How to Calculate How Much Snow Your Roof Can Hold (Pay Attention, Tahoe)

SnowBrains | | BrainsBrains

It’s going down in Tahoe. 

Houses are already buried to the max in Lake Tahoe, and now an atmospheric river is forecast to slam the region and bring additional feet of snow.

But how much can these houses take? They already look like literal igloos—at least the ones you can still see do. 

How to calculate snow load on your roof, according to snow scientist Randall Osterhuber: 

A simple equation is used to calculate the weight of snow on a roof:

HS x density x 62.5 x cosine(roof angle) -HS is the height of the snow (feet). Density is the average density of the entire snow height profile (expressed as a fraction) Density is the percent of any snow volume that is occupied by ice. There are particular tools to measure this, none of which you will own. But you can make a go of it. You’re probably really good with your shovel by now.

Cut a nice, clean cube of snow, one foot on a side. 12” x 12” x 12”. Weigh it. The weight, divided by 62.5 will be the density of that snow sample. If your block weighs 21 pounds, its density will be .34; if the block weighs 30 pounds the sample has a density of .48. Density varies throughout the height of the snow profile, so if you can cut several blocks and get an average, that helps.

Example: You have 98 inches of snow on your roof or 8.2 feet. You measure the average density at .39. Your roof is sloped 20 degrees above horizontal. 8.2 x .39 x 62.5 x .94 = 188 lb/ft2 If your house has a roof 40 feet x 40 feet in area (1600 ft2), it’s holding up 300,800 pounds of snow. Some roof lines are simple, many are complex. HS will vary, as will density.

Snow loads on roofs are not static, they are dynamic. The snow is always moving so the forces it exhibits are always shifting. And many roofs are differentially loaded, meaning there is more snow on some portions than others. This can be caused by wind, sun, tree cover, how much heat is released from the structure below, and the routing of free water (meltwater) flowing through the snow. Differential snow loads exhibit lateral forces to the structure. Though most wood-framed structures are incredibly strong under compressive forces, they are considerably less robust with laterally applied forces. Beware the mid-winter earthquake!

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