What function does shingle underlayment serve?
An underlayment, commonly known as roofing felt, will:
- Protect the roof deck from moisture prior to shingle application
- Provide a degree of backup protection in the event water gets under roofing shingles.
Some local building codes and UL standards require that a shingle underlayment be installed. Also, some manufacturers offer a special underlayment product which prevents leaks caused by water backup from ice dams—a common condition in many winter snow areas. Protection against ice dams can be obtained by using a waterproofing shingle underlayment at the eaves or lower edges of the roof, in addition to installing adequate ventilation and proper insulation in the attic floor.