detaches' from the ridge. On the shallow roof, the wind can flow
more smoothly over the lee side and less snow is deposited in the form of
drifts. Of course, this depends on lot on how the local wind flows
over/around the house.
daestrom
P.S. Do you know the term 'Lake Effect' ???? ;-)
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