What is the main purpose of a roof underlayment?

The main purpose of a roof underlayment is to provide moisture protection. Underlayment acts as a secondary barrier against water infiltration when roofing materials such as shingles are installed. If the outer layer is compromised—due to wind-driven rain, ice dams, or physical damage—the underlayment will help prevent water from penetrating the roof deck and causing damage to the structure underneath.

In addition to moisture protection, underlayment also plays a role in safeguarding against the effects of temperature fluctuations, potentially extending the lifespan of the roofing system. While options such as providing additional insulation or serving as a base for shingles may have a degree of relevance, they do not capture the primary function of underlayment as effectively as moisture protection. Aesthetic appeal is not a function of underlayment, as it is typically not visible once the primary roofing materials are applied.