Do you ever feel as if your contractor is selling you polyisocyanurate foam when what you really needed was a setback thermostat? Here is a handy guide to common (and not so common) construction terms. Scroll through the glossary below or sort by the letter that starts the word you're looking for. For example, to find polyisocyanurate foam, select P and click Submit.
A condition which can occur with snow and freezing conditions. When snow or ice melts on a roof over a heated or partially heated attic space, the melting water may refreeze over an unheated areas such as a roof overhang. This re-frozen water may create a "dam" and allow additional melt water to back up under shingles and cause leaks.(See:Illustration "A").
Solutions include: proper roof venting and insulation (See: illustration "B"), membrane roofing or roofing underlayment, and heat tapes.
Once an ice dam occurs, remedies are difficult and or dangerous. Working on a frozen roof should be avoided, as should the use of any open flames. The use of hot water to melt the ice may help, it may also increase the amount of leakage.
Illustrations are from Wood-Frame Construction by L.O. Anderson - Agriculture Handbook #73 p. 72.