Snow curls

In the right conditions, snow curls as it slides off our roof.

It’s raining today, aka miserable winter weather for a guy who loves snow and snow sports. What little snow we had left is melting quickly. As a fun side effect of the past week’s moderate temperatures (highs just above freezing, lows just below freezing), however, the snow on our garage roof is curling. What the heck? read on.

Snow slowly slides off our garage roof as it melts, rising up where it hits the snow-retainer bars. It froze into this position a week ago! (Photo from two days ago, 14 January.)

The roof held 2-3″ of snow from a snowfall two weeks ago; when the weather warmed above freezing, it started to slide down the roof, but a row of plastic bars hold it in place (see why, below). This past week, the snow was just slick enough to slide over the bars, but just dense enough to hold together, forming what looks like a ski jump. When the temperature dropped below freezing, overnight, the curls freeze in place. This slow process persisted for a week.

With today’s rain and warmer temperature, the process is accelerating, leading one curl to roll up triple! Here, seen today from the opposite end of the roof:

Snow curls on the garage roof; note the third bay from left, rolled up into a tight curl. Photo from today.

I know some of my readers are not from snowy climates, so let me explain. On traditional roofs (asphalt shingles), it’s important for snow not to accumulate, then melt and re-freeze. The snow will not slide off rough asphalt shingles; indeed, it can bond to the shingle as a thick layer of ice. These “ice dams” can cause future rainwater (or meltwater) to back-up and seep into the roof and inside the home. I used to spend a lot of time raking the snow off the edges of the roof, to prevent this expensive damage. So, about ten years ago we replaced our roof with a standing-seam metal roof, and I no longer need to rake the roof. The snow now slides off! 🙂

On the other hand, the snow slides off in massive avalanches. 😦

The snow can still accumulate in cold conditions, perhaps several feet thick; when the weather warms a little, a melt layer forms between the snow and the roof and, all of a sudden, an entire roof-full of snow can slide off and create an avalanche below. (Our cat loves to watch for these, from the safety of a windowsill.) On one side of the house, Pam re-designed the decking to fold up in winter, allowing the avalanches to fall right through and onto the ground below.

Deck-level accommodations for snow avalanches off the west roof.

Great solution! On the other hand, it passes the buck to the floor below, so we set up plywood ‘deflector shields’ to angle the snow away from the siding and foundation.

Ground-level accommodations for snow avalanches off the west roof.

On the other side of the house, however, we have a walkway between the house and the garage; when those roofs let go of their snow, we end up with a densely-packed mass of snow on this walk and our entryway. Today it’s a minor irritation, because there is little snow left to fall off the roof.

Snow falls off the garage roof as it slowly melts.

Next time we get a good dump of snow, I’ll show you what that looks like, and invite you to come shovel it out with me ;-).

Author: dfkotz

David Kotz is an outdoor enthusiast, traveller, husband, and father of three. He is also a Professor of Computer Science at Dartmouth College.

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