The science of the 'urban avalanche'

As Chicagoland digs out, we consult an expert about the physics of the rooftop avalanche.

February 3, 2011

By Gabriel Spitzer

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(Flickr/GinnyRED57)
Roof snow can be an avalanche hazard. But probably not one you have to worry much about.

(This story has been edited to reflect the proper weight of rooftop snow)

As the Chicago region digs out from 20 inches of snow on the ground, keep in mind that a lot of it is still piled up above our heads.

Drifting snow on rooftops can be a problem for building owners, and for passers-by.

WBEZ’s Gabriel Spitzer reports on the science of the urban avalanche.

SPITZER: Science Storms is one of the newest permanent exhibits here at the Museum of Science and Industry, and right when you walk into this big cavernous hall, the first thing you see on your left is a giant movie screen, where there is a very cold-looking guy in a winter hat and a big beard, who is explaining the kinds of conditions in the mountains of Montana that cause avalanches.

ADAMS (RECORDED): For a snow avalanche, we need snow. We need a slope, and we generally have a weak layer sandwiched in between two strong layers. And then we hit it with a trigger, that whole thing can take off.

SPITZER: So what do avalanches have to do with the Chicago area? Well it turns out there are places right here in our area that are subject to very similar avalanche hazards as the mountain west. I’m talking about rooftops. It turns out rooftop avalanches tend to kill one or two people every year in the United States. And you know, it’s not a huge danger. It’s nothing even approaching the number of people killed or injured every year from car accidents on snowy or icy streets. But I wondered what the physics of a rooftop avalanche have to do with what’s going on in the mountains of Montana. So who better to ask then Ed Adams, the guy in this video.

ADAMS: Yeah, I’m Ed Adams, and I’m, professor of engineering mechanics at Montana State University in Bozeman.

Adams says the physics of avalanches in the mountains and off the roof are quite similar.

In each case, there’s a particular slope that’s most hazardous: about 37 degrees.

Now take those ingredients he mentioned in the film: strong layer, weak layer, and trigger.

First, the strong layer: snow on a roof can harden quickly into a slab, especially in the wind.

ADAMS: The wind has the propensity to drift, specifically on the leeward side of a slope or in this case, a roof. So you tend to get buildup on that down slope. And the snow is quite cohesive.

Then there’s the weak layer.

On a building, that often comes from heat radiating up through the roof.

ADAMS: In the urban avalanche, I think it may adhere to your roof for a while. But if you have a melt layer forming between that snow load and the roof itself, as it starts to get warm and that snow starts to get a little wet, you may start to keep your eye open for an avalanche coming down and nailing you as you walk in or out of your building or your house.

And finally, the trigger, which is often human.

ADAMS: We tend to think of it in terms of a skier or a snowmobiler. So you know, probably don’t ski or snowmobile on your roof, is probably a reasonable thing. But shoveling your roof, too, if you’re trying to get rid of that. It’s something to be very careful and maybe put some sort of a belay on, so you’re protected from that.

It’s easy to underestimate how much falling snow weighs.

If you have, say, a 20-by-20 foot roof with 20 inches of snow, depending on how dense it is, that could weigh up to 6 tons.

That’s like three good-size SUVs dangling over your head.

ADAMS: I think the problems there are going to be suffocation, if no one’s around to pull you out, or a broken neck.

Now, the science of all this may be intriguing, but let’s be clear: Rooftop avalanches are not a big problem around here.

The American Avalanche Association reports five U.S. fatalities in the last three years – none in Illinois.

But it’s still a good idea to be careful.

John Rogers runs Rogers Roofing out of Hammond Indiana, and he does quite a bit of snow removal.

His workers use a 14-foot long rake.

ROGERS: It allows you to pull the snow off your roof along the gutter edge, to where you’re going to be safe. When the snow does come down it’s gonna be towards the building and not on you.

Rogers says there’s a much bigger hazard out there: ice dams.

That’s when ice forms at the edge of the roof, so water can’t drain.

It can back up under shingles and cause mayhem.

ROGERS: It can get into your walls, so now you start worrying about electrical. So you’re talking about drywall damage, insulation damage and possibly electrical damage as well.

So your snowy roof may be more likely to bury you in bills than an avalanche.

In any case, Rogers advises homeowners not to remove the snow themselves.

As you’d expect, he says call a pro.

Gabriel Spitzer, WBEZ.