What is the purpose of roof felt underlayment?

The purpose of roof felt underlayment is primarily to keep the roof sheathing dry. This layer is typically installed directly on top of the roof deck before applying shingles. It acts as a barrier against moisture, helping to protect the sheathing and, ultimately, the structure below from water infiltration caused by rain, snow, or ice.

While the other options mention features that may seem related, they do not accurately describe the underlayment's main role. For instance, roof felt is not designed to be a primary waterproof layer like a membrane; it can provide added protection but is more of a secondary measure. It does not provide insulation, as its primary function is not thermal resistance but moisture management. Additionally, it is not intended as a decorative layer since it is typically covered by shingles and not seen once the roof is completed.