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Why Do Ice Dams Form in Roof Valleys?

Homes that have large roof valleys or many roof valleys typically have yuuuge ice dam issues, when compared to homes without valleys or few of them

The home of your dreams may have six glorious roof valleys, so I don’t want to speak ill of it, but those valleys can create a few problems as far as ice dams are concerned:

Problem 1: Snow accumulates in valleys.  There’s almost always more snow in roof valleys than anywhere else on a roof.  The wind blows snow in there, and the snow gets trapped. 

Problem 2: Valleys collect water.  That’s largely by design, so as to allow for runoff.  A valley is formed by two intersecting slopes of a roof.  Any melting water collects right there, and it then gets soaked up by the snow that’s already packed into that space.

Problem 3: The valley is often times shaded by the peaks.  Any part of the roof that’s in the shade can be 10-20 degrees colder than an area that gets direct sunlight.  The lower the space on the roof valley, the colder it will get.

The snow accumulates throughout the roof valleys (from top to bottom).   Near the top of the valleys, where it’s warmer, the snow begins to melt and runs down the valley.  Remember, the hot air in your attic rises, so it’s always going to be warmer near the top of your valleys, and colder near the bottom of your valleys.  As this melted snow runs down the warmer upper part of your valleys, it freezes when it reaches the colder lower part of your valleys.  (This lower part of the valley is packed with even more snow, which helps slow the flow of water and gives it even more time to freeze.)  All this is a perfect recipe for an ice dam.  

The snow is also busy insulating the roof, helping to make it hotter.  This can depend on the density of the snow, but as a rule of thumb snow has at least an R-value of 1, per inch of snow.  Insulation is measured in R-values.  Most insulation has an R-value of 2.2 to 3.8.  That extra bit R-value from the snow can make a big difference.  That’s even more of a problem in roof valleys because there’s more snow in your valleys.

So if you have a choice and if ice dams are a big concern, look for a home without many valleys on the roof—or any valleys at all.  Sometimes they’re unavoidable, but you may be asking for trouble if you buy a home with multiple or big valleys.

If you did recently purchase a home with multiple and/or large roof valleys, do yourself a favor and invest in a roof rake.  Do your best to keep those valleys as snow-free as possible throughout the winter months.

You might also consider having a home energy audit performed, as well as setting aside an emergency ice dam removal fund so you that you can call an ice dam removal professional if and when you get into trouble.

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