Stressed about weight of that snow on your roof? Your roof may be stressed too

A severe snowstorm caused the collapse of the Edmonds Marina roof in 1996. The Port of Edmonds says the rebuilt roof held up fine during the most recent storm. (Photo by Doug Parrott)

That sound of snow sliding off your roof is a welcome sign that temperatures are warming, and the worst of our local winter blast is at last behind us.

But what if you don’t hear it sliding or if your roof is flat? Could it collapse? How much snow is too much?  Remember the 1996 collapse of the Edmonds Marina covered moorage?

As this week’s snow melts, the water content goes up and it’s no longer that light fluffy stuff that fell earlier in the week. It’s heavier and more dense, and has people asking questions like:

– How much weight can my roof support?

– How can I prevent snow roof collapse?

– Should I remove snow myself or call a professional?

This chart compares the relative weights of fresh snow, packed snow and ice, showing the accumulation of each needed to hit the roof collapse danger zone of 20 lbs./square foot. (Graphics courtesy of Insurance Institute for Business and Home Safety)

According to the Insurance Institute for Business and Home Safety (IBHS), most home roofs regardless of location are designed to support 20 pounds per square foot before becoming stressed. While some locations such as New England and mountainous areas that are prone to significant snowfall require higher stress standards, your roof should be safe as long as it’s not damaged or decayed and if loads don’t exceed the 20-pound standard.

Because melting snow tends to run more quickly off sloped roofs, in most cases it will simply slide off before becoming a problem.  But for flat roofs or those sloped less than 3 inches per 12 inches of horizontal distance, it can be a problem. Think carports, lanais and other structures that are often added on, not part of the main structure, and frequently not engineered to support loads.

This graphic compares the amounts fresh snow, packed snow and ice required to equal 5 lbs./square foot of pressure.

But how much snow needs to accumulate to get you to that critical 20 lbs./square foot load (see chart)? It all depends on the kind of snow.

Fresh new snow is fairly light, with a 10-12 inch accumulation equaling about one inch of water. This works out to about 5 lbs./square foot, so it takes about 4 feet of new snow before the roof will become stressed.

Packed snow is heavier, with 3-5 inches equaling one inch of water, so anything more than 2 feet of old snow could be too much for your roof to handle.

Ice is heaviest. With almost the same weight per volume as liquid water, it takes only 4 inches to put your roof in the danger zone.

But, as happened this week, when new snow falls on top of old, the total accumulated weight can go up fast. For example, 2 feet of old snow covered by 2 feet of new snow could weigh as much as 60 lbs./square foot, well beyond the typical load capacity of most roofs.

According to the IBHS, if you are in the “danger zone” with thicknesses of snow and ice that exceed 20 lbs./square foot, you should consider removing it.

You can do it yourself with a long rake or extension arm that will allow you to remove it from the ground. You should make sure your downspout is clear, use a plastic shovel or rake to avoid damaging your roof, and start clearing from the bottom up. But be safe!  Don’t under any circumstances do this if it means getting on the roof.

In these cases, hire a contractor and expect to pay between $250 and $500 for most jobs.

And as for that 1996 Port of Edmonds Marina collapse? Executive Director Bob McChesney says that when the marina was reconstructed, “it was based on specific design improvements including pitched roofs, steel structure, and improved floatation.” During the great snow of 2019, he says, “no big problems to report. No catastrophes. No damage. No injuries.”

— By Larry Vogel

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