General snow load discussion:
1. snow on the roof will typically be less than snow on the
ground due to the influence of wind and the non contiguous
nature of the snowpack on the roof. also, the edge effect
of wind eroding the snow from the roof and the fact that
wind velocities on the roof are higher than those next to
the snow/ground surface.
2. roof orientation with respect to the prevailing winds can
have a dramatic affect on the amount of snow retained on a
roof.
3. microtopography of the roof can determine the size and
location of snow drifts.
4. drift surcharge is typically responsible for the majority
of failures. overall the total roof load isn’t exceeded
very often, it is typically a drift in a specific area or
other unusual circumstances such as Ice buildup or rain on
snow with plugged drains, etc.
5. in deep snow country, avoid gutters, they will just get
ripped off by snow creep, etc. they will also allow the
edge of the roof to be damages, water to seep back through
the eave structure to the inside as they pull off/apart.
6. always put a gable over every entrance to a building such
that the snow falls to each side of the entryway. this will
prevent small scale avalanches from killing/injuring people
as the enter/leave a structure. the sliding snow will be
diverted to each side of the entrance. this also helps with
snow removal, you dont have to shovel as much.
7. complex roofs: those with many gables, levels, dormers,
and protrusions have the highest potential for drift
surcharge and can have the weakest points at the joining
surfaces. They are also the ones that are currently “in
style” and you see more and more very complex roof lines in
various applications. The areas between dormers can easily
fill to the overall roof level, easily doubling the roof
load in those areas, which again are the weakest part of the
overall roof.
8. wind patterns in and around the roof location
dramatically affect the snow catch on the site. Trees
adjacent to the site can either act as a snow fence, adding
to the snow load or as a venturi, eroding the snow from the
roof. Most sites/owners prefer a lot of trees to get a
‘deep forest’ kind of feel and this generally adds to the
roof load. those who are on a mountainside and have a view,
tend to open up the downhill side to get an unobstructed
view of the peasants below, this typically opens up a
venturi effect which will typically erod off one side and
deposit on another yielding unbalanced loads.
9. north south aspects: there is always a disparate snow
load on these two sides of the roof: under no wind
redeposition scenario the north will always have a higher
load than the south and at time, it can be severely
overbalanced with the north having 2 to 4 times as much snow
as the south, particularly at this time of year. This is
the can opener affect where one side of the roof tries to
peel off the opposite side. typically the walls will try to
buckle inward on the heavy side.
10. snow sliding onto lower roof levels. upper decks can
contribute a great deal of snow and ice to lower levels both
through snow sliding and creep as well as meltwater
reforming in ice layers and ice dams on gentler slopes. as
this snow accumulates thru the year, it can easily submerge
the structure adding significant shear stress to the
building as thesnow settles and compresses later in the
season. this shear stress can pull a structure down. this
shear stress is great enough to bend schedule 40 galvanized
steel tubing sections 7 feet long by as much as 6 inches.
10. SNOTEL sites with 20 inches of swe or less, get a
standard fence. those with 20 to 50 inches, get a
reinforced fence and those with over 50 inches get a 3 leg
fence. the force of snow on various structures is immense fences of all kinds can be laid horizontal or stripped off
the posts and laid on the ground.
Snow creep moving down steep slopes can shear off telephone
poles, towers, ski lifts towers, radio fascilities and
anything else that protudes through the snow. At heavenly
valley where we have a 26 foot rain gage, we have to have a
splitter device uphill of the gage so the snow doesnt knock
it over - it is a 6 inch angle iron about 20 feet high with
the angle pointed uphill. we started out with a 4 inch
angle but it was damaged.
11. Shear strength in a snowpack is dependent on its
density, structure and the temperature within the pack.
Very cold, small crystalline structure with high density has
the highest shear strength. large crystals with poor
cohesion and high temperatures has lower shear. Ice has the
highest shear strength and can be a troublsome nuisance such
as when the bear river bay froze over in 84’ with water
backed up to record levels, it covered 2 major electrical
lines to a depth of 1 to 2 feet, froze solid and then thru
expansion, sheared hundreds of power poles off at the base.
12. snow creep can occur, given the right conditions, on
angles as low as 8% slope. They can transmit vertical
forces of over 150% of the ground load and far greater
forces in shear stress.