Weight of ICE and Snow
People become pretty anxious when they see a lot of snow on their roof. While weatherman and citizens are impressed with how deep the snow is, but
what’s of real interest to an Engineer is what is the weight of the snow and what influenced how much was accumulating on a structure.
Buffalo recently had 6-7′ of snow with some documented collapses.
So why did one structure fail and another didn’t?
How much did the freshly fallen snow actually weigh and was the weight above the building code design parameters?
If the weight exceeded the code design, will the structure collapse?
Trained Professional Engineers routinely investigate these claims, and the investigation starts with identifying what snow load can be expected. Collapses that occur below design loads point to either unusual accumulations and unbalanced loads caused by drifts and/or design, maintenance, decay, or construction defects. Design loads are intended to address how much bending is allowed to prevent damage to finishes. They are not intended to be the point where a collapse should occur.
Weight of Snow and Ice
The best way to determine the weight is too actually taken a cylinder of known weight and size and push it into the snow, collecting the snow and weighing it. Simple division of weight divided by volume provides the density and the value is multiplied by the supporting area to determine the weights.
Now I’ll agree, I’d be impressed with 7′ of snow, even though I grew up in Rochester NY. Since I’m now located here in balmy Connecticut and can’t take a sample of newly fallen snow without driving 10 hours in a blizzard to determine what the snow might have weighed in Buffalo, I’m able to estimate what real impact can be expected based on the average weight without drifts.
One way to estimate the snow quality and weight is based on the temperature when the snow fell. During the snow event in Buffalo in November 2014, the average temperature was estimated to be 25F. The newly fallen snow is estimated to be the approximate equivalent of a bit less than 5″ of water (round up to nearest full integer). A square foot of water with an inch depth of water weighs 5.2 psf.
The estimated weight of the snow on the ground would be 26 psf, (5.2 x 5).
Now, if it’s windy, that can cause some drifts and also increases the density of the snow where it accumulates and Engineers will need to help you assess the field conditions.
Snow to liquid ratio (SLR)
There are other ways to estimate snow loads and another commonly used method is the Snow to Liquid Ratio (SLR). This value can be widely variable depending on whether the snow is wet or dry. Average values for a region are based on the following figure.
What does this tell us?
The building codes are based on the ground snow load, such as calculated in the previous paragraph. To keep it simple we will look into the International Residential Code (IRC) for single and two family houses. The IRC is updated about every 3 years and the one that’s important to a given structure is the one that was in effect when the structure was permitted to be built.
Just looking at the Design snow load graphic we’d find that the minimum ground snow design load in the Buffalo region (IRC 2000) was between 35-40 psf. Local and state building officials may raise this value based on their experience but it at least gives us a place to start. Also, when the code is changed, it typically is not retroactive unless new construction is proposed and the addition affects the original structure.
The estimated snow ground load in Buffalo was 26 psf, about 60-70% of the design load. Based on this alone, there shouldn’t be many structures affected. Why? What is a Design Ground Snow Load and what is it protecting? Why are some structures affected and others not?
What happens if the snow stays on the roof and it stays cold with some intermittent snow and rain? The weight of the accumulated snow on the roof will increase until there is sufficient melt off to reduce the weight. Ice often forms on the eaves and if the eaves extend more than 12″, the accumulated weight of ice can cause damage to the framing at the lower edge of the roof.