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Once the floor system was stabilized, it was time to see what was going on with the roof.  In years past, we had a few problems with ice dams.
Ice dams are caused when snow on the roof is melted by warm, interior air temperatures.   The now-liquid water flows down the roof and reaches the eaves, were the roof deck is not warmed by the interior conditions of the house. The water freezes, and  newly frozen ice creates a dam at the edge of the roof.  When additional water flows down the roof, it hits this dam of ice and flows toward the path of least resistance, which often can be back up the roof, under the roof shingles and into the house itself.  In short order this can cause both cosmetic and structural damage to the house.  Typically, the closer the roof deck is to interior temperature of the house, the greater the potential for ice dams.  The temperature of the roof deck is tied to the interior conditions by a combination of low levels of insulation and air leakage into the framing cavity from conditioned space.
On my own house, leaks caused by the ice dams were never properly fixed, and water continued to leak into the roof and wall unseen.  I knew I would be facing some issues when we opened up the the roof, but there was actually more damage than I had anticipated.  When the roof was originally built, the connection to the original house had never been properly detailed and flashed, so I had some new rot to repair, an older “repair” to repair and an unworkable flashing problem created by the poorly designed and implemented addition.  This, coupled with the discovery of a carpenter ant infestation, and signs of mice living in the framing cavity made the decision to fix all my problems easy.  It was to rip the roof off entirely.

The picture shows the existing roof framing, where an older repair had been made to “fix” a leak.  The original rafters were rotten, and then sistered, but the cause of the leak was never addressed.  When the addition was attached to the house, it was not properly detailed and flashed.  The long-term leak created a prime condition for rot, ants and mice.