Wednesday, January 4, 2017

How much does our snow weigh?

Many people are beginning to wonder how December of 2016 compared to the snowiest winter that the Inland Northwest had back in 2008. Along with that, we have received calls from people concerned about the weight of the snow on their roof.

We will first look at how the snow totals for a couple locations across the Inland Northwest from December 2008 to December 2016.

Snow Totals for the Month of December in 2008 and 2016 in inches. Rankings is based on how much snow the entire month of December has received from records dating back to 1892.

We can see that the places such as Spokane and Bonners Ferry did not receive snow totals close to that of 2008. On the other hand, locations such as Kellogg and Stehekin received almost double the amount of snow that they received in 2008.

When  looking for the possibility of a roof collapsing, we want to take into consideration that different weights that snow may hold due to water content. Light, dry snows weight ranges from 1 to 3 lb/ftwhile wet, heavy snow can reach up to 21 lb/ft2.Once the snow has fallen, the snow can accumulate more liquid through humidity, the compression of snow, and any additional rain that may have fallen. Snow tends to act like a sponge and even though it may look like the snow depth is decreasing, the snow is actually absorbing the excess water. In other words, the depth of the snow can be misleading compared to the weight of the snow. 

When looking at the maps below, we can see the weight of the snow (in lbs/square foot) at worst case scenario being placed on the roofs. This means that if there was no day time melting that occurred during that period, there could potentially be up to the stated weight on the roofs. But chances are that there was daytime melting that was occurring bringing the total weight down.

Snow load weight in  lb/ft for December 2008

Snow load weight in  lb/ft for December 2016

Above we can see how the weight of the snow differs from the different years. To get a clearer picture on which locations have increased and decrease, a table has been included for a side to side comparison. Note that the table does not include all locations as many of the locations did not have data for both years.

Red: increase in the weight load from 2008 to 2016
Blue: decrease in the weight load from 2008 to 2016

The storms of 2008 caused 95 structured to fail in some way across the region. 91% of those buildings were at least 20 years old. Newer buildings have to be built to code so that the they can sustain at least the normal amount of snow that the region receives.

Here are some suggested signs of snow load problems.
  • Cracks in the walls that were not previously there or have extended
  • Frequent popping, creaking, or cracking sounds 
  • The roof visibly sagging
  • Doors, windows that are noticeably harder to open or close, especially interior doors

For more information about structural snow loads, feel free to visit FEMA Snow Load Safety Guide

1 comment:

  1. Thanks for posting the Spokane Snow Water Equivalent (SWE) data at