A particularly timely repeat posting given the horrible conditions in the Northeast United States. If you’ve got icicles and lots of snow on your roof, you definitely want to check this out.
Then, once you have learned about the causes and dangers of ice dams, do something about it and build yourself a snow-rake.
Link: Dam ice Dams
Want to learn the ?building science? of ice dams? Building Science corp has an amazing set of notes on building problems and precautions. I consider them the source for building information relating to tricky issues that tend to foil most contractors.