I clearly recall the first time I was given a rhetorical question to ponder; well, maybe not so much rhetorical as it was a word puzzle. My Grandpa Hawke loved teasing us grandkids and I was the ‘victim’ of one when I was about eight years old.
“Davey,” he asked, “I have a problem that maybe a smart kid like you can help me with. I’d like to build a wheelbarrow to carry some stuff around, but am trying figure out how sturdy to build it. What do you think weighs more, a pound of feathers or a pound of coal?”
I fell for it. “Coal, Grandpa!” He smiled and thanked me for my input. It took a while, but later that morning I realized my quick answer was not as brilliant as I first thought it was.
Thus began my curiosity about density, mass, weight, volume, friction, drag, velocity and all things regarding scientific physics.
So let’s update the question to today’s situation: what weighs more, a kilogram of new fluffy snow, or a kilogram of old compressed snow? This is a real concern right now, as many of us have a whole heapin’ helping of snow sitting upon our rooftops. Should we be worried?
Short answer: yes.
The snowfalls we have had over the past couple months have provided many types of snowflakes: some sharply pointed, some round balls, some flat plates. There are about nine different shapes that snowflakes, each determined by the amount of moisture in the air and the temperature at which they formed.
‘Wet’ snow is heavy and packs quickly, whereas ‘dry’ snow is usually spiky and the points interlock with each other forming a fluffy layer. So, which kind should I be leery of ... the wet compacting snow or the light fluffy stuff?
Let’s not lose sight of the fact that snow is water. And water is one of the heaviest elements found in nature. It is dense with very little space between its molecules; even granite rock has tiny air spaces between the grains.
Warning: old school measurements follow: One inch of packed snow weighs about one pound per square foot. One cubic foot of water weighs 62 pounds. That’s the easy part, now it gets complicated.
The density and moisture content of the snow on your roof varies widely ... the deeper it is, the more complex the structure. A foot of new fallen fluffy snow soon compacts to five inches, which then feels the pressure of additional snow on top again, then compacting to one inch of ice.
This compacting process happens each time it snows, and the amount of snow as well as its humidity will vary the rate of compaction. Confused yet? You are not alone. Just to mess with your mind, a foot of light snowfall weighs considerably less than four inches of wet snow.
So where am I going with all this mathematical mumbo-jumbo? Just sharing my anxiety as I look up at my burdened rooftop. There’s about a metre (like around three feet in the old days) of combined new snow, packed snow and ice sitting up there, taunting me as to whether or not it will slide off on its own.
The angle, or pitch, or steepness of the roof is a factor on snow slide, as well as what the roof is covered in ... rough ice-grabbing shingles or smooth slick steel sheets? Or in my case, rusty steel with numerous loose nails sticking up.
When I was in college and taking physics, we had to set up a ramp and set various weights on the top of the slide. At each angle we set the ramp at, how much weight could be placed at the top before gravity pulled the weight down the slide? Obviously the steeper the angle the less weight stayed in place. Therefore, a steep roof should shed snow more quickly than a flat roof.
But what if we nudged that weight a bit? You know, just a finger tap against the ramp to get it sliding along? As soon as the stationary friction between the weight and ramp was changed, it slid more easily. So does that mean I should jiggle my roof a bit to get the snow to ‘let go’? Sounds tempting, but for some reason my wife is frowning at that brilliant idea.
And so I wait for the legendary March thaw to cause a bit of melt water to ‘grease’ the snow layer on the roof and to “let ‘er rip” with a whoosh and a thump!
If you are seriously worried about the snow load on your roof, be sure to check out the many websites that provide all sorts of charts and graphs about snow load and roof support. I’m not sure if your insurance company will consider your research ‘due diligence’ but at least you will have a whole new appreciation for building codes.
Now to go outside, stand in the lane, and gaze upon the winter wonderland that is my roof, and wonder which weighs more, a ton of packed snow or a tonne of packed snow? And does it really matter to the 150-year-old rafters below it?