A classic photo of the dreaded ice dam

I had the opportunity to take a photo of a textbook case of ice damming the other day.

So you've got big icicles, and if things are really bad, water is entering the interior near the ceiling.  This is why.  When the temperature hovers around freezing, during the day the attic warms and melts the snow on the roof.  It travels as water to the eaves, where it re-freezes.  As the cycle is repeated, it eventually builds an   "ice dam".
What happens next is that water is trapped behind it and backs up under the shingles causing the leaks.  Leaking may enter the soffit and run down the exterior wall, and in the worst case scenario, the inside of the house.
How to fix it?  Ice guard can be used when a new roof is installed.  It's a rubber membrane that is placed at the lower edges of the roof instead of the familiar tar or felt paper.  It seals around nails and typically is folded over the fascia board, so if water backs up under the shingles, it simply travels over the top of the ice guard to the gutter/fascia.
The other way is to keep the roof cold through proper venting.  The colder the roof/attic space,  it becomes less likely that heat will melt the snow.
More on corrective action and some of the problems that are encountered when doing them in my next blog.