Is a shingle underlayment necessary?
Yes, all shingle manufacturers require underlay (UDL) to validate their warranties. A UDL stops moisture (from condensation or wind-driven rain) from entering your home.
The roof slope (pitch) determines the type of underlay required. Low slope roofs (2/12 to 3/12) require two plies of # 15 felt (organic or fiberglass). Steep slope roofs (4/12 +) require a single ply of manufacturers' approved UDL. The most proven and affordable underlay is # 15 asphalt felt. Synthetic UDL's, mostly PVC-based, have a higher cost, but do not wrinkle when left exposed to the weather for a long period of time.
All UDL's are installed in conjunction with anti-icing eave protection.
Make sure your estimator gives you an option for breathable underlay.