Anatomy Of An Ice Dam


When deep snows happen, ice dams often happen too. 

Notice two words in the left diagram:

Convection and Conduction.

An easy way to remember what they do is this:

Conduction is THROUGH and convection is TO. 

Cooking heat is conducted through a frying pan into the food, which heat also convects to the upper level and the upstairs feels warmer.

Deep snow on a roof creates a condition ripe for ice dams.  There are a lot of battles going on between heat and cold.

This winter I have gotten lots of calls because of ice damming.  And it is a huge problem.  And some of the icicles I have seen are enormous!


This photo shows literally what the diagram is teaching above.

This is a box window on a house I visited on Saturday.  To the right is the garage and the left a box window.  The other side of the box window looks just like this side.

Trapped water is being forced through the siding, soaking into the garage wall and onto the garage floor.

Trapped water is also being forced up into and under the cedar shakes and into the box window ceiling and framing.

This is what my thermal camera saw in that box window:


This is immediately under those shakes you see in the photo to the right. The pattern of water intrusion is clearly visible as lavenders and purples. The darkest purple is 38 degrees F.

The other side of this same box window, about 6' wide, is here:

This moisture is clearly getting into the wood framing of the window as well.

Ice damming is a real problem and happens around here every time we get deep snows.  Fortunately the snow is melting slowly and saving foundations.  Too much water pressure around the house and, well, other problems begin there too!

My recommendation:  Pay attention to your house!  If possible, try to eliminate some of the build up at the bottom of the roof.  Your house will appreciate it.  I bet your gutters will too!


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Jay performs inspections Monday through Saturday, throughout Northern Virginia, from his office in Bristow to Leesburg and Centreville, to Great Falls and Vienna and everywhere in between!