Raking The Roof

28January 2020

I always thought roof raking wasn’t really very helpful. Or so I thought until a couple years ago. I was watching a YouTube video about roofs in which the contributor talked about ice dams. It turns out there is some merit to roof raking – and not just because of the extra weight. Raking off snow helps prevent ice dams and the damage they can cause.

In the long run however, the real cure for ice dams is much safer and easier than raking snow down onto yourself. The real cure is better roof ventilation. That’s it.

It’s fairly common knowledge that ice dams are caused by heat escaping into an attic and melting snow on the roof. The melt water runs down the roof and freezes over the eaves where it isn’t warm. Ice builds up, forming a dam which retains more melt water, and eventually some of it backs up under the shingles and leaks into the house.

What I learned by watching the video is that even if you have a well-insulated attic or roof space, ice dams can still be a problem after a winter storm because snow is such a good insulator – about R-0.5 to R-1.0 per inch.

Consider the following:

  • A roof deck which is above zero can cause melting. Regardless of the amount of insulation installed, if there’s 24 inches of snow on the roof, the roof deck is sandwiched between two layers of insulation… whatever is installed below, and now another R-18 more or less, on top of the roof – in the form of snow.
  • If the temperature difference between inside and outside is 40 degrees (-20C outside and +20C inside) then the roof deck is probably right in the middle, around 0C – at the point where snow can begin to melt. The snow is in fact insulating the roof sufficiently to melt itself. Not every winter is the same, however here in east-central Ontario we can have snow almost every night for a few days, and two or three feet of snow on a roof is not uncommon.
  • You can’t count on insulation alone to control ice damming during heavy snow falls. You need to complement your insulating efforts with suitable ventilation – or Australian ski bums with shovels as the YouTube video pointed out. Roof rakes however, cause less damage than ski bums with shovels.

The Bottom Line

Adequate eaves ventilation is just as important as having a suitable number of roof vents. If you experience ice damming which is not attributable to clogged eaves troughs or frozen down pipes, and you are unsure whether you have adequate ventilation, a quick call to a professional roofing or building contractor, or your home inspector might be a good idea.

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