Fennell Engineering
Call (978) 352-6500
M-F 9-5 or by Appointment
Call (978) 352-6500
M-F 9-5 or by Appointment
Snow Accumulation on Roofs: When to Worry
As Winter progresses and a series of snow storms
deposit an ever increasing depth of snow on the roof of a building, we are
often called by concerned owners wondering how they should determine if their
roof is overloaded with snow and should be shoveled. One of the most overlooked ways that snow will damage
your home is the actual process of removing it! Scraping
snow off your roof can damage the shingles and take years off
of your roof’s life.
Snow accumulation on the roof of a building is
influenced by several factors.
A sloped roof with a pitch of 8:12 or greater tends to
hold less snow than roofs with a lesser pitch. Flatter roofs such as those with
a roof pitch of 4:12 tend to hold more snow.
 
Wind tends to blow snow off of flat roofs and can have
the effect of piling snow on a roof with a parapet. Wind can also remove snow
from the windward side of a pitched roof and deposit the snow on the leeward
side of a sloped roof leading to an unbalanced and potentially hazardous
loading.
The cubic weight of snow has a wide variation
depending on density. Light fluffy snow can have a weight of as little as 7
pounds per cubic foot while wet dense snow can weigh as much as 25 pounds per
cubic foot. Deeper snow tends to have a greater weight per cubic foot as it is
compressed by the snow above.
Roof snow accumulation can be influenced by various
factors such as adjacent buildings, roof projections, mechanical units, changes
in roof elevation, friction of the roofing material and various other factors.
Newer homes that are well insulated tend to have
cooler roof surfaces than older homes.
 
These cooler roof surfaces tend to melt
less snow and as a result have greater snow accumulation that older homes when
all other factors are the same.
Building codes (and more broadly by ASCE 7-10) provide
a structural engineer with a ground snow load that varies depending on
historical snow fall in a geographic region. Ground snow loads in the NorthEast
range from a low of 25 pounds per square foot in some coastal areas to a high
of 100 pounds per square foot in Northern Maine.
An engineer uses that code prescribed ground snow load
to calculate a design roof snow load by applying various factors that take into
account several variables including roof slope, building use, thermal factor,
roof friction and wind exposure.
To determine the actual weight of snow on a roof a
test sample of the roof snow can be removed in a 12”x12” area down to the roof
deck and captured in a container, melted and a volume of water calculated.
Based on this the weight of the snow per square foot on the roof can be
determined. An engineer can evaluate the roof structure and determine its design
capacity. Using this information an educated decision can be made about the
necessity of removing the snow from the roof.
Although every roof is different and an engineer
should be called in if there is a concern, for many roofs in the NorthEast a
rule of thumb is that if a roof has in excess of 18” of snow either an
evaluation should be done by a professional engineer or the snow should be
removed.
An inspection can also reveal potential weak spots or
places where your roof structure, deck, or framing might fail. Additions or modifications can affect your
home’s ability to support snow.
Shoveling snow off of a roof can be a hazardous and
expensive proposition and should be carefully considered before proceeding. All
safety regulations should be taken into account.
Additionally it is important to remove the snow in a
manner that doesn’t make the roof unstable such as removing all the snow from
one side of a gable roof but not the other.
http://newyorkengineering.blogspot.com/2013/12/snow-accumulation-on-roofs-when-to-worry_30.html |
